MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 231

Figure 9.6 Tension build-up and decrease during a rapid maximum voluntary contrac-
tion and relaxation. The time to peak tension can be 200 ms or longer, mainly because
of the recruitment according to the size principle and because of delay between each
motor unit action potential and twitch tension. Note the presence of tension for about
150 ms after the cessation of EMG activity.

recruited until tension has built up in the smaller units. Thus, it can take
several hundred milliseconds to reach maximum tension. During voluntary
relaxation of the same muscle, the drop in tension is governed by the shape
of the trailing edge of the twitch curve. This delay in the drop of tension is
even more pronounced than the rise. This delay, combined with the delay in
dropping out of the motor units themselves according to the size principle,
means that a muscle takes longer to turn off than to turn on. Typical turn-on
times are 200 ms and turn-off times 300 ms, as shown in Figure 9.6, where
maximum rapid turn-on and turn-off are plotted showing the tension curve
and the associated EMG.

9.1 FORCE-LENGTH CHARACTERISTICS OF MUSCLES

As indicated in Section 9.0.1, the muscle consists of an active element, called
the contractile element , and passive connective tissue. The net force-length
characteristics of a muscle are a combination of the force-length characteris-
tics of both active and passive elements.

9.1.1 Force-Length Curve of the Contractile Element
The key to the shape of the force-length curve is the changes of the structure
of the myofibril at the sarcomere level (Gordon et al., 1966). Figure 9.7 is a
schematic representation of the myosin and actin cross-bridge arrangement.