Myoelectric evidence of peripheral muscle fatigue during exercise in severe hypoxia: some references to m. vastus lateralis myosin heavy chain composition |
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Authors: | A D Taylor R Bronks P Smith and B Humphries |
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Institution: | (1) Southern Cross University, PO Box 157, Lismore 2480, New South Wales, Australia, AU |
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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). Biopy 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 · 1−1 and 3.5 (1.1) mmol · 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.
Accepted: 16 June 1996 |
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Keywords: | Hypoxia Muscle fatigue Electromyography Conduction velocity Electromechanical delay |
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