Mechanical and neural activation changes that accompanied muscle isometric training were studied in males. Training and testing sessions consisted of right elbow isometric flexions. Each experimental group was trained during 5 wk at one of the following angles: 25, 80, and 120 degrees. Bipolar surface electromyogram (EMG) was recorded from the biceps brachii and brachioradialis muscles. An improvement of maximal voluntary contraction (MVC) was always found at the training angle and was systematically greater than at the other angles. Moreover, the shorter the muscle length at which the training has been carried out, the more the gain was limited to the training angle. An increase of the maximal integrated EMG of both biceps brachii and brachioradialis frequently accompanied the improvement of MVC at the training angle.
This study analyses the changes in the electromyographic activity (EMG) of six major muscles of the leg during an incremental running test carried out on a treadmill. These muscles, the gluteus maximus (GM), biceps femoris (BF), vastus lateralis (VL), rectus femoris (RF), tibialis anterior (TA) and gastrocnemius (Ga) are known to have quite different functions during running. The aim of this study was to develop a methodology adapted to the analysis of integrated EMG (iEMG) running results, and to test the chronology of the onset of fatigue of the major muscles involved in running. Nine well-trained subjects [VO(2max) 76 (2.9) ml.min(-1).kg(-1)] took part in this study. They completed a running protocol consisting of 4 min stages, incrementally increasing in speed until exhaustion. The EMG signal was recorded during ten bursts of activation analysed separately at 45 s and 3 min 40 s of each stage. During running, consideration of the alteration in stride frequency with either an increase in speed or the onset of fatigue appears to be an indispensable part of the assessment of muscular fatigue. This allows the comparison of muscular activation between the various stage speeds by the use of common working units. Distance seems to be the only working unit that allows this comparison and thus the determination of the appearance of fatigue during running. The biarticular hip-mobilising muscles (RF and BF), which present two different bursts of activation during one running cycle, are the muscles that show the earliest signs of fatigue.
This study analysed the changes in the electromyographic activity (EMG) of the vastus lateralis muscle (VL) during an incremental maximal oxygen uptake test on a treadmill. A breakpoint in the integrated electromyogram (iEMG)-velocity relationship has already been interpreted in two ways: either as a sign of neuromuscular fatigue or as an expression of the iEMG-velocity relationship characteristics. The aim of this study was to test a method of distinguishing fatigue eects from those due to increases in exercise power. Eight well-trained male runners took part in the study. They completed a running protocol consisting of 4-min stages of increments in power output. Between each stage (about 15 s after the start of a minute at rest), the subjects had to maintain a standard eort: a 10-s isometric leg extension contraction [50% isometric maximal voluntary contraction (IMVC)]. The EMG was recorded during the running and isometric protocols, a change in the EMG signal during the isometric exercise being considered as the sign of fatigue. The iEMG-velocity relationships were strongly ®tted by a second-order polynomial function for data taken at both the start (r 0.98) and the end (r 0.98) of the stage. Based on the stability of the 50%IMVC-iEMG relationship noted between stages, the start-iEMG has been identi®ed as expressing the iEMG-velocity relationship without fatigue. The stage after which end-iEMG increased signi®cantly more steeply than start-iEMG was considered as the iEMG threshold and was simultaneous with the ventilatory equivalent for carbon dioxide threshold. The parallel changes of minute ventilation and iEMG would suggest the existence of common regulation stimuli linked either to eort intensity and/or to metabolic conditions. The fall in intracellular [K + ] has been discussed as being one of the main factors in regulating ventilation.
The overload imposed on the neuromuscular system under Electrostimulation (ES) can be expressed by applied current intensity or by Electrically Evoked Torque (EET). The aim of this study was to discern which of these two parameters is the one which is determinant for the efficacy of training by ES. Test and training involved isometric contraction of the flexion maintained at a joint angle of 25 degrees (0 degree = extension). The 16 trained subjects received 15 sessions of 25 electrically evoked contractions, using a monophasic rectangular waveform current at a frequency of 2500 Hz modulated at 90 Hz. Each stimulation lasted for 5 sec at the maximal tolerable current. Contrary to the control group (n = 16), the trained group significantly increased their MVIC (15.6%). The individual strength gains ranged from -5% to 49%. No correlation was found between current level and strength modifications. A direct relation was established between the EET and the strength gains (a minimum threshold of EET must be reached during at least 8 sessions to induce strength increases). The "overload principle", previously described for voluntary contraction strengthening, seems to be suitable for electrical stimulation and concerns the EET shown on the ergometer as the effect of the contraction of agonist and antagonist muscles.
The hypothesis that critical power (CP) is significantly lower than the maximal aerobic power of the knee extensors has been tested in nine endurance-trained subjects, seven gymnasts and seven weight lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and work performed at exhaustion on a knee-extension ergometer. CP was compared with the power output at the end of a progressive knee-extension exercise (P(peak)) and the power outputs corresponding to exhaustion times equal to 4 (P(4 min)), 6 (P(6 min)), 8 (P(8 min)) and 10 min (P(10 min)), calculated according to the linear relationship between work and exhaustion time. The hypothesis that CP corresponds to a steady state in metabolic and physiological parameters was tested in the gymnasts and the weight lifters by comparing CP with the fatigue thresholds of the integrated electromyogram (iEMG(FT)), lactate level (La(FT)), oxygen uptake (VO(2FT)) and heart rate (HR(FT)). The results of the present study demonstrate that the value of CP of a local exercise cannot be considered as the equivalent of the maximal aerobic power for general exercises. The values of P(4 min), P(6 min), P(8 min), P(10 min) and P(peak) were significantly higher than CP, and corresponded to 138, 126, 119, 115 and 151% CP, respectively. The results of the present study indicate that CP can be considered as an index of muscular endurance. Indeed, La(FT), iEMG (FT), VO(2FT) and HR(FT) were not significantly different from CP. All of these fatigue thresholds were significantly correlated with CP (r > 0.92). Moreover, the highest coefficient of correlation (r=0.71; P < 0.01) between the percentage of maximal aerobic power in cycling that corresponds to a blood lactate concentration of 4 mmol x l(-1) (OBLA%) and the different local aerobic indices was observed with CP.
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