sport polymer

Lees Department of Sport & Recreation Studies, Liverpool Polytechnic, Byrom Street, Liverpool L3 3AF, UK Sports equipment encompasses a gamut of devices used in laboratory, training an

E r g o n o m i c s in sport Exercise and sports equipment:

Some ergonomics aspects

T Reilly and A Lees

Department of Sport & Recreation Studies, Liverpool Polytechnic, Byrom Street, Liverpool L3 3AF, UK

Sports equipment encompasses a gamut of devices used in laboratory, training and

competitive contexts and these form the content of this paper Ergometers range in

sophistication from friction braked stationary bicycles to computer controlled

simulators which incorporate exercise modes specific to the athletic user These are

now used in training, as experimental devices and in some instances for competition

purposes Training equipment exhibits a similar emphasis on exercise specificity,

safety being an important aspect of its use Design of projectiles for sporting

activities has mainly reflected their traditional modes of use, the introduction of

synthetic materials having some ergonomics implications Similarly, materials science

and design technology have contributed innovations in equipment for racquet sports

and hitting implements The changes have tended to be associated with availability

of new materials for product construction and have implications for safety and skill

in the transition to using the new products Ski equipment design illustrates ergonomics

factors in interfacing the performer with the sporting environment and how equipment

has progressed by regenerative design processes Enhancement of performance in some

sports must be accompanied by an awareness of safety requirements: where appropriate,

risks to participants should be reduced by use of protective clothing and equipment

Enforced validation of protective equipment is recommended to raise safety levels in

certain sports and the safety of spectators must not be neglected Human factors criteria

can then be applied in monitoring, officiating and spectating at sporting events

Keywords: Exercise, sports equipment, training

Introduction

Consideration of the characteristics of the equipment

used in sport is an important aspect of interfacing the

performer with the sporting environment Historically,

equipment was fabricated to enhance and extend human

capabilities: gradually, as equipment in everyday use became

more extensive and refined, various forms of competition

involving skill in using equipment evolved These equipment

developments varied from vaulting poles, racquets for games,

projectiles, aquatic shells, to locomotory aids such as skis,

skates, cycles and motor propelled vehicles In other sports,

appropriate shoes, clothing and protective equipment have

been fabricated to promote the safety and efficiency of

performance As competition in top-flight sport has grown

increasingly intense, much creativity has been applied by

engineers in attempting to launch new artefacts to be used

by the champion performers Where the quality of new

equipment provided a crucial advantage to the user - as

evident in the introduction of glassfibre vaulting poles,

aerodynamic javelins, turbo-charged Formula 1 racing car

engines or 'jumbo' sized lawn tennis racquets - the novel

feature is generally adopted by competitors to eschew

entering the contest at a disadvantage In other cases the details of the designs, such as the keel of the Australian yacht in the 1983 America's Cup contest, may be kept secret until victory is secure Attention has also been focused on accessory equipment such as sports shoes and clothing and the need to improve their comfort and protective functions Besides, technology is now extensively utilised in the provision of equipment to help officiating at sports contests, enhance training methods, improve information presentation

to the audience and allow precise measurements of human capacities under controlled laboratory conditions

It is apparent that sports equipment can be broadly defined to refer to a gamut of materials used in recreational, training, competition and experimental contexts These areas provide the scope of the current article The requirements

of competition mainly dictate the trends and developments

in the other spheres For competition to be possible and performances to be comparable on a worldwide basis standardisation is necessary, a factor not always mandatory with industrial products National standards are fixed by the governing body appropriate to the sport in question, which invariably has to adopt the standards of the parent

0003-6870/84/04 0259-21 $03.00 Q 1984 Butterworth & Co (Publishers) Ltd Applied Ergonomics December 1984 259

