MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 230

Figure 9.5 Time course of a muscle twitch, modeled as the impulse response of a
second-order critically damped system. Contraction time T varies with each motor unit
type, from about 20 to 100 ms. Effective tension lasts until about 4 T .

These same researchers found that T increased in all muscles as they
were cooled; for example, the biceps brachialis had a contraction time that
increased from 54 ms at 37 ◦ C to 124 ms at 23 ◦ C. It is evident that the cause
of the delayed build-up of tension is the slower metabolic rates and increased
muscle viscosity seen at lower temperatures.
Many other researchers have tested other muscles using different tech-
niques. Milner-Brown et al. (1973 b ) looked at the first dorsal interossei
during voluntary contractions and measured T to average 55 ms ( ± 12 ms),
while Desmedt and Godaux (1978) calculated a mean of 60 ms on the same
muscle but used supramaximal stimulations. These latter researchers, using
the same technique, reported T for the tibialis anterior as 84 ms and for the
soleus as 100 ms. Bellemare et al. (1983), also using supramaximal stimula-
tions, reported soleus T 9ms).

Thus, it appears that the contraction time varies considerably depending on
the muscle, the person, and the experimental technique employed.

= 116 ms ( ± 9 ms) and biceps T = 66 ms ( ±

9.0.6 Shape of Graded Contractions
The shape of a voluntary tension curve depends to a certain extent on the
shape of the individual muscle twitches. For example, if we elicit a maximum
contraction in a certain muscle, the rate of increase of tension depends on
the individual motor units and how they are recruited. Even if all motor
units were turned on at the same instant and fired at their maximum rate,
maximum tension could never be achieved in less than the average contraction
time for that muscle. However, as described in Figure 9.2 and depicted in
Figure 9.3, the recruitment of motor units does not take place all at once
during a voluntary contraction. The smaller slow-twitch units are recruited
first in accordance with the size principle, with the largest units not being