The 3d kinematics of the single leg flat and decline squat

i Keywords Kinematics, biomechanics, single leg squat, physiotherapy screening protocols, lumbopelvic stability, intrinsic injury risk, malalignment, hip strength, ankle dorsiflexion...

T HE 3D KINEMATICS OF THE SINGLE LEG

FLAT AND DECLINE SQUAT

Stephen Timms Bachelor of Applied Science (Human Movements Studies)

Professor Keith Davids, Dr Anthony Shield, Dr Marc Portus

Submitted in fulfilment of the requirements for the degree of

Masters of Science (Research)

School of Human Movements Faculty of Health Queensland University of Technology

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Keywords

Kinematics, biomechanics, single leg squat, physiotherapy screening protocols, lumbopelvic stability, intrinsic injury risk, malalignment, hip strength, ankle dorsiflexion

Abstract

Background: Pre-participation screening is commonly used to measure and assess potential intrinsic injury risk The single leg squat is one such clinical screening measure used to assess lumbopelvic stability and associated intrinsic injury risk With the addition of a decline board, the single leg decline squat (SLDS) has been shown to reduce ankle dorsiflexion restrictions and allowed greater sagittal plane movement of the hip and knee On this basis, the SLDS has been employed in the Cricket Australia physiotherapy screening protocols as a measure of lumbopelvic control in the place of the more traditional single leg flat squat (SLFS) Previous research has failed to demonstrate which squatting technique allows for a more comprehensive assessment of lumbopelvic stability Tenuous links are drawn between kinematics and hip strength measures within the literature for the SLS Formal evaluation of subjective screening methods has also been suggested within the literature

Purpose: This study had several focal points namely 1) to compare the kinematic differences between the two single leg squatting conditions, primarily the five key kinematic variables fundamental to subjectively assess lumbopelvic stability; 2) determine the effect of ankle dorsiflexion range of motion has on squat kinematics in the two squat techniques; 3) examine the association between key kinematics and subjective physiotherapists’ assessment; and finally 4) explore the association between key kinematics and hip strength

Methods: Nineteen (n=19) subjects performed five SLDS and five SLFS on each leg while being filmed by an 8 camera motion analysis system Four hip strength measures (internal/external rotation and abd/adduction) and ankle dorsiflexion range

of motion were measured using a hand held dynamometer and a goniometer respectively on 16 of these subjects The same 16 participants were subjectively assessed by an experienced physiotherapist for lumbopelvic stability Paired samples t-tests were performed on the five predetermined kinematic variables to assess the differences between squat conditions A Bonferroni correction for multiple comparisons was used which adjusted the significance value to p = 0.005 for the paired t-tests Linear regressions were used to assess the relationship between kinematics, ankle range of motion and hip strength measures Bivariate correlations

R2 = 305) kinematics Only the dominant ankle (p = 0.020; R2 = 331) was found to have a relationship with the decline squat 3) Strength measures had tenuous associations with the subjective assessments of lumbopelvic stability with no significant relationships being observed 4) For the non-dominant leg, external rotation strength and abduction strength were found to be significantly correlated with hip rotation kinematics (Newtons r = 0.458 p = 0.049; Normalised for bodyweight: r = 0.469; p = 0.043) and pelvic obliquity (normalised for bodyweight: r

= 0.498 p = 0.030) respectively for the SLFS only No significant relationships were observed in the dominant leg for either squat condition Some elements of the hip strength screening protocols had linear relationships with kinematics of the lower limb, particularly the sagittal plane movements of the knee and ankle Strength measures had tenuous associations with the subjective assessments of lumbopelvic stability with no significant relationships being observed;

Discussion: The key finding of this study illustrated that kinematic differences can occur at the hip without significant kinematic differences at the knee as a result of the introduction of a decline board Further observations reinforce the role of limited ankle dorsiflexion range of motion on sagittal plane movement of the hip and knee and in turn multiplanar kinematics of the lower limb The kinematic differences between conditions have clinical implications for screening protocols that employ frontal plane movement of the knee as a guide for femoral adduction and rotation Subjects who returned stronger hip strength measurements also appeared to squat deeper as characterised by differences in sagittal plane kinematics of the knee and ankle Despite the aforementioned findings, the relationship between hip strength and lower limb kinematics remains largely tenuous in the assessment of the lumbopelvic

stability using the SLS The association between kinematics and the subjective measures of lumbopelvic stability also remain tenuous between and within SLS screening protocols More functional measures of hip strength are needed to further investigate these relationships

Conclusion: The type of SLS (flat or decline) should be taken into account when screening for lumbopelvic stability Changes to lower limb kinematics, especially around the hip and pelvis, were observed with the introduction of a decline board despite no difference in frontal plane knee movements Differences in passive ankle dorsiflexion range of motion yielded variations in knee and ankle kinematics during

a self-selected single leg squatting task Clinical implications of removing posterior ankle restraints and using the knee as a guide to illustrate changes at the hip may result in inaccurate screening of lumbopelvic stability The relationship between sagittal plane lower limb kinematics and hip strength may illustrate that self-selected squat depth may presumably be a useful predictor of the lumbopelvic stability Further research in this area is required

