Limiting factors to high intensity exercise : the role of intramuscular pH and skeletal muscle buffering
Within the context of a metabolic model of fatigue, formulated from a review of the literature, a decrease in intramuscular pH was identified as a potential limiting factor to the performance of high intensity exercise (HIE). This suggested a role for skeletal muscle buffering (B) in retarding the intramuscular acidosis typically incurred during HIE. Previous studies on human muscle, were largely unable to confirm this role, partly due to the lack of a clearly defined and consistently utilised method of measurement of B. The validity of the procedures currently employed in the determination of B was therefore investigated, revealing that many of the individual differences in B, previously attributed physiological significance, may have originated from inappropriate methodology. In a cross-sectional examination of the vastus lateralis muscle of young, active individuals, low correlations (p<0.05) between the muscle carnosine concentration ([CAR]) and in the 'in vitro B' (Bvit) (r=0.30), and between either [CAR] or Bvit and the % type II fibre area were observed. An elevated Bvit was shown to be of importance in minimising the decrement in pH during HIE, but did not per se permit the accumulation of a higher muscle lactate concentration, or allow for an enhanced HIE performance; these factors were more dependent on the % type II fibre area. The muscle pH, following dynamic HIE was highly variable between individuals, and showed a significant (p<0.05) negative correlation with the % type II fibre area. 16 weeks of isokinetic training of the quadriceps resulted in significant improvements (p<0.05) in dynamic HIE performance with no concomitant improvements in [CAR] or Bvit. The data suggest that the muscle buffer value per se, is not a major limiting factor to HIE, and that rather, the glycolytic capacity and pH tolerance of the muscle may be the more important factors to consider.