Myoelectric evidence of peripheral muscle fatigue during exercise in severe hypoxia: some references to m. vastus lateralis myosin heavy chain composition

Abstract

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.

Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans

“…Second, we confirmed the effects of preventing EIAH by showing that moderate arterial hypoxemia during exercise, induced via small reductions in FI O 2 , exacerbated locomotor muscle fatigue. Our findings are consistent with those of Taylor et al (47) in severe, acute hypoxia but not with those of Kayser et al (26) in severe, chronic hypoxia, both of whom measured the activity of the integrated quadriceps EMG over time during constant-load exercise as a test of peripheral muscle fatigue (see Introduction). We added just enough FI O 2 during exercise to raise alveolar and arterial PO 2 to prevent the HbO 2 desaturation from falling below resting levels.…”

“…Alternatively, an inability of the sarcolemma and T tubule to conduct repetitive action potentials may indirectly reduce Ca 2ϩ cycling. The possibility that such peripheral processes are involved in exercise limitation during hypoxemia is suggested by the finding that severe acute hypoxia increases integrated electromyographic (EMG) activity of the vastus lateralis muscle during heavy-intensity constant-load cycling (47). This finding was interpreted to reflect an increase in motor unit recruitment to compensate for the fatigue (47).…”

“…The possibility that such peripheral processes are involved in exercise limitation during hypoxemia is suggested by the finding that severe acute hypoxia increases integrated electromyographic (EMG) activity of the vastus lateralis muscle during heavy-intensity constant-load cycling (47). This finding was interpreted to reflect an increase in motor unit recruitment to compensate for the fatigue (47). In contrast, it has been argued that systemic hypoxemia may reflexively inhibit central motor output to locomotor muscles to ensure that a catastrophic failure of homeostasis does not occur during exercise (3,19).…”

The effects of training in hyperoxia vs. normoxia on skeletal muscle enzyme activities and exercise performance

“…On the other hand, previous work has also suggested that central motor output is inversely related to inspired oxygen concentration (14,23). This suggests that H may lower central neural drive for a given exercise intensity, which may delay the development of neuromuscular fatigue.…”

Electromechanical Delay after ACL Reconstruction: An Innovative Method for Investigating Central and Peripheral Contributions

“…21 Taylor et al 30 showed that fatigue prolonged electromechanical delay and reduced muscle conduction velocity. On the basis of biopsy findings, the degree of these changes was greater in muscles with a higher percentage of type-II fibers.…”