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Table of Contents

Keywords i

Abstract ii

Table of Contents v

List of Figures vii

List of Tables viii

List of Abbreviations ix

Statement of Original Authorship x

Acknowledgments xi

CHAPTER 1: INTRODUCTION 1

1.1 Background 1

1.2 Purposes 4

1.3 Hypotheses 5

1.4 Thesis Outline 5

CHAPTER 2: LITERATURE REVIEW 7

2.1 Methodology 7

2.2 Sport and Exercise Related Injury 7

2.3 Intrinsic Injury Risks of the Lower Limb 9

2.3.1 Strength Deficiencies 9

2.3.1.1 Strength Deficiencies and General Injury Incidence 10

2.3.1.2 Regionally Specific Injuries and Associated Strength Deficits 12

2.3.2 Lower Limb Alignment - The ‘Medial Collapse’ 17

2.3.2.1 Clinical Implications of Medial Collapse 19

2.3.3 Hip Strength, Lower Limb Malalignment and Force Attenuation 22

2.3.3.1 Running 22

2.3.3.2 Landing 23

2.3.3.3 Cricket Fast Bowling 25

2.3.4 Ankle Dorsiflexion Range of Motion 26

2.4 Exercise and Sport Related Epidemiology 30

2.4.1 Cricket Epidemiology 31

2.5 Screening Protocols Used To Assess Risk 35

2.6 Functional Testing to Assess Intrinsic Risk: 38

2.6.1 Trendelenburg Assessment 38

2.6.2 The Squat 39

2.6.3 The Single Leg Squat 40

2.6.4 Single Leg Flat Squat Vs Single Leg Decline Squat 46

2.7 Summary and Implications 53

CHAPTER 3: RESEARCH DESIGN 55

3.1 Participants 55

3.2 Research Design 55

3.2.1 Anthropometry 55

3.2.2 Single Leg Squatting Protocol 55

3.2.3 Single Leg Squatting 3-Dimensional Motion Capture 57

3.2.4 Anatomical Modelling 58

3.2.4.1 Kinematic Modelling and Data Output 58

3.2.4.2 Data Filtering 60

3.2.5 Strength Testing Protocol 60

3.2.6 Ankle Dorsiflexion Range of Motion 63

3.3 Analysis 64

3.3.1 Data Analysis 64

3.3.2 Statistical Analysis 65

3.3.2.1 Comparing SLDS and SLFS kinematics 65

3.3.2.2 Ankle Dorsiflexion Range of Motion and Kinematics 66

3.3.2.3 Subjective Lumbopelvic Screening and Kinematic Comparison 66

3.3.2.4 Strength Measures Vs Kinematics 67

3.4 Ethics and Limitations 68

CHAPTER 4: RESULTS 70

4.1 Subjects 70

4.2 Kinematics of the SLFS and SLDS 70

4.2.1 End of Range Angles 70

4.2.2 Mean Angles 71

4.2.3 Additional Kinematic Observations 72

4.3 Ankle Dorsiflexion Range of Motion 75

4.4 Qualitative and Quantitative Assessment of Pelvic Obliquity and Hip Rotation 77

4.5 Strength Measures 81

CHAPTER 5: DISCUSSION 84

5.1 3D Kinematics of the Single Leg Flat and Decline Squats 84

5.1.1 Pelvic Obliquity 85

5.1.2 Weight Bearing Hip Rotation and Adduction 85

5.1.3 Lateral Flexion of the Lumbar Spine Relative to the Pelvis 87

5.1.4 Frontal Plane Movement of the Knee 87

5.1.5 Additional Kinematic Observations 88

5.2 Ankle Dorsiflexion 90

5.3 Kinematic and Subjective Clinical Assessment 93

5.4 Kinematics and Strength Analysis 95

CHAPTER 6: CONCLUSIONS 99

6.1 Direct Response To Study Hypotheses 99

6.2 Concluding Statements 101

BIBLIOGRAPHY 105

CHAPTER 7: APPENDICES 117

7.1 Appendix A: UWA Model outputs 117

7.2 Supplementary Results 118

[177] 59 Figure 5 A) Illustration of the hip coordination system (XYZ), femoral coordinate system

(xyz), and the Joint Coordinate System (JCS) for the right hip [1] 5 B) Graphical

representation of knee flexion in the sagittal plane 60 Figure 6 Example of the abduction strength test conducted in lying supine during the study

The dynamometer is held against the lateral malleolus and subjects are asked to

build up force against the dynamometer 61 Figure 7 Example of the internal rotation strength test 62 Figure 8 Example of the knee to wall test The angle of the tibia relative to the vertical is

represented by “” and was reported as ankle dorsiflexion The distance (mm) the

hallux was away from the wall is represented by “d” 63

Figure 9 Sagittal plane representations of the SLDS and SLFS conditions at EOR (A and B

respectively) and a direct comparison of the squatting conditions (C) Frontal plane

representations of the SLDS and SLFS conditions at EOR (D and E respectively)

and a direct comparison of the squatting conditions (F) 74 Figure 10 A graphical representation of the ND ankle and knee kinematics with respect to the

clinical measure of ankle dorsiflexion 76 Figure 11 Kinematic scatter plot of the dominant hip rotation and frontal plane knee as

categorised by subjective physiotherapy rating 79 Figure 12 Comparison of the hip external rotation kinematics for the SLFS and SLDS when

plotted against normalised hip external rotation strength The shape “” represents

the SLFS ND hip rotation ( º) whilst the shape “” represents the SLDS ND hip

rotation angles 82 Figure 13 The pelvic obliquity kinematics for both squatting protocols compared with hip

abduction strength The symbol “” represents the SLFS ND pelvic obliquity

angles (º) whilst the symbol “o” represents SLDS ND pelvic obliquity angles (º) 83

List of Tables

Table 1 Reported incidence of cricketing injuries according to injured body region 33 Table 2 Summary of the single leg squat literature 50 Table 3 Basic descriptive statistics of the joint angles at EOR and the results of the paired t-test

comparing squatting conditions 71 Table 4 Basic descriptive statistics of the mean joint angles and the results of the paired t-test

comparing squatting conditions 72 Table 5 Summary of the functional differences from the paired t-test of the SLS conditions 73 Table 6 Correlations between the two measures of ankle dorsiflexion ROM and sagittal plane

movement of the knee and ankle 75 Table 7 The average pelvic obliquity and relative lateral flexion measurements as categorised

by qualitative physiotherapy assessment (“Normal” vs “Excessive” movement) for

the SLDS 77 Table 8 The average pelvic obliquity and relative lateral flexion measurements as categorised

by qualitative physiotherapy assessment (“normal” and “excessive” movers) for the

SLFS 78 Table 9 Mean strength measurements as categorised by qualitative physiotherapy assessment

for each squat condition 80 Table 10 UWA model outputs and practical meanings 117 Table 11 Non-dominant leg strength (in Newtons) and EOR kinematic correlation matrix 118 Table 12 Non-dominant leg strength (normalised to body weight) and EOR kinematic

correlation matrix 118 Table 13 Dominant leg strength (in Newtons) and EOR kinematic correlation matrix 119 Table 14 Dominant leg strength (normalised to body weight) and EOR kinematic correlation

matrix 119 Table 15 Results from independent t-tests comparing the strength measures of normal and

excessive movers in both squat conditions 120 Table 16 Linear regression modelling comparing the clinical measure of ankle dorsiflexion and

sagittal plane 120

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List of Abbreviations

ISB International Society of Biomechanics

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet requirements for an award at this or any other higher education institution To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made

Chapter 1: Introduction

This chapter outlines the background (Section 1.1) of the research, and its purposes (Section 1.2) Section 1.3 outlines the hypotheses of the study and finally Section 1.4 outlines the remaining thesis chapters

Whilst there has been an increased promotion of physically active lifestyles to improve quality of life [2, 3] and reduce the risk of noncommunicable diseases [4], there has been a reluctance to recognise the coupled risk of injury associated with participation in physical activity [5] A large proportion of the population engage in sport and as such, sports injuries are relatively common in modern western societies [6]

Sports injuries are a multifaceted phenomenon and are often difficult and time consuming to treat resulting in serious financial ramifications such as the cost of medical treatment and physiotherapy, loss of work time and the loss of physical function [2, 3, 5-7] It was estimated that the cost of sports injuries in Australia was

$1 billion annually in 1990 [8], $1.65 billion in 2002 [9], and still remains significant [10] Preventative strategies are therefore justified on medical as well as economic grounds [9-12]

To fully appreciate the complexities of the multifaceted concept of injury; the epidemiology, aetiology, risk factors and exact mechanisms associated with injury need to be defined Risk factors are typically differentiated into either extrinsic (environmental) or intrinsic (internal) factors [5, 7, 13, 14] Emphasis has been placed on the role of the intrinsic risk factors [7] as these have been demonstrated to

be more predictive of injury than environmental related factors [15]

