Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 349

flexors, 193 - 197,221,222,231 knee flexors, 173,174,294 hamstring, 179 pectoralis and anterior deltoids (chest
press), 245 and calf and biceps. 136 Whole body movements such as downhill stepping exercise, 187,189
downhill running, 199 or cycling 175 have also been used to create muscle injuries. The amount of acute
resistance for most human studies ranged from 10 to 180 repetitions for the segmental exercise
studies, and as little as 20 min of downhill stepping exercise 187 to produce muscle soreness and evidence
of myofiber injury. Thus, even a low number of repetitions or short exposure to lengthening contractions
can result in a strain injury.
It is interesting to note that exposures that involve both whole body movements and segmental
loading result in muscle damage, loss of force, and delayed onset muscle soreness. Different types of
information can be attained from these two types of exposures. Whole body exercise typically involves
closed kinetic chain movements where the level of exercise (treadmill or cycling speed, metabolic load,
angle of inclination) is controlled but muscle forces or torques, velocity, range of motion, and
number of repetitions are not typically measured. In contrast, most segmental exercise models are
open kinetic chain movements involving isolated loading of the limbs, and are administered by either
isokinetic or isotonic dynamometry (computer-controlled strength testing equipment that operates in
either constant velocity or constant torque mode), or by isoinertial loads (using either free weight or
weight attached to an apparatus). Joint torques or forces, as well as velocity, range of motion, and
number of repetitions are measured.
Prolonged strength loss, as measured by maximal isometric force, is considered to be the best method
to quantify the degree and time course of muscle injury and recovery after exposure to damaging
lengthening contractions. 289 It is also the primary means to quantify muscle function in humans
where muscle function is defined as the ability of the muscle group of interest to generate force over a
prescribed range of motion, or fixed length at a given level of muscle activation. 289 Other measures,
such as biochemical markers in the blood and urine, and level of histological damage as obtained
from biopsy, are not well associated temporally with functional performance. 289
Muscle injury studies with humans have been very beneficial in elucidating the type and intensity of
exercise and muscle actions that produce injury. Those studies have also been beneficial for examining
the resultant myofiber changes after injurious exposure and the recovery time after injury. The corre-
sponding levels of pain perception and muscle soreness after exposure was also consonant with the
degree of performance deficit. 58 However, though many studies of muscle injury with humans have
been conducted in controlled laboratory settings, some experimental questions cannot be fully addressed
using human subjects. Confounding factors such as lifestyle, level of psychological stress, pre-existing
disease states such as diabetes and hypertension, and genetic polymorphisms are difficult to control.
In addition, key issues such as the amount of fiber strain and muscle biomechanics necessary for
injury, the amount and site of resultant muscle injury from longitudinal and cross-sectional tissue
analysis, the effect of structural protein knockouts and inflammatory mediator blockade on muscle
injury and repair, and biochemical analyses of muscle tissue, is either difficult or impossible with
human subjects.
Animal models have been developed that reduce or eliminate these confounding factors as well as
provide for control of the degree of motivation and muscle activation strategies. In depth physiological
questions about the pathways involved in muscle injury and repair, the site and extent of injury, the role
of structural proteins in muscle force transmission and injurious response, and biomechanical loading
signature necessary for injury can be more easily addressed in animal models. Animal models that
have been developed allow for more controlled study from the level of isolated muscle fibers to fully
intact muscle groups that contain intact neural and vascular supplies.
15.3.2 Animal Models of Muscle Performance and Injury
Animal models provide a good platform to study the physiological responses to injurious and noninjur-
ious muscle contractions and the biological aspects underlying muscle injury and adaptation. The
majority of animal models used to investigate skeletal muscle performance and injury have used