international governing body and these largely determine

the constraints on designers of new equipment In baseball

the specifications for the ball were determined in 1872 and

for the bat in 1876, both still applying today In contrast,

specifications in machine sports and aquatic activities are

regularly modified to accommodate the ingenuity of new

designers, who are spurred on by the rewards that accompany

sporting successes Whether governing bodies hinder or

encourage new developments in equipment, designers must

work within the limits of the specifications laid down in the

rules for the sport They also have to consider what

implications use of their product may have for training and

injury occurrence

Many sports activities do not have an inherent heavy call

for equipment in competitive contexts, yet practitioners use

sophisticated hardware in training to optimise their

preparation This applies, for example, to the training of

runners and swimmers Similarly, elaborate analytical

systems are available for measurement of biophysical and

perceptual-motor capacities so that athletic potential may

be predicted and adaptations to training regimes monitored

Measurement of fitness variables has become increasingly

refined with the recognition of the multivariate nature of

fitness (Ostyn et al, 1980) and its specificity for the activity

in question Additionally, laboratory methodologies in the

sports sciences have progressed in sophistication and these

have overlapped into training contexts with an increased

use of ergometers for specific training purposes as welt as

for testing The transfer illustrates the possibility of cross-

fertilisation of ideas from theory to practice

Ergometers

Conventionally, practitioners used field tests such as the

Cooper 12-min run test and a maximum weight lift to

measure endurance and muscular strength capacity

respectively, and for monitoring improvement with training

regimes (Reiliy, 1981) Such tests were relatively simple to

execute and were closely related to sporting activities

Similarly the Harvard Step Test (Brouha, 1943) required

only simple equipment and so for some time was widely

adopted for use in military, industrial and sports institutions

The introduction of cycle ergometers into exercise physiology

laboratories fostered a sharper interest in fitness testing and

medical screening under exercise conditions where the rate

of work could be quantified and controlled and the

physiological reaction to that work rate could be monitored

The outcome was that numerous facilities became available

in developed countries for exercise stress testing

The possibility of estimating human power output and

energy expenditure during stationary cycling led to the

acceptance of friction or hydraulically braked bicycle

ergometers in training for positive health and body-weight

control Use of cycle ergometers flourished in commercially

based fitness clubs to promote physical well-being of

members changing from a sedentary to a more active use

of leisure time These ergometers were also the chosen mode

of exercise when tests of aerobic and physical working

capacity, such as VO 2 max and PWC 170 (Sjostrand, 1960),

were conducted ThougIa'2he Monark cycle ergometer is

still a fundamental item of equipment for general use in

exercise testing, its limitations for specific purposes are now

recognised and many alternatives are commercially available

Electrically braked cycles overcome the difficulties of acquiring precise control o f the work rate encountered in using their friction-braked predecessors by not having to depend on dictating the pedal frequency Power output

is f'txed; the resistance, which is provided by the electro- magnetic field set up by rotating copper discs, accommodates

to changes in pedalling frequency so that it alters inversely with changes in the pedal rate to maintain power output constant Even so, changing the pedalling rate while keeping power output constant will affect metabolic responses to exercise (Hughes et al, 1982) The force bicycle used by Davies e t al (1984) has the cranks drawn by a variable speed motor so that maximum force production and power output at pre-determined pedalling speeds can be calculated when the cycle is linked to a computer The Fitron cycle developed by Cybex Inc (Ronkonkoma, NY) is isokinetic, allowing the pedalling rate to be f'Lxed in advance, the resistance accommodating to the effort exerted by the individual The resistance in the Dynavit (Garrick Co, London) cycle (Fig 1) is computer-controlled so that an incremental work test to maximum effort or a steady-rate regime can be set pre-start An ear plethysmographic system linked to the cycle permits concomitant monitoring and recording of the heart rate Other ergometric systems utilise feedback from the user's physiological responses to exercise, as is the case in 'heart-rate controlled ergometry' This facility is enormously helpful to clinicians and to

Fig 1 Computer controlled cycle ergorneter with accessory

pulse rate and blood pressure monitor

supervisors of exercise programmes prescribed to cardiac

patients and coronary risk individuals: it is now available in

the current range of treadmills as well as in cycle ergometers

Motorised running treadmills provide an alternative mode

of precisely controlling the exercise load The VO 2 • a x is

generally found to be about 8% higher when measurer~ on a

treadmill compared with that measured on a cycle ergometer

(~,strand and Rodahl, 1977) Current treadmills have a

number of improved features including more compliant belts

for greater shock absorption on landing, easy-to-use safety

harnesses and emergency stop controls A facility for

declining as well as inclining the belt has promoted

experimental interest in downhill locomotion (Pimental et al,

1982) Self-propelled belts compare favourably with

electrically driven treadmills in enabling maximum running

effort to be elicited (Davies e t al, 1981) and this has helped

their acceptance for general fitness training

Recognition of the specificity of fitness has led to the

development of a range of ergometers appropriate to

demands integral to individual sports Astrand and Englesson

(1972) reported the design of a swimming flume for use in

physiological testing of swimmers which has subsequently

been utilised for fundamental investigations of swimming

actions (Holmer, 1972; Clarys and Lewillie, 1975) Similarly,

tanks with rotating water channels are used in studies of

oarsmen and canoeists, or of their respective crafts These

laboratory facilities are expensive and are available in but a

few well-equipped sports science research centres such as

the University of Tsukuba, Tokyo University, and research

institutions in Stockholm, Buffalo, Milan and Brussels The

facilities may also be used for fundamental studies of the

behaviour of new water-sports equipment such as sailboards

(Gregory and I~es, 1981)

Where research funds are less generously provided, dry

land simulators of water sports may be employed A

'Biokinetic Swim Bench' (Isokinetics Inc, Mountain View,

CA, Fig 2) which partially accommodates to the input of

effort and which allows duplication of the pattern of arm and shoulder involvement in pulling through the water in the swimming stroke has been used for fitness testing (Sharp

et al, 1982) Similarly, an isokinetic swim bench can be utilised for fundamental research in the physiology of swimming (Swaine and Reilly, 1983) How closely the hydrodynamic forces during the pulling action are simulated has not been demonstrated Rowing and canoe ergometers which incorporate hydraulic, mechanical or electrical resistance mechanisms and which mimic the actions of the competitive events are also being utilised (Cooper, 1982; Dal Monte et al, 1981) Competitive cyclists can use their own machines on rollers and a wind load simulator for more realistic reproduction of road conditions in the laboratory (Firth, 1981) For skiers, treadmill rails have been modified

to accommodate work with ski poles as well as leg movements; for proficient cross-country skiers, VO 2 max values in excess of their treadmill running VO 2 max are then found (Bergh, 1982) Arm ergometry is now recognised

as necessary for specific function testing of arm-trained athletes, modes including arm cranking, turning and bending being employed (Mangum et al, 1983), though it is also possible to modify conventional cycle ergometers as occasion demands Mobile underwater ergometers permit control and measurement of divers' work output during swimming at ocean depths up to 30 m and are becoming more reliable during extended use (Dwyer, 1977)

