The effects of work strategy and a rest condition on the physiological changes of the erector spinae muscle were studied. Eleven volunteers repeated fatiguing static contractions of holding an industrial box in 30° trunk flexion for a repetition of 12 times interrupted by rests for which the duration was equal to the duration of each preceding contraction. Each contraction was stopped at two fatigue sensation levels; moderate or strong, which corresponded to Borg's CR-10 3 and 5, as the work strategy conditions. The repeated contractions were performed for a total of eight conditions combined with the two rest conditions, with and without stretch exercise, and two contraction levels of 10 and 40% maximum pulling force in flexed posture (MVC) on separate days. Near-infrared spectroscopy and surface electromyography (EMG) were recorded. Amplitude and median frequency (MF) of EMG, oxy-hemoglobin, and deoxy-hemoglobin were calculated. MF related to fatigue sensation most closely. Oxy-hemoglobin decreased during each contraction. It, however, increased with repetition especially at 10% MVC and the increase was interpreted as adaptation to the work. The effect of the work strategy and the rest conditions was weak. MF decreased more in a strong work strategy condition than in a moderate one.
Relations between surface electromyogram (EMG) and fatigue sensation was compared between the first fatiguing contraction and the following contractions with insufficient rests. Six male subjects performed static contractions of the elbow flexors at 8 and 13% MVC with the forearm semipronated and at 13% MVC with the forearm supinated. Contractions were repeated 6 (13% MVC) or (8% MVC) times (C1, C2, C3, C4, C5, C6) with rests of 5 min (13% MVC) or 10 min (8% MVC). During the contractions the subjects reported values of rated fatigue sensation (VRFS) whose scale was from 0.0 (no fatigue sensation) to 4.0 (apparent pain and moderate severity) at every 30 sec. C1 ended between 3.5 and 4.0 of VRFS or at 18 min (13% MVC) or 20 min (8% MVC) of contracting time. C2-C5 ended when fatigue sensation became that at the end of C1 or 5 min had passed. Bipolar and monopolar surface EMG was recorded from 6 synergists of elbow flexors. Mean amplitude (AEMG) and relative power below 22Hz (RPWL20) were calculated. Though AEMGs were positively correlated with VRFSs in C1, the correlations were often obscure in C2-C6. Most AEMGs at the same VRFS were larger in C2-C6 than in C1. AEMGs at the start of C2-C6 were sometimes larger than those at the end of C1. RPWL20s of monopolar EMG positively correlated with VRFSs. The RPWL20 correlations were often reserved in all contractions. The increases of AEMGs in C2-C6 at the same VRFS cannot be explained by the migration of the activities among synergists, since the increases in AEMGs were seen in most synergists simultaneously. The compensation of the failure of muscle contractibility were not the only cause of the increases in the AEMGs during fatiguing contractions. The compression of EMG spectrum towards lower frequencies was usable to estimate the change of fatigue sensation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.