Pre-participation (or baseline) screening is a commonly used method to assess potential intrinsic injury risk factors by identifying characteristics of the musculoskeletal system that may predispose an athlete to injury, or to identify incomplete recovery from a previous injury [16, 17] In addition to injury risk management strategies, screening is concurrently promoted as part of a performance enhancement strategy [18] Screening tests are thought to highlight an athlete’s predisposition to injury but the validity of a majority of the current protocols have yet to fully established due to the paucity of quality injury risk factor studies [18, 19] Moreover, there is almost no reliable evidence base to support the validity of these tests in predicting injury risk [20, 21]

The ability to clearly identify injury risk or performance enhancing factors is reliant on the accuracy with which measurements are made Furthermore, establishing the reliability and validity of commonly used clinical assessment tools is

a key issue encountered by studies of intrinsic injury risk factors [17] Issues surrounding screening reliability can be alleviated by biomechanically investigating the accuracy of screening protocols A level of formal evaluation such as motion analysis clarifies the association between the clinical practices and the quantitative methods [22] Whilst research has been conducted in assessing the reliability of lower extremity clinical screening tests [17, 19, 23], it focussed on inter-rater and test-retest reliability The reliability of functionally orientated tasks has been investigated [18] but many of these are yet to be validated

There is almost a universal agreement within the literature that a lack of physical fitness is an intrinsic risk factor for musculoskeletal injury during physical activity [5, 7, 24] Nevertheless, the link between muscular strength and lower injury risk is not fully understood [25] Athletes must possess sufficient strength to provide joint stability in all three planes of motion [26] to maximise athletic function [27] as well as reducing the incidence of injury [14, 25, 26, 28-34]

Evidence is beginning to emerge that highlights a relationship between certain screening tests and the incidence of lower limb injuries, particularly in the sporting

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demographic [35] One such screening test that warrants further investigation is the single leg squat (SLS) which replicates an athletic position commonly assumed in sport requiring multi-plane control of the trunk and pelvis on the weight bearing femur [25, 27, 36] The SLS is used clinically as a functional measure of lumbopelvic stability [25, 27, 36] as it is argued that this test has a greater ability to highlight those with poor lumbopelvic stability [37] than the standard two legged squat With the addition of a decline board, the single leg decline squat (SLDS) is also widely used as a targeted rehabilitation intervention for patellar tendinopathy due to an increased loading of the patellar tendon [38-42] Increased loading of the patellar tendon is achieved by significantly reducing any posterior ankle constraints allowing greater squat depth [43] Greater squat depth is presumably the reason why the SLDS has been employed in the Cricket Australia (CA) physiotherapy screening protocols as a measure of lumbopelvic control in the place of the more traditional single leg flat squat (SLFS) The greater squat depth conceivably promotes a more challenging position for the participant and supposedly allows for a superior lumbopelvic screening tool However, the assumption of a deeper squat created by the decline board promoting a more challenging position has yet to be tested

Previous research has investigated the kinematic differences between the SLDS and SLFS both in 2D [44] and 3D [43] focussing mainly on the differences in sagittal plane knee kinematics These researchers were unable to demonstrate clear differences between the two conditions relating to the kinematics of the torso and weight bearing hip [43, 44] In particular, it is not known whether the two techniques differ with regards to hip internal rotation and adduction, obliquity of the pelvis and torso lateral flexion A better understanding of the differences between the two conditions around the weight bearing hip is an important aspect of interpreting the SLDS in the CA physiotherapy protocol

Investigating the relationship between the clinical and field based testing procedures [23] and the more sophisticated 3D kinematic analysis is an important step in validating the use of field based tests to predict injury Understanding this relationship would facilitate the development of standardised musculoskeletal screening protocols This standardisation would conceivably yield more reliable and

accurate screening protocols allowing for appropriate musculoskeletal interventions [18] Appropriate interventions assist the management of any predispositions to injury, which in turn may influence the incidence of lower limb musculoskeletal injury in cricket

This study had numerous goals The first was to compare the kinematic differences between the flat and decline squatting conditions, primarily the five key kinematic variables fundamental to subjectively assess lumbopelvic stability These variables were; 1) pelvic obliquity; 2) hip abduction/ adduction angles of the weight bearing (WB) hip; 3) hip internal/external rotation angles of the WB hip; 4) the degree of lateral flexion of lumbar spine relative to pelvis and 5) frontal plane excursion of the knee on the weight bearing limb

The second aspect was centred on the basis for the employment of a decline board for the single leg squat Determining the effect of ankle dorsiflexion range of motion has on squat kinematics is vitally important in determining the future role that that decline board has in screening for lumbopelvic stability with the single leg squat

The third aspect was to examine the association between squat kinematics and the associated subject clinical assessment Understanding the terms of agreement between qualitative and quantitative measurements of movement is an essential element in validating such tests

The fourth and final facet was to assess what relationship, if any, the aforementioned key kinematic variables had with measures of hip strength An understanding of the relationship between kinematics and strength may provide insight into pathomechanics patterns highlighted by functional screening tools such

as the single leg squat

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So to summarise, the numerous focal points of this study were namely;

1 To compare the kinematic differences between the two single leg squatting conditions, primarily the five key kinematic variables fundamental to subjectively assess lumbopelvic stability;

2 Determine the effect of ankle dorsiflexion range of motion has on squat kinematics;

3 Examine the association between key kinematics and subjective physiotherapists’ assessment;

4 Explore the association between key kinematics and hip strength;

The subsequent chapters of this thesis reviews the literature (Chapter 2) regarding sport and exercise related injury, intrinsic injury risks such as strength

deficits, lower limb malalignment, associated biomechanical changes and clinical implications of lower limb malalignment, force attenuation through the lower limb and ankle dorsiflexion; cricket epidemiology, functional screening tools and their association with assessing intrinsic injury risk and finally, the employment of the single leg squat in screening protocols Chapter 3 outlines the research design and protocols employed in this study Results from the study are outlined and discussed

in Chapters 4 and 5 respectively Finally the implications and concluding statements can be located in Chapter 6 Chapter 7 (Appendix) also contain a guide to the kinematic terms used in this thesis that may assist the reader in understanding what each of the kinematic terms represent Supplementary tables from the results section are also located here for the perusal of the reader

Chapter 2: Literature Review

The methodology employed for the collection of relevant literature pertaining

to this review was chiefly through papers available to QUT students through electronic databases namely, Academic Search Elite, Mediline, Cinahl, SportDiscus and Academic Search Premier which are all subsidiaries of EBSCOhost Some papers were obtained from within the bounds of the QUT library periodicals section

if not accessible electronically at the previously mentioned databases All papers were peer reviewed

The key words that were predominantly used to search for journal articles included: kinematics, biomechanics, single leg squat, physiotherapy screening protocols, lumbopelvic stability, intrinsic injury risk, injury, malalignment, hip strength and ankle dorsiflexion

