Neural Correlates of Effort during Exercise

Abstract

Changes in movement-related cortical potentials (MRCPs) with increasing workload and muscle weakness could be related to effort. However, there is a lack of MRCP studies that have experimentally manipulated effort and correlated MRCP components with subjective effort ratings. PURPOSE: To determine whether the MRCP is a neurophysiological correlate of effort by experimentally manipulating effort during a weightlifting task with exercise induced muscle weakness. METHODS: Ten right-handed participants had one arm (randomly counterbalanced) eccentrically pre-fatigued. Participants performed four blocks of 50 quasi isometric elbow flexions (± 2 s each) in a randomized order. They lifted a light weight (20% of one-repetition maximum (1RM)) and a heavy weight (35% 1RM) with the fatigued (FAT) and the non-fatigued (NF) arm with 6 s rest between lifts. Ratings of perceived effort, biceps brachii electromyogram (EMG) and 62-channel electroencephalogram (EEG) were recorded. EEG was segmented and time-locked to EMG onset (0 ms), and artefact free trials were averaged within each block. Comparisons between blocks were made using two-way ANOVA with repeated measures. Means and standard deviations and p-levels of paired differences are reported. RESULTS: The fatigued arm was on average 29% weaker than the non-fatigued arm (p = 0.001). Effort ratings were significantly higher for the fatigued arm compared to the non-fatigued arm for both weights (Light FAT 3.1 ± 1.2 vs. NF 1.9 ± 0.7, p = 0.003; Heavy FAT 6.0 ± 1.5 vs. NF 3.9 ± 1.2, p < 0.001). Similar effects were found for biceps EMG amplitude (Light FAT 282 ± 222 vs. NF 139 ± 108 μV, p = 0.012; Heavy FAT 422 ± 332 vs. NF 192 ± 136 μV, p = 0.008). Effects of fatigue on MRCPs were found during and after the movement at selected electrode sites (e.g. Cz 0-1000 ms: Heavy FAT -12.9 ± 5.9 vs. NF -10.5 ± 4.4 μV, p = 0.038; C1' 1000-2000 ms: Heavy FAT -5.7 ± 3.4 vs. NF -7.0 ± 3.2 μV, p = 0.013; Pz 2000-3000 ms: Heavy FAT 5.6 ± 5.2 vs. NF 4.3 ± 4.9 μV, p = 0.023). CONCLUSION: Certain MRCP components reflect experimentally induced changes in effort during weightlifting exercise. No effects of fatigue were found before EMG onset in Cz, C1', and Pz. The supplementary motor area, primary motor cortex and parietal cortex might be involved in the perception of effort.