The linking of automatic systems for expiratory gas analysis to conventional ergometers has considerably enhanced the productivity of many sports research laboratories On-line systems using dedicated microprocessors make measurement of VO 2 max and VE max routine, consecutive computations being performed every 30 s or every 60 s Additionally, it has been suggested, sub-maximal indices of the ability to sustain high intensity exercise may

be obtained from the disproportionate increase in minute ventilation with increased work loads (Wasserman e t al, 1973) though the interpretation of the break-point in ventilation

Fig 2 Swim bench simulator used

for research, fitness testing and training of swimmers

Applied Ergonomics December 1984 261

and its relationship to metabolic acidosis are still the subject

of controversy (Hughes et al, 1982; Brooks and Fahey, 1984)

Similarly, fast-response analysers have permitted measurement

of breath-by-breath expirations and so accelerated the

understanding of the kinetics of the acute response to exercise

The Wilmore-Costill adaptation to standard automatic systems

allows for calibration of analytical equipment concomitant

with measurements during exercise, thereby easing the task

of the exercise test technician

The importance of anaerobic power in explosive

activities has directed the attention of exercise scientists

towards its measurement Margaria and co-workers (1966)

described a stair run test for calculation of alactacid

anaerobic power, the equipment needed comprising a

stairway of suitable dimensions and switch pads or photo

cell assemblies connected to electronic timers on appropriate

steps A modification of this test for use on a cycle

ergometer was developed by Pirnay and Crielaard (1980)

Dal Monte (1980) developed a similar protocol for use on

the treadmill by strain gauging the front raft which was

gripped by the subject as he ran Lakomy (1984) described

how a non-motorised treadmill could be instrumented for

measuring the power generated during sprinting: the applied

force and the belt speed are measured, while a harness is

passed around the waist to secure the subject to the treadmill

Alternatively, peak power and corollary observations can be

obtained by means of a force platform, the methodology for

its use as an ergometer being described by Cavagna (1975)

The Wingate 'anaerobic test' (Bar-Or, 1981) can be

performed on a cycle ergometer to distinguish two aspects

of anaerobic power: accessory transducers enable

measurement to be made of peak power over 5 s and mean

power over a 30 s all-out test These are taken as representative

of maximal alactacid and lactacid anaerobic power respectively

Fig 3 shows how results can be presented by means of a

computer with measurements of power production for each

second A study of 18 young male swimmers demonstrated

high correlations for both mean power (r = 0.81) and peak

power (r = 0-81) with sprint swimming velocity when the

Wingate test was modified for use on a Biokinetic Swim

Bench: the relationships were still significant when data

were normalised for age and body weight, though the

relationship weakened as the distance swum was increased

(Bayley and Reilly, 1984)

Measurement of muscular strength can be important in

identifying predispositions to injury (Reilly, 1981) as well

as in indicating fitness status Conventional methods of

testing have employed dynamometers, cable tensiometry or

strain gauging assemblies to measure peak isometric tension

Ergometers have been designed for eccentric as well as

concentric muscular exercise: exercise on the model reported

by Knuttgen and co-workers (1982) consists of cycling

activity of the legs against the rotational movements of an

electric motor, the direction in which the pedal crankshaft

of the ergometer is driven being dependent on whether

concentric or eccentric work is desired Use of isokinetic

equipment such as that of Cybex Inc allows dynamic

measurements to be made and at a variety of speeds of

contraction The 'Kin-Com' computer controlled system

(Chattecx Corp, Chattenooga, TN) overcomes some of the

torque overshoot problems (Sapega etal, 1982) associated

with earlier variable resistance machines and permits control

Fig 3 Results of 'Wingate Anaerobic Test' on one subject

with power production computed for every second

of the 30 s test

of torque as well as speed in concentric and eccentric actions Variable resistance has been incorporated into a range of ergometers by Cybex Inc, following the commercial success

of its knee extension model; the Ornithron, Uppe body ergometer' and the Fitron cycle are examples The pattern of changes in torque and peak power with increasing speeds of contraction on isokinetic equipment is Used in some laboratories to estimate fibre type distributions in the involved muscles The extremely high correlation between sprint free style swimming and power achieved on a

'Biokinetic Swim Bench' (Sharp et al, [982)demonstrates

the potential of such ergometric apparatus for fitness testing The techniques are readily ac~ptable to sports practitioners for monitoring biophysical capacities since the methods are non-invasive

In summary, ergometers now ran_ ge according to the type

of muscle action permissible Increasing emphasis is also placed on the need to match the etgemetric mode to specific needs of the user Computer linked systems in er[ometry enhance the ease of exercise testing and reduce the risks attendant on strenuous exercise to the subject

Training equipment

Increased systemisation of training regimes for elite

athletes has spawned the design of various training machines

and items of equipment The most prominent is the use of

resistance modes to accustom active muscles to work against

supra-normal loads Weight training has been adopted by

most sportsmen, astute choice of the training stimulus being

made to match the activity in question The risks of incurring

back, knee or wrist injuries with heavy weight training are

documented (Reilly, 1978) and various types of ergometers

have been used as alternatives The risk of injury is not

totally eliminated during heavy exercise if free weights are

replaced by a weight-machine: a study by Andrews e t a!

