Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 351

contraction-induced injury provides a more physiologically representative preparation than the
ischemic, noninnervated in vitro model.
In situ models have been used to study injury mechanics in mice EDL muscles, 35,37 - 39,65,72,75,129,177,178,313
rat EDL, 176,276 - 278 rat adductor longus, 268,281 rabbit tibialis anterior (TA), 27,161,162,165,191,192,201,206 rabbit
soleus muscle, 23 rabbit EDL, 219 and rabbit triceps surae. 264 The parameters used to produce injury in
those studies ranged from single stretch models that consisted of stretch outside of the typical phys-
iological range (usually greater than 130% of the resting or optimal muscle length, l o )ofthe
muscle 27,38,39,129,191,192,201,219 upto 1800 contractions within the normal physiological range (typically
70% l to 130% l o ). 161,165
In situ models are well suited for acute muscle injury studies where changes in contractile forces sub-
sequent to an injurious perturbation are of interest. Exact length changes and lengthening velocity of
the muscle-tendon group can be controlled and monitored during testing. However, the invasive
nature of this model precludes it from being applicable to repetitive injury models because the target
muscle-tendon complex cannot be left exposed for more than a single session. Also, in most in situ
studies, the forces or torques are not tested about the normal joint axis where effects of synergist
muscles will have an effect on the resultant forces or torques about the joint axis. Also, the transmission
of muscle-tendon forces through the joint axis to the target output limb via the mechanical advantage of
that joint could not be studied. However, results from in situ studies have provided much information
about the causal factors in acute muscle injury and the resultant physiological responses.

15.3.3.3 Animal Models

It was apparent from in vitro and in situ findings that it would be beneficial to investigate muscle response
and injury mechanics by testing about the normal joint axis of the target muscle and also be concerned
with the invasiveness of the procedure. By using a noninvasive procedure, the confounding effects
implicit in the required surgical procedure of in situ preparations are removed, and the temporal
response after exposure can be examined. Also, a noninvasive preparation would be ideally suited for
the study of muscle response from repeated exposures. In vivo models address these issues by facilitating
testing about the normal joint axis in a noninvasive manner, with intact neural and vascular systems,
and intact muscle-tendon systems.
Most in vivo models can be categorized as either volitional or nonvolitional models. Volitional models
are those in which the movement tasks are performed voluntarily using different types of motivational
tools. In contrast, nonvolitional models are those in which the animal typically is anesthesized and
muscle contraction is initiated and controlled by an external electrical source.

In Vivo

15.3.3.3.1 Nonvolitional Models
In order to fulfill a need for more control and quantitation of in vivo muscle function, Wong and Booth
in the late 1980s and early 1990s used electrical stimulation of the rat plantar flexors and a weighted
pulley bar apparatus that would provide isotonic resistance to the plantar flexors. 302 - 304 This approach
controlled the number of repetitions, the activation of the muscle group, the temporal arrangement of
the repetitions, and loading of the plantar flexors about the joint axis. However, this model did not
use a servomotor to control the range of motion or velocity and acceleration of the movement, and
did not measure dynamic forces of the plantar flexors.
This approach was refined in the early 1990s to provide better control of the kinematics of the move-
ment by use of an electrical servomotor. In vivo dynamometry incorporated electrical servomotors,
load cells to measure forces, and potentiometers and tachometers to measure the kinematics of the move-
ment to comprise a total testing system 63 (Figure 15.6). Dynamometry can be used to control and measure
the biomechanical loading signature in real-time via control of muscle activation levels, range of motion
of the muscle action, type of muscle action (isometric, shortening contractions, lengthening contractions,
or stretch-shortening), velocity, acceleration, number of repetitions, duty cycle, and exposure duration. 63
The main difference between dynamometry and the Wong and Booth model is that the electrical