The reference list of this review was emailed to all members of the research team as to ensure no prominent omissions from the literature review

Physical inactivity is among the leading causes of the major noncommunicable diseases, including cardiovascular disease, type 2 diabetes and certain types of cancer, contributing substantially to the global burden of disease, death, disability and injury [4] Previously, physically active lifestyles were linked with more vigorous working and labour intensive domestic environments Currently however, technological advancements have reduced incidental physical activity and thus yielded a more sedentary society As a consequence, sport and exercise related physical activity has been undertaken by a significant proportion of the population either recreationally or competitively [5] It has been well documented that physical

activities in the form of sport and exercise have positive effects on the physical and psychological health and wellbeing of individuals [2-4, 45] It is recognised internationally that physical activity, such as sport, exercise and active travel, may help to prevent a number of global public health problems [45] such as those previously outlined [4] This ethos has consequently encouraged the World Health Organisation (WHO) as well as many governments to set formal physical activity guidelines encouraging citizens to engage in physical activity through sport and active travel [5]

Whilst there has been an increased promotion of physically active lifestyles to improve quality of life [2, 3] and reduce the risk of noncommunicable diseases [4], there has been a reluctance to recognise the coupled risk of injury associated with participation in physical activity [5] A large proportion of the population engage in sport and as such sports injuries are relatively common in the modern western societies [6] In the US, sports-related injuries account for 2.6 million visits to the emergency room made by children and young adults (aged 5–24 years) [46] Injuries sustained by high-school athletes currently result in 500 000 doctor visits, 30 000 hospitalisations and a total cost to the healthcare system of nearly $2 billion per year [46] In the UK, there are an estimated 19.3 million sport and exercise related injuries annually [3] whilst one in five Australians are prevented from being more physically active due to injury or disability [8] Sports injuries are a multifaceted phenomenon and as such are often difficult and time consuming to treat resulting in serious financial ramifications such as the cost of medical treatment and physiotherapy, loss

of work time, and notwithstanding the loss of physical function [2, 3, 5-7] It was estimated that the cost of sports injuries in Australia was $1 billion annually in 1990 [8] $1.65 billion in 2002 [9], and still remains significant [10] Preventative strategies are therefore justified on medical as well as economic grounds

To fully appreciate the complexities of the multifaceted concept of injury; the epidemiology, aetiology, risk factors and exact mechanisms associated with injury need to be defined Risk factors for example are usually differentiated into either extrinsic or intrinsic factors [5, 7, 13, 14] Extrinsic risk factors are those that originate external to the body These are described within the literature as elements

such as level of competition and skill level, intensity and frequency of activity, shoe type, playing surface, environmental conditions and external contact from equipment and other players [5] Conversely, intrinsic injury risk factors can be defined as those that are internal to the body in forms such as age, gender, musculoskeletal alignment, previous history of injury, somatotype, strength, range of motion and biomechanics [13] The varying aetiologies of lower limb injuries are typically a manifestation of one or numerous risk factors interacting together at a given time More recently, emphasis has been placed on the role of the intrinsic risk factors [7] as these have been demonstrated to be more predictive of injury than environmental related factors [15] As such, for the purposes of this literature review intrinsic risk factors such as lower limb strength deficits, alignment, biomechanics, landing kinematics and ankle dorsiflexion range of motion will be focussed on to illustrate their association with lower limb injuries

2.3.1 STRENGTH DEFICIENCIES

Considering the wide variety of movements associated with athletic function, athletes must possess sufficient strength to provide joint stability in all three planes

of motion [26] Whilst there is almost a universal agreement within the literature that

a lack of physical fitness is a risk factor for musculoskeletal injury during physical activity [5, 7, 24], the link between muscular strength and lower injury risk is not fully understood [25] When the musculoskeletal system works effectively, the result

is the appropriate distribution of forces, optimal control and efficiency of movement, adequate absorption of ground-impact forces and an absence of excessive compressive, translational, or shearing forces on the joints of the kinetic chain [33] Stability through the pelvis and hips, proximal lower limb, spine and abdominal structures creates several advantages for integration of proximal and distal segments

in generating and controlling forces to maximise athletic function [27]

The gluteal muscles are stabilisers of the trunk over a planted leg which generate a great deal of power for athletic activities [27] Moreover, the hip

abductors (gluteus maximus, posterior gluteus medius, biceps femoris) and external rotators (piriformis, gemellus superior, obturator internus, gemellus inferior, obturator externus and quadratus femoris) play an important role in lower extremity alignment in ambulatory activities as they assist in the maintenance of a level pelvis [47] and are involved in the prevention of hip adduction and internal rotation during single limb support [26, 48, 49] Conventional wisdom asserts that strength deficits would presumably contribute to inadequacies in the aforementioned elements of an effective system resulting in poor physical performance, elevated injury risk, or both

An increase in injury risk varies depending on the anatomical location, as muscles of the peri-pelvic region and lower limb have numerous individual and synergistic roles

in lower limb movements

2.3.1.1 STRENGTH DEFICIENCIES AND GENERAL INJURY INCIDENCE

The association between weakness of the hip musculature and injuries of the lower limb has been investigated by numerous studies [14, 25, 26, 28-34, 50] An early study by Nicholas, Strizak and Veras [34] attempted to define the existing relationships between an injured part of the lower extremity and muscle groups far removed anatomically from the site of injury These researchers classified 134 injured patients into a seven categories according to the nature of their musculoskeletal disease or injury These injury groups were named ankle and foot-, back-, knee ligamentous instability-, intraarticular defect-, patella-, arthritis- and control-group [34] The control group was derived from the all patients’ legs that were uninvolved by the aforementioned injury processes and were matched against the affected of symptomatic leg [34] Generally speaking, the data revealed that the more distal the injury site, the greater the total weakness in the affected limb Patients with ankle injuries revealed consistent weaknesses in their hip abductor and adductor muscle group [34] Ipsilateral quadriceps weakness was significantly associated with ligamentous instability of the knee, patellar lesions, intraarticular defects and back complaints (P < 0.025, 0.01, 0.005 and 0.05 respectively) [34] The researchers concluded that the strength of the lower body is an integrated unit, which can be affected in many different areas, some quite remote form the site of pathology, by a single pathological disorder [34] A clear limitation of this study is the retrospective aspect of data collection as it is difficult to ascertain whether

weakness contributed to the injury, exacerbated it symptoms, or is a product of the injury

An attempt was made by Lysens and colleagues [7] to understand the physical and psychological profiles of the accident prone and overuse prone athletes In a one year prospective study, 185 physical education students (118 males; 67 females) of the same age (18.3 ± 0.5 years) trained under the same conditions and were exposed

to similar extrinsic risk factors [7] Numerous physical intrinsic risk factors were profiled including anthropometric data, physical fitness parameters, flexibility aspects and malalignments of the lower extremities in addition to 16 personality traits [7] Concerning the overuse proneness, a lack of static strength, ligamentous laxity and muscle tightness predisposed students to injury, presumably due to the compromised function of associated muscles and ligaments [7] These effects were amplified by large body weight and height, a high explosive strength and lower limb malalignment [7] Researchers also noted that psychosomatic factors such as a degree of carefulness, dedication, vitality and hypochondria are prominent in the pathogenesis and management of an overuse injury [7]

