Reflex regulation during sustained and intermittent submaximal contractions in humans

Abstract

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.

Figure 1. Hoffmann (H) reflex and long latency reflex (LLR) recordings in the abductor pollicis brevis (APB)

Typical traces (average of 36 sweeps) recorded in one subject in response to the electrical stimulation of the median nerve at the wrist before (A) and after (B) fatigue caused by a submaximal (25 % MVC) contraction which was sustained until the endurance limit, and after 5 min of recovery (C). The dotted vertical line represents the onset of the stimulation artefact. On each trace, the horizontal line denotes the average background EMG level and the small arrows indicate the onset of the two reflex responses.

Figure 4. Changes in voluntary EMG and M-wave amplitudes during submaximal contractions and recovery

A, average EMG (means ± s.e.m.; n = 10) obtained during sustained contractions at 25 % (•) and 50 % MVC (▴), and during intermittent contractions at 25 % MVC (○) as a function of endurance time. All data were first normalised to the corresponding M-wave amplitude and thereafter expressed as a percentage of the pre-fatigue value. B, M-wave amplitude (means ± s.e.m.; n = 10), normalized to the pre-fatigue value during the three fatigue tests as a function of endurance time. Significant differences from pre-fatigue value : *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 2. Changes in H reflex amplitude during submaximal contractions and recovery

Values (means ± s.e.m.; n = 10) obtained during sustained contractions at 25 % (•) and 50 % MVC (▴), and during intermittent contractions at 25 % MVC (○) as a function of endurance time. All data were first normalised to the corresponding M-wave amplitude and thereafter expressed as a percentage of the pre-fatigue value. Significant differences from pre-fatigue value: *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 3. Changes in LLR amplitude during submaximal contractions and recovery

Values (means ± s.e.m.; n = 10) obtained during sustained contractions at 25 % (•) and 50 % MVC (▴), and during intermittent contractions at 25 % MVC (○) as a function of endurance time. All data were first normalised to the corresponding M-wave amplitude and thereafter expressed as a percentage of the pre-fatigue value. Significant differences from pre-fatigue value : *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 5. Changes in reflexes amplitudes as a function of the contraction intensity during the sustained fatigue tests

Data (means ± s.e.m.) for 25 % and 50 % MVC are from the present study, whereas those of the 100 % MVC are from a previous study (Duchateau & Hainaut, 1993). The equations of the linear regressions are : y = 0.11x+ 54.8 (r²= 0.35) and y = 0.42x+ 50.8 (r²= 0.99) for the H reflex and the LLR, respectively. The regression lines indicate a significant (P < 0.001) association between the changes in reflex amplitude and the intensity of the contraction for LLR but not for the H reflex (P > 0.05).