(1983) showed that though greater strength development

may accompany the use of a typical squat machine compared

with a similar barbell exercise, a greater shear force at

certain joints may also be involved

Free weights cannot present the maximum training

stimulus throughout the complete range of movements

Isokinetic equipment, which incorporates a speed governor

in the apparatus to allow variable resistance according to the

effort produced, has proved superior to conventional

progressive resistance and to isometric strength training

methods (Thistle e t al, 1967) An alternative practice is to

use cams designed according to typical human strength

curves so that the resistance is altered with the angle of

contraction at whatever joint is involved The 'Nautilus'

(De Land, Florida) and 'Polaris' (San Diego, CA) systems

incorporate this principle of accommodating resistance and

overcome a limiting feature of the current isokinetic

machines in that eccentric as well as concentric actions are

permitted Weight throwers and swimmers have traditionally

employed weighted pulleys which permit exercising with the

specific patterning of the muscle actions of the competitive

event while the current 'Biokinetic Swim Benches' provide feedback on the forces produced and the work accomplished Running harnesses have been used by runners to condition their muscles to work against supra-normal resistance, while weighted clothing and weighted boots have been employed

by jumpers, sprinters and gymnasts

The versatility of resistance training is illustrated by the utilisation of multi-station equipment, such systems now being designed on ergonomics criteria (Fig 4) Typically a twelve-station apparatus incorporates stations for arm, leg and trunk work which are adaptable for flexion or extension exercise while resistance is provided by either body weight, loaded stacks, isokinetic machines or pulley systems (Reilly and Thomas, 1978) The use of circuit weight-training has been validated for training the cardiovascular system (Gettman e t al, 1982; Reilly, 1983) Multi-station equipment can be used for training specific muscle groups or for general conditioning of team squads The Masolet (Oslo) 'sequence' system described by MacDonald (1983) demonstrates how agility and joint flexibility can also be improved Additionally, the multi-station machines are relatively safe for novices to use since free weights are not involved and the apparatus has proved effective in fitness training of industrial workers on- site as well as in training competitive athletes (Reilly, 1979a) Practitioners recognise the difficulty of isolating a speed training stimulus from that producing effects on muscular strength or muscle power Nevertheless, various regimes and apparatus have been designed for improving speed of limb movements Boxing speedballs have been utilised by sprinters reputedly for speeding up arm movements with a positive transfer to leg speed, the existence of the neurophysiological mechanism for effecting the transfer being unclear

Experimental work has shown that forced fast movements, induced on a treadmill or cycle ergometer, can result in

equipment (Reilly and

Thomas, 1978)

increased speeds of contraction when the assistance is

withdrawn (Dintiman, 1974) Downhill ramps are used in

indoor arenas in the training of long jumpers to permit a

faster acceleration in the run-up and achieve an economy of

energy output over a complete training session In these cases

the principle is one of speed assistance rather than provision

of resistance to increase the load on contracting muscles

Similarly, the exploitation of energy stored in eccentric

contractions for a subsequent concentric action has been

incorporated into the design of training machines, so

permitting the development of muscular power and speed of

contraction Consequently various bounding and depth-

jumping drills, collectively known as plyometrics, are

practised by many athletes (Reilly, 1981) For this reason

weight training racks may include a facility to concentrate

on weight lowering exercises where greater loads can be

handled than in lifting, and many multi-station devices allow

eccentric as well as concentric actions An adverse after-

effect is the muscle stiffness associated with vigorous

eccentric contractions

A range of artefacts has also been developed for training

of individuals, temporarily or permanently impaired

Hydrotherapy pools are generally accepted as important

means of providing resistance to muscle action in water As

body mass is buoyed up by water, this is a useful training

mode for handicapped and injured athletes since the strain

involved in lifting body weight against gravity is avoided

Harnesses are used for suspending the individual in water

while simulating the running action; for example, to gave

the injured runner an opportunity to maintain training status

Glaser et al (1980) reported the use and validation of wheel-

chair ergometers which permitted progressive arm training

for semiplegics Special lightweight wheelchairs have been

designed for wheelchair marathon racing, thereby reducing

the energy demands of propulsion and enhancing performance

in the competitive event Passive methods of inducing training

effects, such as electrical stimulation of muscle (Hudlicka

1983), or cycloid vibration for joint flexibility (Atha and

Wheatley, 1976) have potential applications to the injured

as well as to top competitors whose active regimes can

thereby be augmented

Another aspect of equipment design which has captivated

sports engineers is how skills training can be optimised by

use of simulators The golfer's groove, designed to forcibly

guide the club head in a predetermined path, has not stood

up well to validation studies (Skrinar and Hoffmann, 1977)

The 'golf trainer', a device which permits measurement of

club head accelerations and decelerations in driving, and

prediction of resultant ball flight characteristics, provides

useful immediate feedback to the golfer Comparable benefit

is offered if portable video-recording systems are used in

field conditions, the visual feedback being most effective

when employed in conjunction with expert comments of a

mentor Relays of lights which are tripped in sequence are

used by the sides o f running tracks and swimming lanes as

aids to pace judgement, though the extent to which this

transfers to competition has not been documented

Some simulators avoid the discomfort, risk or expense of

realistic practices Golf driving ranges are popular in Japan

where entry to a golf course is both difficult to gain and

expensive Sailboarding can be simulated on dry land with

artificial provision of wind at selected velocities and directions

on the sail, so the individual can learn without the discomfort

of repeated immersions Similarly, a man-powered flight simulator was designed (Evans and Reilly, 1979) to enable optimal control configurations to be determined betore the Kroemer prize for man-powered flight was won by the Gossamer machine Team skills such as rugby scrummaging may be practised on scrummaging racks whose designs range

in complexity from makeshift wooden structures to sophisticated hydraulic systems with force transducers attached Such devices cannot adequately represent the finer points of the game when the effort of the squad is co- ordinated as in coping with a wheel or turn of the scrum Individual tackle dummies are widely used in teaching tackling skills in American football as well as in Rugby football, and injuries due to imperfect execution of practice tackles can thereby be reduced

