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373
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:
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
374 J.A. Faulkner et al.
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
375Role of Contraction-Induced Injury in Age-Related Muscle Wasting and Weakness
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
(%L
f
)
Force
(%P
o
)
a
d
e
bc
Fig. 1 The three types of contractions that single fibers, motor units and whole skeletal muscles
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. L
f
, fiber length that results in maxi-
mum force; P
o
, maximum isometric tetanic force (Reprinted with permission from Faulkner et al.
2007, Wiley)