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G.S Lynch (ed.), Sarcopenia – Age-Related Muscle Wasting and Weakness,

DOI 10.1007/978-90-481-9713-2_16, © Springer Science+Business Media B.V 2011

Abstract In mammalian taxonomy, skeletal muscle constitutes a remarkable tissue not only in its innate capacity to generate force while shortening, remaining isomet-ric, or lengthening, but in its capacity to adapt through atrophy or hypertrophy

in response to decreased or increased loads, respectively and regenerate when

injured The chapter begins with Section 1 on the Structure of Skeletal Muscles

and Skeletal Muscle Fibers Section 2 describes Types of Contractions, shortening,

isometric, and lengthening and the differences in the force development by each

The interactive roles of decreased usage and aging are covered in Section 3:

Age-Related Muscle Wasting and Muscle Weakness and the condition of physical frailty

is discussed Section 4 focuses on Late-Onset Muscle Soreness described by Hough

in 1902 and gaining widespread attention in the 1980s The development of the

concepts: Contraction-Induced Injury and Force Deficit are discussed in Section 5 Section 6 clarifies The Cause of the Contraction Induced Injury as a function of

interactions between homogeneity of sarcomere strengths within a muscle and the severity of lengthening contraction protocols Section 7 elaborates on the

sig-nificance of the stability of the sarcomeres within fibers and the Contribution of

Lateral Transmission of Force to Contraction-Induced Injury Section 8, the Role

of Contraction-Induced Injury in Wasting and Weakness contrasts the impact of contraction-induced injury on young and healthy and on elderly and frail subjects

J.A Faulkner (*) and S.V Brooks

Departments of Biomedical Engineering and Molecular and Integrative Physiology,

University of Michigan, Ann Arbor, MI 48109-2200, USA

e-mail: jafaulk@umich.edu

C.L Mendias

Departments of Orthopaedic Surgery and the School of Kinesiology, University of Michigan, Ann Arbor, MI 48109-2200, USA

C.S Davis

Department of Molecular and Integrative Physiology, University of Michigan,

Ann Arbor, MI 48109-2200, USA

Role of Contraction-Induced Injury

in Age-Related Muscle Wasting and Weakness John A Faulkner, Christopher L Mendias, Carol S Davis,

and Susan V Brooks

The final Section 9: Measures to Prevent Contraction-Induced Injury emphasizes

the positive aspects of utilizing lengthening contractions in training programs for both young and old participants.

Keywords Contraction-induced injury • Delayed-onset muscle soreness • Muscle wasting • Muscle weakness • Lengthening contraction • Eccentric contraction

• Muscle repair • Muscle regeneration • Force deficit • Muscle conditioning

1 Structure of Skeletal Muscles and Skeletal Muscle Fibers

Skeletal muscles are composed of muscle fibers organized into motor units innervated by a motor nerve In humans, single muscles range from small finger flexor muscles in the hands with fewer than 100 motor units and around 100 muscle fibers per motor unit on average to the gastrocnemius muscles in the lower leg composed of almost 600 motor units and close to 2,000 fibers per motor unit (Feinstein et al 1955 ) Each individual muscle fiber within a motor unit contains myofibrils that consist of myosin filaments surrounded by and overlapping with thin actin filaments that are anchored in the z-discs at either end of sarcomeres The globular head of the myosin molecules are capable of binding to sites on the thin actin filaments when a muscle fiber receives an action potential and there is a release of calcium from intracellular calcium stores The myosin cross-bridges then proceed through a driving stroke that, under circumstances when the muscle is unloaded, or loaded with a resistance that can be moved, draws the thin filaments past the thick filaments in a movement that brings the z-discs together and shortens the length of sarcomeres If the muscle is held at a fixed length and activated, cross-bridges cycle generating force without filament sliding and sarcomere shortening Finally, if while activated, the muscle is stretched by a load greater than that generated by the cycling cross-bridges, cross-bridges are strained prior to release and re-attachment.

2 Types of Contractions

When a muscle is activated by action potentials, the muscle fibers in the activated motor units attempt to shorten Whether the fibers actually shorten, remain at the same length, or are lengthened depends on the interaction between the force gener-ated by the muscle and the load on the muscle Consequently, skeletal muscles

make three types of contractions – a shortening contraction, wherein the load on

the muscle is less than the force generated by the muscle and the activated muscle fibers shorten (Fig 1 , Panel a); an isometric contraction, wherein the load on the

muscle is either immoveable, or equivalent to the force generated by the muscle

and the activated muscle remains activated at a fixed length (Fig 1 , Panel b); or a

lengthening contraction, wherein the load on the muscle is greater than the force generated by the muscle and the muscle is lengthened (Fig 1 , Panel c) The terms concentric and eccentric contractions are now in wide usage for shortening and lengthening contractions, respectively Although the terms concentric and eccentric contractions are useful clinically, these terms have no intrinsic meaning in terms of the characteristics of the contractions that limb muscles make Thus, throughout this chapter the terms shortening, isometric, and lengthening will be used to describe the type of a specific contraction.

shortening isometric lengthening

Biceps muscle shortens during contraction

Biceps muscle remains at fixed length during contraction

Biceps muscle lengthens during contraction

Force > Load Force = Load Force < Load

90

100

110

0

100

Length

(%Lf)

Force

(%Po)

a

d

e

are able to perform are dependent on the interaction of the force developed by the muscle and the

load against which the muscle is attempting to shorten A shortening contraction (a) occurs when

the force is greater than the load During a shortening contraction, the velocity of shortening is load dependent, with the greater the load the lower the velocity of shortening During a shortening

contraction, a muscle performs ‘work’ An isometric contraction (b) occurs when the force

devel-oped by the muscle equals the load or under conditions when the load is immovable A

lengthen-ing contraction (c) results when the load on the muscle is greater than the force developed by the

muscle (Modified from Vander, Sherman 2001, Luciano Human Physiology, Figs 11–31, page

320, McGraw-Hill Reproduced with permission of The McGraw-Hill Companies.) The changes

in the lengths of the muscle are displayed during each of the three types of contractions Tracings

of the displacements initiated by a servo motor lever arm (d) and the forces developed (e) by a

maximally activated muscle measured by a force transducer Lf, fiber length that results in maxi-mum force; Po, maximum isometric tetanic force (Reprinted with permission from Faulkner et al

2007, Wiley)