Leetun and colleagues [26] prospectively studied collegiate athletes who participated in running and jumping sports comparing core stability measures between genders in addition to comparing injured and uninjured athletes Findings unearthed that athletes who sustained an injury over the course of a season demonstrated significantly lower measures of hip abduction and external rotation strength [26] Moreover, backwards logistic regression revealed that external rotation strength was the sole variable that predicted injury status for the athletes in the study [26] Studies such as Lysens and colleagues [7], Nicholas, Strizak and Veras [25] as well as Leetun and colleagues [26] demonstrated the relationship between proximal strength deficiencies and the general incidence of injury in the lower limbs

2.3.1.2 REGIONALLY SPECIFIC INJURIES AND ASSOCIATED STRENGTH

50-A study by Cichanowski and colleagues [31] determined the strength differences of hip muscle groups in collegiate female athletes diagnosed with unilateral patellofemoral pain and subsequently compared the strength measures with the unaffected leg and non-injured sport-matched controls Results illustrated that hip abductors and external rotators were significantly weaker between the injured and unaffected legs of the injured athletes [31] Moreover, injured collegiate female athletes exhibited global hip weakness compared with age- and sport-matched asymptomatic controls [31] Ireland and colleagues [32] also measured a number of hip strength measures using hand held dynamometry and demonstrated that participants with PFPS demonstrated 26% less hip abduction strength (p<.001) and 36% less external rotation strength (P<.001) than similar age-matched controls Baldon and colleagues [50] observed females with PFPS had higher hip adduction to abduction torque ratio in addition to a diminished capacity to generate eccentric hip abduction torque The greater incidence of PFPS in these studies was presumably due

to insufficient muscular strength to protect the knee from excessive internal rotation and knee valgus moments which may predispose to the further development of PFPS [31, 32, 51, 55]

Overuse knee injuries such as illiotibial band syndrome (ITBS) have also been investigated The causative mechanisms of injury for ITBS include extrinsic factors such as spikes in workload and downhill running in addition to intrinsic risk factors such as illiotibial band (ITB) tightness [56] and abnormal biomechanics [57] In addition, Fredericson and colleagues [28] observed that distance runners with ITBS had weaker hip abduction strength in the affected leg compared with their unaffected leg and with unaffected long distance runners [28] Their findings surrounding the relationship between hip abduction strength and ITBS were augmented when a six-week stretching and strengthening intervention program prescribed to all injured runners reduced the symptoms of ITBS [28] The researchers concluded that symptom improvement in addition to a successful return to the pre-injury training program, accompanied improvement in hip abductor strength [28] The suggestion that symptom improvement reflected improvements in hip abductor strength is congruent with a more recent study by Arab and Nourbakhsh [56] which reported that lower back pain participants with and without ITB tightness had significantly lower hip abductor muscle strength compared to participants without lower back pain [56]

Whilst the relationship between hip strength weakness and injury has been examined, Niemuth and colleagues [29] have proposed that a relationship exists between hip muscle imbalance and injury patterns Their study demonstrated that injured runners exhibited significant side-to-side differences in muscle strength in three hip groups (hip abduction, adduction and flexion), compared to non-injured counterparts [29] The injured runners’ side hip flexors and abductors were significantly weaker whilst their adductors were significantly stronger than their uninjured side muscles [29] As a point of comparison, non-injured runners did not show any side-to-side differences in hip strength This was the first study to show an association between hip abductor, adductor, and flexor muscle group strength imbalance and lower extremity overuse injuries in runners [29] Although no cause and effect relationship between weakness and injury was established, this study identified an association not widely recognised in the contemporary literature for the analysis and treatment of running injuries This study in conjunction with those

previously mentioned emphasized the substantive role of the hip external rotators and abductors in healthy and pathological knee function

THE HAMSTRINGS:

Moving distally from the stabilising proximal hip musculature, the hamstring muscle group plays a more significant role during activities such as running and jumping [58] The hamstrings contract eccentrically when they slow the forward swing of the leg to prevent overextension of the knee and flexion of the hips typical

of such movements as sprinting and when kicking a ball [59] Not surprisingly, hamstring strains are a common injury in sports that demand high intensity sprinting efforts such as athletics or numerous football codes [30, 60, 61] The total amount of missed playing time as a result of a hamstring injury has accounted for 16% in the Australian Football League (AFL) [62], between 10 and 23% in soccer [63] and almost 18% in cricket [64] There appears to be a consensus within the literature that

a vicious circle of recurrent hamstring injuries is not uncommon, resulting in a chronic problem with significant morbidity in terms of symptoms, reduced performance, and time loss from sports [15, 60, 61, 63, 65, 66] Additional causative factors for hamstring muscle strains have been studied extensively revealing that muscle fatigue, age and muscle weakness are the most commonly postulated intrinsic risk factors [19, 59, 60, 63, 65, 66] Studies have also shown that the addition of specific preseason strength training for the hamstrings – including eccentric overloading – would be beneficial for elite soccer players, both from an injury prevention and from performance enhancement perspectives [67]

THE LUMBAR SPINE

The dysfunction of the lumbar spine musculature plays a significant role in the aetiology of lower back pain in general population [68] The osteo-ligamentous lumbar spine is inherently unstable since, in vitro, it buckles under compressional loading of only 90N or 20lbs [69] The lumbar vertebrae tend to be most susceptible given their load dissipating attributes during trunk motion which in turn requires stabilization via the coordination of a number of mechanisms [27] This critical role

is undertaken by the complex interplay of both superficial and deep muscles around

the spine and demonstrates how vital core musculature is for generation and attenuation of energy during movement [27, 69] Deeper muscles primarily provide postural stability and consist of the quadratus lumborum (QL), iliocostalis lumborum, longissimus lumborum and the lumbar multifidus These muscles can have a direct influence on segmental stability and control of the lumbar spine due to their attachments to the spinal column [70] Coordinated, co-contraction of the lumbar paraspinal muscles with the abdominal wall muscles such as transversus abdominus is suggested to provide single joint stabilization that in turn allows multi-joint muscles to work more efficiently to control spine movements [27, 69] Consequently, this mechanism is assumed to provide a stable and safe platform for trunk and limb movement in addition to load dissipation [71]

Muscles of the lumbar region are reported to be major stabiliser of the lumbar spine [55] with a prime example being the QL The QL has been described as a major stabilizer of the lumbar spine by working dynamically in union with more passive structures such as bone and ligament [27, 69] in addition to being active during activities that require lateral flexion, axial rotation and extension of the trunk such as javelin throwing and fast bowling in cricket [72, 73] Understandably, any mechanisms that alter the functionality of any of the structures of the lumbar spine such as muscular asymmetry or weakness are likely to have detrimental effects on the loading characteristics and thus likelihood of injury [72-74] Muscular asymmetry in side-to-side strength of the hip extensors and abductors was found in athletes with a previous history of lower extremity injury or lower back pain in a study by Nadler and colleagues [74], implying a lateral dominance effect Furthermore, these same injured athletes were shown to have decrements in hip strength as compared with athletes without injury [74]