Simulation of environmental conditions can also be a useful strategy for training purposes Climbing walls allow difficult manoeuvres to be attempted in the gymnasium and afford an opportunity of testing new mountaineering equipment without the necessity of incurring the risks attendant on high mountain climbs Artificial ski slopes provide the novice with the opportunity of acquiring basic skills prior to experiencing the ski slopes and are a popular recreation facility in sports centres in the United Kingdom where good ski conditions are rare In neither case has the magnitude of the transfer of skill to the realistic condition been thoroughly established A portable simulator of altitude hypoxia may be worn as a back-pack in preparation for skiing, mountaineering, or athletic performance at moderate to high altitudes In a previously unpublished study b y Clucus, McClean and Reilly at Liverpool Polytechnic the use of such equipment (PO 2 Aerobic Exerciser Inspir Air Corporation, Westlake Village, CA)whilst cycling on an ergometer at a toad

of 200W was found to elevate various physiological responses significantly over normal conditions The heart rate

increased by a mean value of 8 beats/min (n = 12), perceived exertion was raised and blood PO 2 was reduced to a value compatible with expectations at an altitude of 2300 m The system recirculates expired air and mixes it with inspired ambient air to reduce the partial pressure of inspired O 2 ,

CO 2 being absorbed in a canister on the back pack

Respiratory responses to wearing the equipment were shown

by Berryhill and Williams (1984) to be dissimilar to those associated with exercise at altitude, suggesting that hypoxic stress was incompletely simulated The validity of its use to improve performance potential at sea level is even more contentious Simulators of heat stress (e g, environmental chamber or sauna bath) can also be effective in promoting adaptive responses that benefit endurance performers when that exercise is conducted in the heat Use of sweat suits by American footballers to promote sweating and weight loss in early season practices is dangerous because of the risk of heat injury involved when evaporative heat loss is hindered Compression chambers comprise an important facility in the training of divers and are an essential mode of therapy in the event of decompression sickness

Acceptance of simulators is the basis of many machines used in holiday and gaming resorts Ofteal their popularity spawns competitions in their own right Hence ski competitions are held on artificial ski slopes or on grass courses while competitive climbs are held on 'climbing walls'

Recent examples of the progression of ergometers from

experimental to training and competition use are the tread-

mill relay races inaugurated by 'Power-jog' machines in 1983

A typical competition involves team work to cover the

maximum distance in a set time Efficient performance

involves skills specific to the simulator so that pronounced

changes in belt speed are minimised as individual team

members alternate on the treadmill

EQUIPMENT FOR PLA Y

Intrinsic to many sports is the use of equipment as an

essential element in play Ergonomic factors in the design

and use of selected categories of products are now considered

Projectiles

Projectiles are used in sports to demonstrate individual

abilities in achieving distance or accuracy or both Their

design and construction are based on either practical grounds

or historical tradition, and often reflect their mode of use

Where permissible, the developments which have occurred

have been to enhance performance, or reduce injury potential

The projectiles used in athletics are the discus, shot,

javelin and hammer, and are all based on historical design

Little change in their shape, size and mass has been allowed

by the sport's governing body The exception to this has

been the development of the aerodynamic javelin in which

modifications to its shape and weight distribution have

occurred This has been largely as a result of the use of new

materials ( aluminium alloy) for its construction The shot

and hammer are of heavy metal (usually iron) construction

and of uniform density They have changed little over the

years The discus is of wooden construction, with a

protective thin metal rim and weighted centre, which enables

a higher rotational velocity to be achieved at release A

discus of rubber construction is available for practice, giving

similar mass and inertia properties to its equivalent for

competition, but improving durability Special versions of

both shot (loose shot) and discus (rubber) are manufactured

so that they are suitable for use indoors The rules of

competition allow different weights to be used by junior and

female athletes, and are a reflection of the differences in the

strength and size of these participant groups The weights

and sizes allowed have been chosen arbitrarily and not on

the basis of scientific investigation

Archery is another historic event with its origins extending

back to primitive man, and the bow and arrow have

received considerable development and refinement over the

intervening years The modern wooden arrow has a tip of

steel construction for penetration and durability, and a

plastic nock at the rear which is durable and provides a

means for orientating the arrow correctly Three feathers

are used, and these are glued to the shaft at a slight angle

so that a spin about the longitudinal axis is caused during

flight, producing spin stability (Butler, 1968) The target

arrow is made as light as possible so that a given kinetic

energy wilt result in a higher release velocity, and a smaller

drop of the arrow from its initial line of flight, thus aiding

accuracy in target shooting Field or hunting arrows need to

be of sturdier construction, and this is achieved by increasing

the shaft's diameter The extra weight of the arrow demands

Fig 5 A modern bow

that larger feathers are used for aerodynamic stability, which also increases air resistance Glassfibre and aluminium are two other materials used for the construction of arrows and the consistency possible in their manufacture means that they can be produced to be both robust and highly accurate Aluminium arrows are considered to be the best for accuracy (Butler, 1968)

