Front Matter

Chapter 3 Musculoskeletal Structure and Physiology 51

with bone fracture or tendon rupture. The area
under the curve represents the total amount of
energy introduced into the material to cause
failure. The maximum strength of the material
is indicated by mu, the level of applied stress at
the point of failure.
Musculoskeletal tissues are anisotropic and
viscoelastic. Mechanical properties of aniso­
tropic materials vary depending upon the
direction of applied stress. For example,
tendons are capable of resisting high tensile
loads, but deform readily when subjected to
compressive stress. Mechanical properties of
viscoelastic materials vary depending upon the
rate at which stress is applied. Viscoelasticity is
a melding of the basic properties of viscosity
and elasticity. Viscosity is a property of fluids
that reflects the resistance of molecules to strain
(flow) in response to a given force. Highly
viscous fluids are resistant to flow, but will flow
linearly and irreversibly over time in response
to a constant force. Elasticity is a property of
solids that deform instantly in response to an
applied stress but that return to their original
state following removal of the stress. Viscoelastic
materials demonstrate elastic deformation in
response to rapid loading but undergo perma­
nent change of shape in response to sustained
stress. The progressive strain that occurs in
viscoelastic materials in response to a constant
stress is called creep. Stress relaxation is a
related property that refers to the gradual dis­
sipation of internal stress that occurs in response
to a constant strain.
The viscoelasticity of musculoskeletal tissues
derives from their composite structure: within
the ECM, collagen and elastin fibrils form the
solid component and impart tensile resistance
and elasticity, while the hydrated proteoglycan‐
rich extrafibrillar matrix behaves as a viscous
fluid that allows gradual interfibrillar shear
and permanent strain in response to sustained
stress (Elliott et al., 2003). Under normal condi­
tions, the responses of musculoskeletal tissues
to physiological stresses are primarily elastic.
Strains experienced by the ECM are neverthe­
less sensed by resident cell populations and
trigger constitutive ECM remodeling activity
by which the structural integrity of a given tis­
sue is maintained (Kjaer, 2004). In tissues such
as muscle, tendon, and ligament, sustained or
high‐intensity activity, such as occurs during

athletic training or performance, may cause

greater degrees of plastic deformation of the

ECM. This type of strain represents a form

of  ECM microinjury (Kjaer, 2004; Mackey

et  al., 2008). Mild inflammatory responses

may occur in vascularized tissues in response

to stress‐induced plastic deformations of the

ECM. Inflammatory mediators may in turn

trigger anabolic cellular responses that result

in adaptive changes in ECM composition and

a net gain in tissue strength (Vierck et  al.,

2000). This type of cellular response underlies

the physiological adaptation of musculos­

keletal tissues and is the basis for athletic

conditioning.

Skeletal muscle

Organization and motor activity

Collectively, the skeletal musculature is the

largest organ in the body. Skeletal muscle is a

complex and highly organized tissue composed

of repeating units called sarcomeres. Individual

muscles are organized into beds containing

numerous fascicles surrounded by perimysium,

and each fascicle is composed of multinucleated

myofibers enclosed by an endomysium

(Figure 3.6). The epimysium is the collagen‐rich

sheath that surrounds a whole muscle;

epimysium is continuous with the dense

aponeurotic fascial sheets that enclose some

muscle groups. The perimysium is the primary

component of the ECM through which the force

of contraction is transferred from a muscle to

its  associated tendon.

Single myofibers act in concert through the

sliding filament model of actin and myosin

within organized sarcomeres (Figure  3.7).

Within the sarcomere, structural proteins

anchor actin, troponins and tropomyosin to

the  Z‐line that contains actinin. Multimeric

myosin proteins are suspended as intercalating

filamentous strands with globular head units

that function as ratchets along adjacent actin

filaments. Binding of acetylcholine to receptors

on the surface of the myofiber sets off a cascade

of intracellular signaling events resulting

in  calcium release from the sarcoplasmic

reticulum. Intracellular free calcium interacts

with troponin C, which is bound to the actin,

troponin complex (I, C, and T), and tropomyosin