Physical Education - Basic muscle physiology: firing pattern
View the video of a tennis serve: the muscle firing pattern changes as the
intensity of the action changes.
If we relate firing pattern to exercise intensity, we see this pattern. At
low exercise intensities, like walking or slow running, slow-twitch fibres
are selectively utilised because they have the lowest threshold for
recruitment. If we suddenly increase the pace to a sprint, the larger fast
units will be recruited. In general, as the intensity of exercise increases
in any muscle, the contribution of the fast fibres will increase.
For the muscle, intensity translates to force per contraction and
contraction frequency per minute. Motor unit recruitment is regulated by
required force. In the unfatigued muscle, a sufficient number of motor
units will be recruited to supply the desired force. Initially, desired
force may be accomplished with little or no involvement of fast motor
units. However, as slow units become fatigued and fail to produce force,
fast units will be recruited as the brain attempts to maintain desired
force production by recruiting more motor units. Consequently, the same
force production in fatigued muscle will require a greater number of motor
units. This additional recruitment brings in fast, fatigable motor units.
Consequently, fatigue will be accelerated toward the end of long or severe
bouts, due to the increased lactate produced by the late recruitment of
Specific athletic groups may differ in the control of the motor units. Top
athletes in the explosive sports like Olympic weightlifting or the high
jump appear to have the ability to recruit nearly all of their motor units
in a simultaneous or synchronous fashion. In contrast, the firing pattern
of endurance athletes becomes more asynchronous. During continuous
contractions, some units are firing while others recover, providing a
built-in recovery period. Initial gains in strength associated with a
weight-training program are due to improved recruitment, not muscle
hypertrophy (increase in size).
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