The manner in which an arrow is constructed is one factor affecting the accuracy to which it can be shot The characteristics of the bow and the archer are also important The bow is designed to help impart a consistent quantity of energy to the arrow, and to help in its aim at the target The length of draw can be determined by the position of the arrow on the arrow rest, and it can be set perpendicular to the bow string by marking a 'nocking-point' at the appropriate place The angle at which it is released can be determined by using the bow sight on the grip of the bow The modern bow is distinguished by large weights attached

to the bow by long rods (Fig 5) Their purpose is to increase the bar's moment of inertia to rotation resulting from the recoil of the bow string during the release of the arrow This

is in part a compensation for the asymmetrical bow body, caused by the need to have a clear view along the arrow and through the bow sights These additions to the bow vary from archer to archer They represent a level of 'fine tuning'

of the bow to the archer which is essential in a sport that demands consistent high-accuracy performance It is then

left to the archer to make the necessary adjustments in

order to cope with changing environmental conditions

The speed and strength at which the arrow is released

may lead to injury of the archer from the recoiling bow

string Archery gloves are used to protect the fingers of the

pulling hand from the pressures associated with the high

tension in the bow string and its small area of contact Arm

guards are used to protect the supporting ann from being

grazed by the bowstring Traditionally leather has been used

but as the speed and strength of the bow has increased it

has been necessary to add steel reinforcements to gloves

and arm guards

Soong (1974)has suggested a bow design which enables

the maximum amount of strain energy stored in the bow to

be imparted to the arrow This theoretical treatment enabled

him to comment on the mass of the bow, its initial curvature,

stiffness distribution, length of the bow and the length of

the bowstring The generation of maximum velocity has

implications for accuracy as mentioned already

Man himself is a projectile in many sports events even

though he is not often thought of as such and this sometimes

has an impact on sports equipment In the pole vault the

introduction of the glassfibre pole enabled performance to

be enhanced and Hay (1971) has analysed the energy transfers

that occur when these poles are used The other jumping

events rely on the athlete himself and the rules prohibit

any attempt to enhance performance artificially; thus the

maximum thickness of the shoe sole is 25 mm in the high

jump and any springs or elastic type materials in the shoe

are not allowed With these restrictions the enhancement

of performance has occurred as a result of improved fitness

and better jumping techniques rather than by equipment

Dyson (1977) described the evolution of techniques and

the effect that this has had on the height cleared The most

notable change in technique was the introduction of the

Fosbury Flop, which enabled Richard Fosbury to win the

Olympic Gold Medal in 1968 (Arlott, 1975) The technique

is now well known but an important consideration is that

after the jump the landing must be taken on the back Only

when sufficiently soft surfaces for landing on were

introduced to this event on a widespread basis could this

technique have been practised and used Injuries still result

from this type of landing due to inadequate landing beds

or their faulty positioning

Trampolining and gymnastics are also sports in which man

projects himself The equipment used reflects the ever

increasing difficulty of the moves the performer wishes to

achieve In trampolining the bed construction and spring

strengths govern what height the performer may achieve

This in turn gives him more time in order to perform the

more complex stunts Trampolining is a relatively new sport

(the first official championships were in the USA in 1954)

and the materials used now have not changed greatly since

the sport's inception

In gymnastics and acrobatics, a sprung floor is an

essential feature of contemporary floor routines The

resilience of the floor reduces the magnitude of the impact

forces (a positive safety feature) but also allows energy to

be stored and utilised by the gymnast in his performance

Similar principles are utilised in the spring board or

trampette for enhancing take off in vaulting events As a

consequence of the greater heights achieved a soft energy absorbing surface or mattress for landing is requiredin order to avoid unnecessary injury The ultimate landing surface is that provided by a safety net which avoids all contact with hard surfaces

The ability of man to perform complex manoeuvres is beyond his understanding of how they are performed, The computer has been an essential toot for enabling

biomechanical analyses to be undertaken in order to uncover the scientific principles of performance Van Gheluwe and Duquet (1977)have investigated ~mnastic movements, Yeadon (1981 ) has analysed trampoline somersaults, while Ramey (1979) has investigated athletics jumps Hatze (1983) has attempted to simulate many types of human motion including long jumping, and Miller (1971) has performed a useful simulation of springboard diving While the computer

is not used directly, its processing power has enabled these studies to throw some light on the main mechanical factors which are involved in the performance of complex skills This knowledge has been of use to both performers and their coaches to help not only in the understanding of the actions which they perform but also in improving their performances

Sports ball design and i~rfornmne~

A sports ball is a general term given to an object which

is thrown, hit, kicked or passed between persons as an essential ingredient of a game or sport; Often sports balls are spherical in shape, but this need not necessarily be the case Common examples of spherical sports balls are those used in soccer, tennis and golf Unusual examples of sports balls are the ellipsoid shaped ball used in rugby, the American football and the puck used in ice hockey The size, shape and weight of sports balls have often been determined by the nature of the gam e during its evolution, and in many cases the available materials provided certain limitations on ball construction, and hence design: A further restriction has invariably been imposed by the governing bodies of the respective sports, who have placed limits to factors such as ball diameter, weight, resilience, mode of construction and colour

The shape of a sports ball is closely associated with its function The most common shape is one which is spherical, and this allows the ball to roll easily, to be projected through the air, and to be thrown, hit and caught easily without any regard to the orientation of the ball It gives the best air flow characteristics for a non-specialised aerodynamic shape and can take advantage of spin about any axis both in movement through the air or in bouncing off a rigid surface Shapes which deviate from this reflect the special requirements of the game For example, in rugby the ellipsoid ball promotes its handling during the game, and in ice hockey the puck is required to slide rather than roll over the ice