The notion of muscular asymmetry and weakness has been illustrated in numerous cricket studies investigating the force attenuation role of the QL in relation

to stress fractures of the pars interarticularis In a mechanical sense the pars acts as a fulcrum for the facet joints which lie to the posterior and are vital in preventing excessive lumbar spine movement [75, 76] Without the sufficient strength and activation of the lumbar musculature in symphony with numerous other lumbopelvic

mechanisms, the likelihood of a defect or fracture in this narrow portion of bone is elevated [73] Such fractures are referred to as a spondylolysis [77]

Engstrom and co-workers [72] have previously prospectively linked asymmetry of the QL muscle to lumbar spondylolysis in 51 adolescent bowlers They used MRI annually to measure and quantify QL asymmetry and for also identifying spondylolysis and compared QL asymmetry to that of a control group of swimmers (n=18) It was concluded that there was a strong association between QL asymmetry and the development of symptomatic unilateral spondylolysis [72] An appealing association was evident between the mechanical couplings of repetitive forces associated with symptomatic spondylolysis and the substantial asymmetry of QL in the injured fast bowlers [72] Asymmetry of the QL conceivably reflected an adaptive preferential hypertrophy of QL in response to the loading milieu and thus escalating susceptibility for pathogenesis of spondylolysis [72] This viewpoint is congruent with a study conducted by Visser and colleagues [73] who hypothesized that that the bowling technique of some cricketers caused unilateral hypertrophy of the QL indicating a technique that transmits abnormal stresses upon the lumbar musculature According to this study, the longer a cricketer has been exposed to a compromised technique that produces high stresses in the pars, the more likely the establishment of a cause-effect relationship between an bowling specific large asymmetry and a fracture [73] The discrimination between bowlers with and without symptomatic pars lesions provides a rational basis for using QL asymmetry as a potential clinical screening tool for investigating suspected spondylolysis [73]

THE ANKLE

The link between muscular strength, imbalance, and flexibility of the muscles acting on the ankle are frequently mentioned in the literature as possible intrinsic risk factors [78] However, due to the lack of quality prospective studies, the conclusions that can be drawn regarding the possible injuries are tenuous [78] Mahieu and colleagues [78] however, have prospectively investigated numerous intrinsic injury risk factors on the rate of Achilles overuse injuries in a military recruit population Almost 15% of the studied population suffered an injury with the analysis revealing

that male recruits with lower plantar flexor strength and increased dorsiflexion excursion were at a greater risk of Achilles tendon overuse injury [78] An isometric plantar flexor strength of lower than 50 Nm and dorsiflexion range of motion higher than 9.0° were possible thresholds for developing an Achilles tendon overuse injury [78] It was concluded by the research team that greater muscle strength produced stronger tendons that could deal better with high loads [78] These results reiterate how adequate muscular strength facilitates force attenuation and the associated reduction of injury risk

When all of the aforementioned literature is integrated, it indicates that movements of the lower limb involve a series of synergistic muscular contributions

of the entire kinetic chain to achieve the desired locomotor or performance outcome Any disruptions to this system manifest themselves in the form of an injury and can

be accredited to intrinsic and/or extrinsic injury risk factors Intrinsic risk factors such as deficits in muscular strength and balance have consistently been associated with the aetiology of lower limb injuries throughout all parts of the lower limbs and lumbopelvic region The literature in this area particularly demonstrates the importance of proximal stabilization for lower extremity injury prevention [26] particularly the knee [31, 32, 34, 51, 79] It appears that adequate lumbopelvic-femur muscle function may conceivably reduce exposure to other intrinsic risk factors such

as inefficient force attenuation, unstable movement patterns and lower limb malalignments [25, 80]

2.3.2 LOWER LIMB ALIGNMENT - THE ‘MEDIAL COLLAPSE’

The intersegmental joint forces and the structures that resist them, such as articular surfaces, ligaments and musculature, are associated through the anatomical alignment of the joints and skeletal system [14] Lower limb skeletal malalignments have been proposed as a risk factor for acute and chronic lower extremity injuries [5, 81] and may even be the primary cause of musculoskeletal patient problems [82] Biomechanical abnormalities associated with malalignments of the lower limb have frequently been implicated as a causative factor for lower limb injuries as a result of

intensive exercise [5], in addition to exacerbating the presence of a musculoskeletal injury that have some other causal mechanism [82]

The quadriceps angle or Q-angle is defined as the angle formed by a line from the anterior superior iliac spine to the patella centre and a line from the patella centre

to the tibial tuberosity and is often associated with malalignments of the lower limb [81, 83] Numerous studies have postulated that it is this structural difference between males and females that may contribute to an altered lower extremity movement pattern [84], and in turn, contribute to a gender injury bias [25, 26, 84, 85] Non-contact anterior cruciate ligament (ACL) injury rates, for example, have been reported to be six times higher in women especially in jumping sports [85-87] Whilst the aetiology of this type of injury is multifactorial [88], the most common mechanism of injury has been proposed to involve rapid deceleration of the lower extremity such as when landing from a jump or a rapid change in direction whilst running [87, 88] During activities such as rapid changes in direction or landing, the greater Q-angle in the female athlete may predispose the knee to more vulnerable positions which in turn places greater strain on the ACL [86, 88, 89]

Excessive frontal- or transverse-plane hip motion during single-limb weight bearing may be associated with excessive femoral adduction, an internal rotation leading to knee valgus, tibial internal rotation and excessive foot pronation This series of postural malalignments has been described as medial collapse [90, 91] Such alignments have been associated with insufficient muscular control and can alter the joint load distribution and, consequently, joint contact pressure of adjacent

or distant joints [92] Accounting for the alignment of the entire extremity in this context, rather than a single segment, may more accurately describe the relationship between anatomic alignment and the risk of lower extremity injury, since one alignment characteristic may interact with or cause compensations at the other bony segments [81, 82] Whilst this viewpoint remains largely theoretical [91], it appears more plausible when clinical interventions are designed and successfully implemented to reduce symptoms by addressing the underlying pathological malalignment and biomechanics [82]

The potential for an interactive effect between joint segments has been explored by Nguyen and Shultz [81] A factor analysis approach was employed in attempt to use a number of lower extremity alignment variables (femoral anteversion, quadriceps angle, tibiofemoral angle, genu recurvatum, tibial torsion and pelvic angle – the angle formed by a line between ASIS and PSIS relative to the horizontal plane)

to examine whether relationships could be identified among these variables [81] The analysis identified three distinct lower extremity alignment factors namely a valgus (greater anterior pelvic, quadriceps, and tibiofemoral angles), pronated (greater genu recurvatum and navicular drop and less outward tibial torsion) and femoral anteversion factor which demonstrated the potential interaction among lower extremity alignment variables [81] A factor of particular relevance to this review was the relative valgus alignment characterised by increased pelvic angle, quadriceps angle and tibiofemoral angle as this collective posture insinuates a medial collapse of the knee The medial collapse alignment may reflect an interaction between the pelvis and knee angles as increased anterior pelvic tilt has been associated with internal rotation at the hip [93]

