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The Journal of Neurophysiology Vol. 81 No. 4 April 1999, pp. 1718-1729
Copyright ©1999 by the American Physiological Society
John B. Pierce Laboratory, New Haven, Connecticut 06519
Fuglevand, Andrew J.,
Vaughan G. Macefield, and
Brenda Bigland-Ritchie.
Force-frequency and fatigue properties of motor units in muscles that
control digits of the human hand. Modulation of motor unit
activation rate is a fundamental process by which the mammalian nervous
system encodes muscle force. To identify how rate coding of force may
change as a consequence of fatigue, intraneural microstimulation of
motor axons was used to elicit twitch and force-frequency responses before and after 2 min of intermittent stimulation (40-Hz train for 330 ms, 1 train/s) in single motor units of human long finger flexor
muscles and intrinsic hand muscles. Before fatigue, two groups of units
could be distinguished based on the stimulus frequency needed to elicit
half-maximal force; group 1 (n = 8) required 9.1 ± 0.5 Hz (means ± SD), and group 2 (n = 5) required 15.5 ± 1.1 Hz. Twitch contraction times were
significantly different between these two groups (group 1 = 66. 5 ms; group 2 = 45.9 ms). Overall 18% of the units were fatigue
resistant [fatigue index (FI) > 0.75], 64% had intermediate fatigue
sensitivity (0.25 
FI 0.75), and 18% were fatigable
(FI < 0.25). However, fatigability and tetanic force were not
significantly different among groups. Therefore unlike findings in some
other mammals, fast-contracting motor units were neither stronger nor
more susceptible to fatigue than slowly contracting units. Fatigue,
however, was found to be greatest in those units that initially exerted
the largest forces. Despite significant slowing of contractile
responses, fatigue caused the force-frequency relation to become
displaced toward higher frequencies (44 ± 41% increase in
frequency for half-maximal force). Moreover, the greatest shift in the
force-frequency relation occurred among those units exhibiting the
largest force loss. A selective deficit in force at low frequencies of
stimulation persisted for several minutes after the fatigue task.
Overall, these findings suggest that with fatigue higher activation
rates must be delivered to motor units to maintain the same relative level of force. Questions regarding classification of motor units and
possible mechanisms by which fatigue-related slowing might coexist with
a shift in the force-frequency curve toward higher frequencies are discussed.
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