The mass of a sports ball is closely related to that part of the human body or striking implement which is used to catch or propel it Those balls which are used primarily

by the hand have a mass which is easily controlled by the musculature of the upper body If a ball is too light the arm muscles involved in the throwing action can contract quickly

but with little force and so the power delivered to the ball

is small (Wilkie, 1976) If the ball is too heavy the

musculature of the arm is used to support the ball and is

generally too weak to propel it effectively Kunz (1974) has

shown that there is an optimum ball mass for the generation

of maximum release velocity, which lies between 0.1 and

0-2 kg

The size of the ball for hand throws determines the

nature of the throwing action and consequently the release

velocity The baseball and the cricket ball are convenient

sizes for holding in one hand and thus enable the user to

take full advantage of the sequential nature of the body

rotations involved in throwing for maximum release velocity

Hoshikawa and Toyoshima (1976) have shown that skilled

one-hand throw relies on considerable body rotation and

that the sequence begins with hip rotation which can be

initiated by a step forward with the leg contra-lateral to the

throwing arm, followed by rotation of the shoulder and

elbow about the body and finally arm extension With this

action release velocities for skilled men have been shown to

be above 40 m/s (Atwater, 1979)

As the size of the ball increases beyond about 0.15 m

diameter, modifications to the hand and arm position

supporting the ball and throwing action are required The

fingers cannot grasp the ball firmly and the hand must now

be placed underneath the ball to give it support, and to

ensure that when thrown the propelling force is directed

close to the centre of mass of the ball This is observed in

the games of handball, netball and basketball As the

diameter of the ball is increased it becomes more difficult

to produce the lateral rotation of the forearm (which results

in a 'leading elbow') necessary for high velocity throws

(Cooper and Glassow, 1976) and so the ball is pushed rather

than thrown, with the hand at all times above the level of

the elbow In games where the rules allow it, two hands are

often preferred This gives security in catching the ball, but

restricts the throwing action by forcibly bringing the ball

closer to the body and reducing the radius of rotation of

the ball from the rotational axis through the body (Lees,

1984) Thus as a result of the ball size, release velocities are

reduced and the ranges of ball flight are restricted This

means that less space is required for these games and in

particular games such as netball, volleyball and basketball

can all take place within a relatively small indoor area Large

heavy 'medicine' balls have in the past been popular for

exercise and strength training

To aid catching in wet or cold conditions, gloves are

often worn The goalkeeper in soccer has long been observed

to do this, as have players in Gaelic football More recently

the New Zealand national rugby team were observed to

wear gloves during an international match against the

British Lions Protective mitts are also worn in baseball,

softball, and gauntlets in hockey in order to protect against

the forces of impact with the ball

Sports balls which are used primarily with the feet tend

to be of the size and weight suitable for foot contact The

leg is stronger, and can move less quickly than the hand,

therefore the weight of the kicked ball can reasonably be

greater than the thrown ball The soccer ball is a good

example With a diameter of 0.22 m and a mass of 0-435 kg

it allows the characteristics of the kicking foot to be

optimally used Similar shapes and masses are found in the

Rugby ball and American football, although the latter is lighter and is used also by the hand for one-handed throws Release velocities in all three sports have generated velocities

of 30 m/s in soccer (Plagenhoef, 1971), 34 m/s in rugby (Aitchison and Lees, 1983) and 30 m/s in American football (Kermond, 1979) The size of the playing areas are similar in all three cases, and until recently this meant that they were all constrained to outdoor locations

Of ergonomics interest is the variation in the ball size used for younger players Both the ball size and mass are reduced for junior teams in the field games just referred to, the full sized ball only being used for adult teams In most cases changes in ball size and mass cannot be made to account for individual differences in hand size

The ellipsoid shape of the Rugby and American football has another advantage Introduced primarily for ease of handling in a game in which the ball is both handled and kicked, its shape enables a degree of stability to be generated

in flight Spin about the longitudinal axis is responsible for 'spin stability' and is a characteristic feature of skilled kicking in rugby and skilled throwing in American football The use of racquets and other striking implements adds a new dimension to the size and mass requirements of sports balls It has already been remarked that the hand can generate higher velocities than the foot, and this is because

of the greater number of segments involved in the sequential build-up of velocity With an implement extended to the arm

as an extra segment the velocities generated are potentially even greater In golf, for example, the club head velocity at impact is 70 m/s (Daish, 1972)

Such high velocity impacts would produce enormous forces and torques on the musculature of the body if it were not for the mass of the ball being kept low Thus in golf, squash, racquetball, badminton and tennis (0.06 kg) the ball mass is considerably lower than in hand or foot sports

In the case of baseball and cricket, the mass of the ball is higher (0" 16 kg) but the striking action is two-handed and the length of the bat is restricted

Despite the reduced ball mass, forces and torques generated can be high Hatze (1976) found that torques generated in baseline shots in tennis can be as large as 23 Nm

In competitive racquet games these forces and torques can lead to injury, and a common complaint is 'tennis elbow' -

an inflammation of the lateral epicondyle of the humerus The usual cure is rest, but its occurrence can be prevented

by using a racquet and style of play which reduce the force loadings on the forearm