2.3.2.1 CLINICAL IMPLICATIONS OF MEDIAL COLLAPSE

The clinical implications of a medial collapse of the knee on lower limb injuries have been investigated with numerous plausible explanations Numerous studies [31, 32, 50-54, 94] have theorised that deficits in hip musculature strength contribute to the mechanisms of patellofemoral pain, particularly through alteration

of lower limb kinematics Specifically, deficiencies in hip external rotation and abduction strength presumably contribute to excessive femoral adduction and internal rotation during weight bearing activities [25, 31, 32], which has been shown

to promote increased lateral retropatellar contact pressure in cadaveric studies [32] Riegger-Kruch and Keysor [92] rationalised that skeletal malalignments can alter soft tissue loading of adjacent or distal joints Altered loading can be demonstrated

by using excessive genu valgus as an example In this instance, the quadriceps group may become less effective as a knee extensor if the quadriceps tendon is altered in a direction with more of the resultant force pulling the patella laterally and less of the force pulling the patella proximally [92] By altering the line of pull of the

quadriceps muscle, there would be a tendency for the patellar to be more laterally displaced resulting in a reduction of knee extension force [92]

The relationship between knee valgus and hip muscle function is of particular importance Hollman and colleagues [90] explored the relationships among frontal plane hip and knee angles such as knee valgus, hip muscle strength, and electromyographic (EMG) recruitment in women during a step-down Strong correlations were found between knee valgus and hip adduction angles (r = 755, P < 001) further demonstrating the findings of collective kinematic alignments Gluteus maximus recruitment was moderately and negatively correlated (r = -.451) with knee valgus, accounting for 20% of the variance in knee valgus [90] An unexpected finding was that there was a significant positive relationship between abduction isometric force-production values and greater knee valgus angles during the step down task [90] These findings were explained in part by the secondary role of the gluteus medius Though primarily a hip abductor, gluteus medius also functionally assists in internal rotation due to its increased moment arm during greater levels of hip flexion [95] This observation is in line with the theory of Gottashalk and colleagues [49] who have postulated that the gluteus medius functions primarily as a hip stabiliser and pelvic rotator, rather than a hip abductor when the hip is less flexed

The hip abductors and external rotators play an important role in lower extremity alignment and ambulatory activities as they assist in the maintenance of a level pelvis [47] and are involved in the prevention of hip adduction and internal rotation during single limb support [26, 48, 49, 95] Consequently, movements into hip internal rotation and adduction may be due to weakness in the muscles controlling eccentric hip internal rotation [25, 26] This notion was supported by the findings of Willson and colleagues [25] in a study which evaluated the association between core strength (trunk, hip and knee) and the orientation of the lower extremity during a single leg squat among male and female athletes The findings indicated that females generated lower trunk, hip and knee torques than males which was coupled with greater frontal plane projection angles, or knee valgus [25] Additionally, the association between external rotation strength and frontal plane

projection angles was both statistically and clinically significant [25] Participants with greater hip external rotation strength may be better suited to resist internal rotation moments [25]

When investigating excessive movements of internal rotation of the hip, Delp and colleagues [95] noted that rotational moment arms of the hip musculature should

be considered, especially when the hip is flexed Through the development of a three-dimensional computer model of the hip muscles, they were able to compare the rotational moment arms of the hip musculature during varying stages of hip flexion Their experimental results demonstrated that the internal rotation moment arms of some muscle increased; the external rotation moment arms of other muscles decreased, and some muscles switched from external rotators to internal rotators as hip flexion increased [95] The trend toward internal rotation with hip flexion was apparent in 15 of the 18 muscle compartments, suggesting that internal rotation is exacerbated by hip flexion [95] This observation has obvious implications for activities that involve elevated hip flexion angles such as in landing and other shock absorbing activities as there may be a tendency for medial collapse

Whilst lower limb alignment has been shown to alter the joint load distribution and, therefore, contact pressure of adjacent and/or distant joints [5, 81], lower extremity malalignment may be secondary to inferior proximal hip musculature function [25, 26, 32, 88] Inadequate musculoskeletal strength, malalignment and biomechanics of the lower limb have all been associated as an intrinsic injury risk and may additionally cause an inability to efficiently attenuate the forces associated with ground impact Consequently, it is also pertinent to review any literature that describes the interrelated aspects of hip strength and lower limb malalignment to ascertain the influence these factors have on force attenuation in the lower limb

2.3.3 HIP STRENGTH, LOWER LIMB MALALIGNMENT AND FORCE

ATTENUATION

2.3.3.1 RUNNING

Most recreational sporting enthusiasts engage in running based sports which involve repetitive high magnitude foot impacts with the ground [96] These athletic activities can impose extreme loads on the musculoskeletal system and may contribute to musculoskeletal injury development [97, 98] Deficits in hip musculature have been shown to play a role in postural malalignments and injury aetiology [25] The ability of the lower limb musculature to resist medial collapse presumably allows the lower limb to attenuate forces through the kinetic chain with greater efficacy Numerous studies have explored the relationship between lower limb alignment [99], strength of the hip musculature and the force attenuation properties of the lower limb during weight bearing activity

McClean and colleagues [99] examined the relationship between peak knee valgus moment and lower extremity postures for men and women at impact during a sidestep cutting task Results of this study revealed that females had significantly larger normalised peak valgus moments and a greater initial contact hip flexion and internal rotation position than males during the sidestepping movements [99] The authors hypothesised that increased hip internal rotation and/or flexion at initial contact therefore, may compromise the ability of hip internal rotators and other medial muscles to adequately support resultant knee valgus loads [99] Greater levels

of hip flexion has been shown, theoretically to exacerbate movements into hip internal rotation as a consequence of altered hip musculature moment arms [95] As a result, hip neuromuscular training has been suggested to increase control at the hip joint as this may ultimately reduce the likelihood of lower limb injury via a valgus loading mechanism during sidestepping, especially in females [36, 99]

The hip muscles are capable in balancing a number of biomechanical forces in the body [29] During running activities the trunk laterally flexes towards the same side as the foot strike and the pelvis is upwardly oblique primarily as a shock absorption mechanism [29, 100] which is in turn stabilised by an equalising contraction of the hip abductors [100] A study by Snyder and colleagues [101]

illustrated that strength training of the hip abductors and external rotators favourably altered the lower extremity biomechanics and joint loading in running [101] This study revealed that rear foot eversion range of motion, hip internal rotation range of motion, knee abduction and rear foot inversion joint moments were reduced following six weeks of hip muscle strengthening [101] The authors suggested that the hip strengthening intervention employed in this study may alter knee joint and ankle joint loading and thus be useful in treating patients with lower extremity injuries [101]

