Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 352

FIGURE 15.6 In vivo dynamometer with an anaesthetized rat on the heated X-Y positioning table. The left foot is
secured in the load cell fixture and the knee is secured in 90 8 flexion.

servomotor (in the dynamometer) controls the movement kinematics and measures the resultant
muscle response during those movements. In the Wong and Booth model, the muscle forces produced
the shortening-only movement about the joint axis where the kinematics was not controlled.
The first reported in vivo rodent dynamometer was developed by Ashton-Miller for the study of
biomechanical behavior of the plantar and dorsi flexor muscles of the mouse hindlimb. 12 This approach
also has been used to study rabbit dorsiflexors, 26,86,163,164,206 rat dorsiflexors, 62,97,116,152,154,156,157 mouse
dorsiflexors, 132,168,283 and rat plantar flexors. 63,296 - 299 Typical exposures ranged from 20 repetitions of
the rat plantar flexors 298 to 900 repetitions in the rabbit dorsiflexors. 86,163,164,206 Typical angular velocities
were based on the animal being tested: 75 deg

sec for rabbit dorsiflexors, 163 upto 500 deg

/ / sec for rat

dorsiflexors, 97,152 and upto 2000 deg sec for mouse dorsiflexors. 132,168,283 Ranges of angular velocities

were selected based on the volitional capability of the muscle group and animal species in order to be
physiologically representative.
The major benefit of nonvolitional in vivo models is the ability to study muscle function and injury
mechanics about the joint axis of the target muscles. Thus, the normal muscle, tendon, and bone attach-
ments are intact as well as the neural and vascular supplies. The synergistic function of muscle agonists
and lateral transmission of adjacent muscle forces is also preserved.
One major limitation of nonvolitional models is the use of artificial electrical stimulation to invoke
muscle contractions. Unlike voluntary contractile activity, which is submaximal and characterized by
a selective recruitment of motor units, nonvolitional contractile activity is typically supramaximal
because electrical stimulation involves the activation of all motor units of the target muscle. Thus,
caution must be exercised when making inferences from comparisons between muscle responses from
supramaximal electrical stimulation and voluntary submaximal contractions.

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15.3.3.3.2 Volitional Models
Volitional in vivo models represent a more physiologically representative animal model for the study of
muscle injury and adaptation. Volitional models differ from nonvolitional models in that normal muscle
recruitment is employed via normal central nervous system control, and the pace of the activity is
controlled by the animal, not the testing equipment or the investigator. One of the earliest reported