Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 353

models of volitional lifting was developed by Gordon in 1967 using weights attached to the back of rats
during vertical crawling and other exercises. 104 - 107 This type of work was furthered by Stone et al. 261,262
and Ho et al. 120 in rats. Gonyea et al. extended this model to cats, also using weight lifting exercise. 100 - 102
Weights also have been added to wing muscles of chickens, roosters, and other birds to produce an
overload model designed to study the skeletal muscle response to persistent overload. 3,83,166,246
Animal treadmill models were developed in the early 1980s as a way to invoke voluntary repetitive
eccentric muscle actions capable of producing muscle injury. 10,234 It was found that downhill treadmill
locomotion produced an eccentric bias on the soleus muscle in the plantar flexor group that resulted in
distinguishable signs of injury. 10,66,202,234 Treadmill exposure has been used to study a wide variety of
physiological variables and typical exposures range from 30 to 150 min per session for a single session
in rats 10,66,153,202,277 upto five sessions per week for 10 weeks duration, 16,109 and upto 9 h exposure in
a single session for mice 185 to study acute injury response and adaptation and reduction of injury
susceptibility after repetitive exposure. The treadmill studies are similar to those conducted in humans
although the animal exposures are typically longer in duration.
In volitional treadmill and resistance training models, the exposure biomechanic, such as muscle forces
or torques, or the number of muscle contractions (repetitions) are not controlled or quantified during
the activity. This lack of quantitation makes it difficult to relate physiological outcomes to specific
parameters of performance and loading history, which can differ widely across individual animals.
Some researchers have employed operant conditioning procedures to produce the kinds of repetitive
muscle loadings that are relevant for the study of exercise-induced physiological responses. In these
approaches, voluntary responses were motivated by various consequences, such as food
rewards, 102,141,308 intracranial stimulation, 94 or electric shock to the tail or feet. 71,84,120,266 The species,
target muscle groups, and training protocols, however, differed widely among these models. For
example, Gonyea et al. 98,101,102 trained cats to grasp and move a weighted bar with the forelimb repeat-
edly in 30-min sessions conducted 5 days per week for upto 87 weeks. Barbe et al. 19 trained rats upto 8
weeks to repeatedly reach their forelimbs into a small tube to retrieve food pellets. Yarasheski et al. 308
trained rats over an 8-week period to climb a wire-mesh ladder with weights secured to their tails,
and Klitgaard 141 trained rats over a 36-week period to enter a vertical tube and use their hindlimbs to
lift a weighted ring. In other approaches, rats wore weighted jackets and were trained in sessions
conducted 8 to 16 weeks to rear up on their hindlimbs to avoid an electric shock 71,120,266 or to receive
brain stimulation. 94
Many of these approaches have been developed for the study of adaptive or regenerative processes.
For example, there is considerable evidence that under some conditions, voluntary repetitive exertions
performed over several weeks or months can lead to muscle hypertrophy as evidenced by increases in
either myofiber number or size. 71,101,266,301 Under other conditions, however, similar patterns of repeti-
tive exertions have resulted in degenerative morphology 98 and inflammatory responses. 19 Unfortunately,
many of these approaches lacked the necessary control and quantitation of the biomechanics of the
movements to allow for thorough assessments of external and internal loadings that would be necessary
to characterize dose-response relationships. In addition, few studies have specifically examined the
effects of external loads on both internal loads and tolerances (as measured by biomechanical per-
formance and physiological force tremor) and physiological or biochemical processes. Thus, little is
known about the relation between specific parameters or changes in performance or physiological
force tremor and physiological or biochemical processes.
15.4 Injury Mechanisms
15.4.1 Acute Muscle Injury
Skeletal muscle is a unique tissue in the body since it has both passive and active properties that are
exhibited during muscle contraction. Because skeletal muscle can generate force during contraction