Force-frequency and fatigue properties of motor units in muscles that control digits of the human hand

Force-frequency and fatigue properties of motor units in muscles that contr ol digits of the human hand. J. Neuro-, 81: 1718-1729, 1999. Modulation of motor unit activation rate is a fundamental process by which the mammalian nervous system encodes muscle ford, To identify how rate ending of force ma y change as a consequence of fatigue, intraneural microstimulation of motor axons was used to elicit twitch and force-frequency responses before and a fter 7 min of intermittent stimulation (IO-Hz train for 330 ms, I train/s) in single motor units of human long finger flexor muscles and intrinsic han d muscles. Before fatigue, two groups of units could he 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 betwee n these two groups (group I = 66. 5 ms: group 2 = 45.9 ms). Overall 18% of the units were fatigue resistant [fatigue index (FI) > 0.75], 64% had inter mediate fatigue sensitivity (0.25 less than or equal to FI less than or equ al to 0.75), and 18% were fatigable(FI < 0.25). However, fatigability and t etanic farce were not significantly different among groups. Therefore unlik e findings in some other mammals, fast-contracting motor units were neither stranger nor more susceptible to fatigue than slowly contracting units. Fa tigue, however, was found to be greatest in those units that initially exer ted the largest forces. Despite significant slowing of contractile response s, fatigue caused the force-frequency relation to become displaced toward h igher frequencies ( 44 +/- 41% increase in frequency for half-maximal force ). Moreover, the greatest shift in the force-frequency relation occurred am ong those units exhibiting thr largest force loss. A selective deficit in f orce at lots frequencies of stimulation persisted for several minutes after the fatigue task;. Overall, these findings suggest that with fatigue highe r activation rates must he delivered to motor units to maintain the same re lative level of force. Questions regarding classification of motor units an d possible mechanisms by which fatigue-related slowing might coexist with a shift in the force-frequency curve toward higher frequencies are discussed .