Front Matter

50 Canine Sports Medicine and Rehabilitation

Structure–function relationships
of musculoskeletal tissues

Basic concepts

The tissues of the musculoskeletal system
are  highly responsive to mechanical strains.
From a biomechanical perspective, physiologic
adaptation refers to the process by which
musculoskeletal tissues adjust their biochemi­
cal composition, molecular architecture, and
mechanical properties in accordance with the
demands placed upon them. Both athletic
training and rehabilitative therapy involve
the application of controlled forces to the mus­
culoskeletal system to facilitate physiologic
adaptations that are specifically suited to a
given athletic or therapeutic objective. Here
we introduce several basic concepts related to
the mechanical properties and functional
adaptations of musculoskeletal tissues.
The body is subject to two basic categories of
force. Intrinsic forces originate from within the
body due to muscular contractions or the
inherent elasticity of connective tissues such as
ligaments or myofasciae. Extrinsic forces arise
from outside the body and include ground
reaction forces due to gravity as well as forces
imposed on the body through contact with
other objects.

When subjected to force, objects deform. This

relationship can be represented graphically as a

force‐deformation curve, which describes the

structural properties of a given object. When

normalized to the cross‐sectional area of an

object, the applied force and resulting deforma­

tion are referred to as stress and strain, respec­

tively. Stress–strain relationships thus describe

the mechanical properties of a particular mate­

rial. An idealized stress–strain curve pertaining

to musculoskeletal tissues is illustrated in

(Figure 3.5). The initial nonlinear portion of the

curve is the toe region and represents an early

phase of deformation that occurs as stress is

first applied to the material. In connective

tissues, the toe region typically reflects the

straightening of crimped collagen fibrils, or the

elongation of elastin fibers. The linear portion

of the curve represents a zone of elastic defor­

mation, where the material will return to its

original state upon removal of the applied

stress. The slope of this region of the curve is

called the modulus, and describes the stiffness

of the material. The yield point represents the

transition from elastic to plastic deformation;

beyond the yield point, structural alterations

within the material prevent it from returning to

its original state despite removal of the applied

stress. The failure point represents complete

loss of structural integrity and macroscopic

breakdown of the material such as would occur

Figure 3.4 Canine bone marrow‐derived mesenchymal
stem cells in primary culture show characteristic
mesenchymal morphology. These cells area capable of
chondrogenic, osteogenic, adipogenic, or neurogenic
differentiation in response to specific signals.

Stress

Physiological range

Yield point

Failure point (μ)

Linear region

Toe region

Strain

Figure 3.5 Idealized stress–strain curve highlighting key

mechanical parameters of musculoskeletal tissues. The

slope of the linear region describes the stiffness (modulus)

of the material. The area under the curve (shaded region)

represents the total energy introduced into the material to

cause mechanical failure.