Biomedical Engineering Reference
In-Depth Information
Successive recruitment can be described as follows. The smallest motor unit
(MU-1) is recruited first—usually at an initial frequency ranging from about 5 to
13 Hz. Tension increases as MU-1 fires more rapidly until a certain tension is
reached; at this point MU-2 is recruited. Here, MU-2 starts firing at its initial low
rate and further tension is achieved by the increased firing of MU-1 and MU-2. At
a certain tension, MU-1 will reach its maximum firing range (15-60 Hz) and will
therefore be generating its maximum tension. This process of increasing tension
reaching new thresholds and recruiting another larger motor unit continues until
maximum voluntary contraction is reached. At that point, all motor units will be
firing at their maximum frequencies.
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