In games where the ball mass or projectile is extremely light (table tennis and the shuttlecock in badminton) the forces and torques generated on the body are considerably reduced, but, as a consequence, both games are dominated

by the aerodynamic forces acting on the projectile in flight The performance characteristics in these cases are a direct result of the materials used for their manufacture

Materials and construction

Many sports balls have traditionally used the materials and methods of construction available at the time of the conception of the game, although recent advances in materials technology have enabled changes to be made,

often improving the performance of the ball and at the

same time reducing its cost

Materials

Leather has always been a traditional material and its

continued popularity in many forms of manufactured goods

is a testament to its versatility It is often favoured due to

its combination of durability, pliability, softness of feel

and resistance to wear, tearing and splitting Ball softness is

an important quality where contact is likely with the

unprotected parts of the body Leather used to construct

modern sports balls is often treated to prevent water

absorption, fungal growth and in the case of the rugby ball

to enhance grip when wet Synthetic laminate materials are

now seriously challenging the dominance that leather has

had for ball manufacture, as similar qualities can be produced

in these materials Plastics have been used for many years to

mass-produce cheap versions of sports balls but they have

generally lacked the weight or softness of the traditional

materials

In games where either leather or synthetic materials are

allowed by the rules, investigations as to their various merits

have been undertaken Mathers and Flatten (1982)

measured the rebound resilience of leather and synthetic

basketballs on four different types o f playing surface They

found that although the leather ball rebounded significantly

higher on all surfaces than the synthetic ball, both male and

female subjects were unable to perceive any visual differences

between two types of bali In a similar experiment, Eccles

(unpublished observations, Liverpool Polytechnic 1980)

investigated the rebound resilience of plastic and two

weights of leather netballs It was found that there was no

significant difference between the heavy leather and plastic

balls, and the light leather ball rebounded less than the others

However, the internal pressure of the ball affected its

resilience and in netball this is specified only in vague terms

When blown to a higher pressure the plastic ball produced a

greater rebound height than did the leather ones

Two games deserve special mention with regard to materials

used In badminton the projectile has traditionally been made

of leather covered cork and feathers Shuttlecocks of these

materials are still favoured by tournament players, although

the cheapness and robustness of the modem plastic and foam

version is preferred for practice This dual use is typical of

the regard given to the different materials used and has

important implications for players The two types of

shuttlecock have different flight characteristics and so skills

are developed under one set of conditions while the skills are

used in competitive matches under a different set of conditions

The differences may be small but become more and more

important as the playing level increases A lack of consistency

in manufacture may also produce similar effects

Golf has had an interesting history of ball materials and

construction which is well described by Cochran and Stobbs

(1968) Before the game had come under the guidance of a

controlling body, the golf ball was a leather pouch tightly

packed with feathers The cheaper version of the time was a

ball shaped from a special type of wood This had better

flight properties, and being cheaper soon became accepted

The modern golfball appeared in about 1900, and it was

constructed with rubber thread tightly wound around a solid

core This was encased in a durable cover, which is now plastic Variations in the flight characteristics of the ball can be produced by varying the tightness of the rubber windings Balls which are more tightly wound are termed high- compression balls They not only fly further but they produce

a harder impact and an unpleasant shock force which is transmitted to the hand

Construction

Sports balls are constructed so that they are either hollow

or solid Hollow balls have a large size to mass ratio They can

be made large as in the case of the basketball, to enable catching and reduce throwing velocities, without incurring a penalty of increased mass Hollow balls generally have little form or structure of their own and are pressurisod to provide this The degree of internal air pressure also affects their resilience, i e, the height to which they will bounce In virtually all cases the laws of the game def'me the acceptable range of internal pressure, and the resilience of the ball is quantified by its coefficient of restitution This value for a wide variety of sports balls is given by Plagcnhoef (1971) and Hay (1973) The limits set for the coefficient of restitution refer to the playability of the ball at the lower level, and to safety considerations at the higher level If the ball is insufficiently resilient then too much energy is required to play with it (as with a squash ball on a cold day) A bali which

is too resilient is also too hard and this will cause large impact forces when in contact with any unprotected part of the body Balls constructed with a solid interior tend to be small in size, so that their size to mass ratio is appropriate for the game Golf, hockey, baseballs and cricket balls are good examples The baseball and cricket balls are both involved with striking and hand catching, and with the high velocities generated, injuries to the hand (as well as other parts of the body if mis- caught) can occur In baseball padded gloves are used to protect the hand, while in cricket special catching techniques are adopted in order to reduce the impact force In indoor racquet sports where the bali is small, eye injuries are becoming more common Existing eye protection devices often fail to prevent the ball from entering the eye socket

As a result an American Society for Testing Materials standard has been issued to cover eye protectors (Sports Medicine Bulletin 1983)

The golf ball is characterised by two official sizes, the American ball of diameter 41-1 mm and the English ball of diameter 42-7 mm In order to conform to size and mass restrictions, the size of the inner core used must vary This

in turn will lead to a difference in moment of inertia of the ball which will affect its rotational velocity when struck off centre Cochran and Stobbs (1968) considered the various merits of each and concluded that the larger ball tends to fly higher and as a consequence carry less far

Although small differences in performance do exist, it was felt that these are marginal compared with the day-to-day variations in a player's own performance This is one difference in design which should not show any resultant differences in practice

The surface characteristics of a ball may have a marked effect on performance The golf ball described above has a dimple depth which is very carefully controlled so as to give

it optimum aerodynamic characteristics The backspin