Integrated electromyography (iEMG) of the m. vastus lateralis was analysed during cycle ergometry in male subjects (n = 8). Two work trials were conducted, one under normoxia (N), the other under environmental normobaric hypoxia (EH in which the oxygen fraction in inspired gas = 0.116), each trial lasting 10 min. The absolute power output (180 W) was the same for both trials and was equivalent to 77 (4)% of maximum heart rate in trial N. Maximal voluntary isometric contractions were performed after each trial to assess changes in force, muscle fibre conduction velocity (MFCV), electromechanical delay (EMD), median frequency of EMG (MF) and maximal iEMG (iEMGmax). Biopsy samples of muscle were obtained from the m. vastus medialis before testing. Myosin heavy chain (MHC) differences were determined through sodium dodecyl-polyacrylamide gel electrophoresis followed by densitometric analysis. No differences in submaximal iEMG were observed between EH and N trials during the first minute of work. At the end of both work trials iEMG was significantly elevated compared with starting values, however the iEMG recorded in EH exceeded N values by 15%. At the end of the EH trials the following were observed: a decrease in isometric force, MFCV and MF with an increase in EMD and the iEMGmax/force ratio. The iEMGmax was unchanged. No differences in any of these variables were observed after the N trial. Mean (SD) lactate concentrations following EH and N trials were 9.2 (4.4) mmol x 1(-1) and 3.5 (1.1) mmol x 1(-1), respectively. Results indicate that an increased motor unit recruitment and rate coding was needed in EH to maintain the required power output. The increased motor unit recruitment and rate coding were associated with myoelectric evidence of "peripheral" muscle fatigue. Subjects with higher compositions of type II MHC accumulated more lactate and displayed greater reductions in MF and MFCV during fatigue.

This study analysed the changes in electromyographic (EMG) activity of the vastus lateralis, biceps femoris and gastrocnemius muscles during incremental treadmill running. The changes in EMG were related to the lactate and ventilatory thresholds. Ten trained subjects participated in the study. Minute ventilation, oxygen consumption, carbon dioxide expired and the fraction of oxygen in the expired gas were recorded continuously. Venous blood samples were collected at each exercise intensity and analysed for lactate concentration. The EMG were recorded at the end of each exercise intensity using surface electrodes. The EMG were quantified through integration (iEMG) and by calculating the mean power frequency (MPF). The iEMG measurements were characterized by a breakpoint in the vastus lateralis and/or gastrocnemius muscles in eight of the subjects tested. However, the results indicated that blood lactate concentrations had already begun to increase in a nonlinear fashion before the iEMG breakpoint had been surpassed. Consequently, the occurence of the lactate threshold cannot be attributed solely to the change in motor unit recruitment or rate coding patterns demonstrated by the iEMG breakpoint. The ventilatory threshold was shown to be a far more reliable and convenient noninvasive predictor of the lactate threshold in comparison with EMG techniques. In conclusion, the EMG measurements used in this study (i.e. iEMG and MPF) were not considered to be viable noninvasive determinants of the aerobic-anaerobic transition phase in treadmill running.

The principle aims of this research were, firstly, to determine if the relationship between integrated electromyography (iEMG) and exercise intensity was linear or threshold-like, and secondly, to determine if the relationship between iEMG and exercise intensity was repeatable on different test occasions. A group of 20 trained male subjects participated in the study. Each subject completed two incremental exercise tests on a Monark cycle ergometer. The tests were identical and separated from each other by a mean period of 42 (SD 12) h. The EMG signals were recorded from the vastus lateralis, rectus femoris and vastus medialis muscles at each intensity using surface electrodes. The relationship between iEMG and intensity was shown to be linear (r = 0.95 to r = 0.98) with no obvious iEMG thresholds present. The gradients of simple regression lines fitted to the iEMG compared to intensity were not significantly different on the retest occasion (CV 9%-12%). In summary, the findings of this study indicated that, during incremental exercise, the relationship between iEMG of the quadriceps musculature and exercise intensity was linear and not threshold-like. Furthermore, the linear relationship between iEMG and workload was repeatable on different test occasions.

This investigation analysed the effects of environmental hypoxia (EU) on changes in quadriceps integrated electromyogram (iEMG) and metabolite accumulation during incremental cycle ergometry. Trained male subjects (n = 14) were required to complete two maximal oxygen uptake (VO(2max)) tests, one test during EH (F(1)0(2) = 0.135), the other during normoxia (F(1)0(2) = 0.2093). The EMG were recorded at each exercise intensity from the vastus lateralis, rectus femoris and vastus medialis muscles over 60 cycle revolutions. Mean integral values were then calculated. Blood was collected from the radial vein of consenting subjects (n = 8) at the end of each exercise intensity. Oxygen saturation of arterial blood (S(a)O(2)) was estimated using pulse oximetry. Gas exchange variables were collected on-line every 15 s. The results indicated that, without exception, EH significantly reduced total exercise time. Mean time to exhaustion in EH was 26.34 (SD 2.58) min compared with 35.25 (SD 4.21) min during N. The S(a)O(2) values indicated that severe arterial desaturation had been achieved by EH. Mean values for VO(2max) obtained in EH were 49 ml*kg* min(-1), compared with 59 ml*kg*min(-1) attained in N. Plasma lactate and ammonia concentrations were both significantly higher in EH. Increases in lactate and ammonia concentration were highly correlated in both N and EH. The onset of plasma lactate and ammonia accumulation occurred at the same exercise intensity in N. The iEMG responses of all three quadriceps muscles tended to be greater in the EH trials, although this difference was not significant. The basis for iEMG nonsignificance may have been related to large within sample variation in iEMG, sample size and the severity of the hypoxia induced.

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