ABSTRACT
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.
Subject(s)
Back , Hemodynamics/physiology , Muscle Contraction/physiology , Muscle Fatigue/physiology , Muscle, Skeletal/blood supply , Muscle, Skeletal/physiology , Adult , Back/physiology , Electromyography , Exercise/physiology , Humans , Male , Rest/physiology , Sensation/physiology , Work/physiology , Young AdultABSTRACT
Surface electromyograms(EMG) during test contractions (TCs) were studied to assess the muscle strain in simulated mushroom picking. Additionally, the duration of the TC for the effective assessment was investigated. Nine female subjects performed standardized shoulder abduction and a stooped posture for one minute as TCs. Each experiment consisted of a 60-min rest, three work periods (W1-W3), a 30-min rest, and two work periods (W4 and W5) separated by a 30-min rest period. The duration of each work period was about 20 min. A total of 18 TCs was performed between the work periods and every 10 minutes in the rest periods. EMGs were recorded from the trapezius, infraspinatus, deltoid, and erector spinae muscles. The amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated. Each TC was divided equally into three parts. Ratings of perceived exertion (RPE) in the neck, shoulder and low-back were reported during TCs. The work increased RPE of all the parts. AEMG and RPE were increased and MPF was decreased by W1, W2 and W3 in the neck and shoulder muscles. MPF of the erector spinae was increased by the work. The results were not affected by the duration of TCs and the parts during the TCs. AEMG and MPF fluctuated before W1 although the changes of RPE were small. Averaging several TCs was recommended to get stable results from TCs. EMG changes and appropriate TC conditions were discussed in relation to the adaptation in fatiguing contractions.
Subject(s)
Electromyography , Muscle Contraction , Muscle, Skeletal/physiology , Posture , Adult , Back , Female , Humans , Muscle Fatigue/physiology , Neck Muscles/physiology , Shoulder , Young AdultABSTRACT
Muscle strain was assessed with surface EMG during simulated mushroom picking. Nine female subjects performed five periods of work (W1-W5). The duration of each period was about 20 min. W1, W2 and W3 were separated by a short break of several minutes. W3, W4 and W5 were separated by a rest period of about 43 min. EMGs were recorded from the muscles of trapezius, infraspinatus, deltoid and erector spinae muscles. Amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated every minute. Ratings of perceived exertion (RPE) were recorded for every 5 min. RPE increased during each work period and with the advance of W1-3. AEMG increased during some of the work periods in the trapezius and infraspinatus. AEMG decreased in a few of the work periods in the other muscles. MPF decreased during some of the work periods in the infraspinatus, deltoid and erector spinae. The increase of AEMG in the trapezius was interpreted as spare capacity for the work. The infraspinatus and deltoid were assessed to be more fatigued than the trapezius. The fatigue level of the erector spinae was low.