2.3.3.2 LANDING

During landing, the lower extremity joints function to reduce and control the downward momentum acquired during the flight phase through joint flexion [102] Different landing strategies have been shown to exist between genders with females having larger frontal plane movements of the knee [85, 88, 89], more erect landing posture, utilising more hip and ankle joint range of motion and joint angular velocities compared to males [102] It has been argued that females may choose these kinematic characteristics to maximise the energy absorption from the joints most proximal to ground contact [102]

A force attenuation strategy based around increased knee valgus and greater lower limb stiffness is considered to be a contributing factor to the aetiology of noncontact ACL injuries for females [85, 89, 103] Hewett and colleagues [89] prospectively screened 205 female adolescent soccer, basketball, and volleyball players via three-dimensional biomechanical analyses in a jump-landing task before their respective seasons Joint angles and moments were measured to help delineate whether lower limb neuromuscular control parameters could be used to predict ACL injury risk in female athletes [89] Of these 205 athletes, nine had confirmed ACL rupture and exhibited significantly different knee posture than the 196 that did not have an ACL rupture Knee abduction angle (P < 05) at landing was 8° greater in ACL-injured than in uninjured athletes The ACL-injured athletes also had 2.5 times greater knee abduction moment (P < 001) and 20% higher ground reaction force (P

< 05), whereas stance time was 16% shorter [89] As a consequence, the

ACL-injured participants were characterised as having increased motion, force, and moments occurring in a smaller amount of time than the non-injured [89] The findings of this study elude to increased valgus motion and valgus moments at the knee joint during the impact phase of jump-landing tasks being key predictors of the increased potential for ACL injury in females [89]

The influence of hip-muscle function on knee joint kinematics during landing and the influence of fatigue has been investigated by Carcia and colleagues [103] Frontal plane tibiofemoral landing angle, excursion and vertical ground reaction forces were recorded from a drop jump under prefatigue, postfatigue and recovery conditions on twenty recreationally active college aged students A bilateral fatiguing protocol was employed which involved a maximal voluntary isometric contraction against a dynamometer Bilaterally fatiguing the hip abductors elicited larger knee valgus but no differences in frontal plane excursion or vertical ground reaction forces

in double leg drop landings when compared to the non-fatigued state [103] The results from this study further illustrate that proximal hip musculature influences the kinematics at the tibiofemoral joint Moreover, fatigue in the proximal musculature might increase the injury risk to the knee during landing [103]

A recent study was conducted to evaluate the relationship between ankle dorsiflexion and landing biomechanics by Fong and colleagues [104] Thirty five healthy volunteers (17 male and 18 female) were recruited Landing biomechanics were measured by an optical motion-capture system interfaced with a force plate Results observed significant correlation between ankle dorsiflexion and knee flexion displacement (r = 0.646, P = 0.029) and vertical (r = -0.411, P = 0.014) and posterior (r = -0.412, P = 0.014) ground reaction forces The researchers suggested that greater knee displacement and smaller ground reaction forces during landing were indicative

of a landing posture consistent with reduced ACL injury risk by limiting the forces the lower limb must absorb

2.3.3.3 CRICKET FAST BOWLING

Fast bowling, by its very nature is a dynamic, multi-planar and forceful activity that produces considerable mechanical loads to the spine which can be repeated as often as 300 to 500 times per week [72, 77, 105] Amongst cricketers, fast bowlers have consistently been identified as having the greatest risk of injury due to the characteristic chronic loading of the musculoskeletal system [106] The enormous intensity of the activity can overwhelm the normal repair process of the soft tissue and bone alike and cause microscopic defects to form and propagate resulting in lower extremity injuries [107] The absorption of these forces is significant in the aetiology and pathogenesis of injuries to the lower limb and vertebral spine as they can reach four to nine times body weight during delivery [108, 109] Excessively frequent exposure to large forces in combination with predisposing factors that include poor technique [110, 111], substandard physical preparation [112], musculoskeletal immaturity [75, 105, 113] and muscular asymmetries [72, 106, 112] consequently demarcates a multifactorial pathogenesis, observed in such injuries such as stress fractures in fast bowlers [72, 75, 77, 106, 108, 109]

The ground reaction forces that are observed during bowling are high magnitude and high frequency forces The coupling of these mechanical components

is instrumental in the development of lower extremity injuries The magnitude of force generated and absorbed in the fast bowlers delivery stride is substantial and transmitted and dissipated through the various loading mechanisms of the lower limbs [105] The process of force attenuation places immense levels of stress upon the osseous structures of the lower limb, hip, pelvis and spine [114, 115] These forces are all generally lower than the critical limit of the specific tissue and combine

to produce a fatigue effect over time, predisposing the tissue to overuse pathologies such as tendonitis, bursitis, fasciitis, fracture or neuritis and cricket specific injuries such as vertebral disk degeneration [105] and spondylolysis [77, 106, 107]

Certain factors contribute to the differences in ground reaction forces Hurrion and colleagues [108] simultaneously measured the back and front foot ground reaction forces of fast bowlers during a delivery stride Results suggested that the

stride length and alignment influence ground force in the delivery stride [108] These factors where described as dependent upon the velocity at which the bowler impacts the crease with the front foot, which also determines the magnitude of force [116] Results also concluded that the highest front foot strike forces are a by-product of a fully extended, or even hyper-extended knee however there is less conclusive evidence to link the straight front leg technique to injury [108]

Previous sections have highlighted the interrelation of proximal strength, malalignment, force attenuation and the resultant genesis of injury However, the more distal ankle joint, particularly range of motion deficits, also contribute significantly to the pathogenesis of lower limb injuries Consequently, reviewing the literature relevant to the ankle dorsiflexion range of motion will add further understanding of the multifaceted nature of intrinsic injury risk

2.3.4 ANKLE DORSIFLEXION RANGE OF MOTION

The flexibility of a joint is determined by the geometry of the articular surfaces and by muscle, tendon, ligament and joint capsule laxity [82] The literature is divided on the influence of range of motion (ROM) has on injury [14] However, decreases in ankle ROM, particularly dorsiflexion, have been implicated in several studies to impaired function and injury [5, 17, 65, 66, 78, 82, 117-122] Adequate dorsiflexion of the talocrural joint is required for the normal performance of functional activities such as walking, running, stair climbing and squatting [119] in addition to adequate force development and attenuation during foot contact [117, 123] The point of maximal ankle dorsiflexion during human gait is approximately 10° and occurs during stance phase just prior to heel rise [124] For this reason, numerous studies advocate testing ankle dorsiflexion range of motion and associated restrictions during full knee extension to accurately assess functional dorsiflexion range of motion [5, 82] Restrictions of ankle dorsiflexion may be caused by a tight gastrocnemius, soleous, capsular tissue or abnormal ossesous formation of the ankle [82, 117] or prolonged immobilization due to injury [117, 118, 122]