Human muscle fatigue during maximum dynamic exercise lasting ten seconds

This research was carried out by the talented Christina Karatzaferi who was a PhD student directed by Professor Anthony J Sargeant. It is another contribution to a whole series of research papers which have their origin in a 1981 research paper of Tony Sargeant’s. In that seminal paper he described for the first time an isokinetic cycle ergometer which made it possible to measure maximum human muscle power in whole body dynamic exercise lasting for a few seconds under controlled velocity conditions.
The technique was applied in the present research to examine the changes in the high energy phosphates (ATP and PCr) which provide the energy for brief maximum exercise. Using needle biopsy of the leg muscles single muscle fibre fragments were then isolated from the biopsy and characterised for muscle fibre type using techniques previously developed in Professor Sargeant’s laboratory. The high energy phosphate content of the different fibre types was then determined by HPLC. The results show that as a consequence of the higher rate of cross-bridge cycling in the faster fibre types there is much greater depletion of the immediate source of  energy supply with surprisingly low levels of ATP in the very fastest fibre types. Thus the loss of dynamic muscle power over the 10 seconds of exercise was associated with the compromised energy status of a relatively small population of the very fastest (but also the most powerful) fibre types at the controlled pedalling rate chosen to generate maximum muscle power in these cycling experiments.

Abstract Changes in high-energy phosphate levels in single human skeletal muscle fibres after 10 s of maximal (all-out) dynamic exercise were investigated. Muscle biopsies from vastus lateralis of two volunteers were collected at rest and immediately post exercise. Single muscle fibres were dissected from dry muscle and were assigned into one of four groups according to their myosin heavy chain (MyHC) isoform content: that is type I, IIA, IIAx and IIXa (the latter two groups containing either less or more than 50% IIX MyHC).

Fragments of characterised fibres were analysed by HPLC for ATP, inosine-monophosphate (IMP), phosphocreatine (PCr) and creatine levels. After 10 s of exercise, PCr content ([PCr]) declined by approximately 46, 53, 62 and 59 % in type I, IIA, IIAx and IIXa fibres, respectively (P < 0.01 from rest). [ATP] declined only in type II fibres, especially in IIAx and IIXa fibres in which [IMP] reached mean values of 16 +/- 1 and 18 +/- 4 mmol (kg dry mass)(-1), respectively. While [PCr] was reduced in all fibre types during the brief maximal dynamic exercise, it was apparent that type II fibres expressing the IIX myosin heavy chain isoform were under a greatest metabolic stress as indicated by the reductions in [ATP].


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