3. Although the central drive intensified during the fatigue test, as indicated by an increase in surface electromyogram (EMG), the discharge rate of the motor units during the hold phase of each contraction decreased progressively over the course of the task for motor units that were recruited at the beginning of the test, especially the low-threshold units. In contrast, the discharge rates of newly activated units first increased and then decreased.4. Such divergent behaviour of low-and high-threshold motor units could not be individually controlled by the central drive to the motoneurone pool. Rather, the different behaviours must be the consequence of variable contributions from motoneurone adaptation and afferent feedback from the muscle during the fatiguing contraction.
To investigate whether the intensity and duration of a sustained contraction influences reflex regulation, we compared sustained fatiguing contractions at 25 % and 50 % of maximal voluntary contraction (MVC) force in the human abductor pollicis brevis (APB) muscle. Because the activation of motoneurones during fatigue may be reflexively controlled by the metabolic status of the muscle, we also compared reflex activities during sustained and intermittent (6 s contraction, 4 s rest) contractions at 25 % MVC for an identical duration. The short‐latency Hoffmann(H) reflex and the long‐latency reflex (LLR) were recorded during voluntary contractions, before, during and after the fatigue tests, with each response normalised to the compound muscle action potential (M‐wave). The results showed that fatigue during sustained contractions was inversely related to the intensity, and hence the duration, of the effort. The MVC force and associated surface electromyogram (EMG) declined by 26.2 % and 35.2 %, respectively, after the sustained contraction at 50 % MVC, and by 34.2 % and 44.2 % after the sustained contraction at 25 % MVC. Although the average EMG increased progressively with time during the two sustained fatiguing contractions, the amplitudes of the H and LLR reflexes decreased significantly. Combined with previous data (Duchateau & Hainaut, 1993), the results show that the effect on the H reflex is independent of the intensity of the sustained contraction, whereas the decline in the LLR is closely related to the duration of the contraction. Because there were no changes in the intermittent test at 25 % MVC, the results indicate that the net excitatory spinal and supraspinal reflex‐mediated input to the motoneurone pool is reduced. This decline in excitation to the motoneurones, however, can be temporarily compensated by an enhancement of the central drive.
This study examines the effect of a change in fascicle length on motor unit recruitment and discharge rate in the human tibialis anterior during shortening and lengthening contractions that involved a similar change in torque. The dorsiflexor torque and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior were recorded in eight subjects. The behaviour of the same motor unit (n = 63) was compared during submaximal shortening and lengthening contractions performed at a constant velocity (10 deg s −1 ) with the dorsiflexor muscles over a 20 deg range of motion around the ankle neutral position. Muscle fascicle length was measured non-invasively using ultrasonography. Motor units that were active during a shortening contraction were always active during the subsequent lengthening contraction. Furthermore, additional motor units (n = 18) of higher force threshold that were recruited during the shortening contraction to maintain the required torque were derecruited first during the following lengthening contraction. Although the change in fascicle length was linear (r 2 > 0.99), and similar for both shortening and lengthening contractions, modulation of discharge rate differed during the two contractions. Compared with an initial isometric contraction at short (11.9 ± 2.4 Hz) or long (11.7 ± 2.2 Hz) muscle length, discharge rate increased only slightly and stayed nearly constant throughout the lengthening contraction (12.6 ± 2.0 Hz; P < 0.05) whereas it augmented progressively and more substantially during the shortening contraction, reaching 14.5 ± 2.5 Hz (P < 0.001) at the end of the movement. In conclusion, these observations indicate a clear difference in motor unit discharge rate modulation with no change in their recruitment order between shortening and lengthening contractions when performed with a similar change in muscle fascicle length and torque.
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