MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 244

( a )
( b )
Figure 9.17 ( a ) Two equivalent series/parallel arrangements of linear elements of
a muscle model. ( b ) Model showing contractile element acting on viscous elastic
elements. Twitch tension of the tendon F t results only if we assume an exponential
activation tension from the contractile element F c .
from the contractile component is sometimes referred to as its active state ,
and this is quite often assumed to be an exponential response to a stimulus.
Figure 9.17 b shows the contractile component combined with the passive
components along with the time course of the active state, F c , and the resul-
tant tendon tension, F t . More complex models have been developed, one of
which is presented in the next section.
9.3.1 Example of a Model — EMG Driven
Any realistic model of muscle must have a valid input to represent the
motoneuron drive. It would be desirable to record the output of the motoneu-
ron pool (summation of all recruited motor units) to any given muscle.
Unfortunately, this is impossible to do experimentally, so the best compromise
is to record the EMG from the muscle. Students not familiar with the record-
ing and interpretation of the EMG should read Chapter 10 before reviewing
this example of muscle modeling.
Surface EMG has been shown to be more reliable than the EMG recorded
from indwelling electrodes (Komi and Buskirk, 1970), and the number of
motor units in the pick-up zone for surface electrodes is considerably larger
than that for indwelling electrodes. These advantages, combined with the ease
of application, make the surface EMG a valid signal to represent the average
motor unit activity of most superficial muscles.
When each motor unit contracts, a characteristic action potential results
and also an impulse of force is generated by the activated muscle fibers.
There is controversy regarding the shape of the tension waveform generated
at the cross-bridges versus the twitch waveform seen at the tendon. Some
researchers assume the internal force waveform (sometimes called active
state ) to be a first-order exponential (Hatze, 1978; Gottlieb and Agarwal,