The effect of pH on fatigue during submaximal isometric contractions of the human calf muscle

Purpose This study examined whether changes in pH throughout the physiologic range would have a differential effect on central and peripheral factors associated with fatigue and force production during submaximal lower limb isometric exercise to task failure. Methods Eight males completed three experimental trials [0.2 g kg−1 ammonia chloride (ACD); 0.3 g kg−1 calcium carbonate (PLA); or 0.3 g kg−1 sodium bicarbonate (ALK)], each consisting of submaximal calf contractions at 55% of maximal voluntary contraction (MVC) to task failure. Every minute of the task subjects performed an MVC, coupled with stimulation of the tibial nerve during and immediately post MVC. Results Time to task failure was not different between trials (ACD 531 ± 166 s, PLA 592 ± 163 s, ALK 596 ± 150 s; p = 0.31). MVC force in all trials declined 29% from the start of exercise to the fifth minute (mean decline of 371 ± 26 N; p < 0.001), however was not different between trials (p = 0.21). Fatigue was mediated in all trials by central and peripheral factors, as declines in voluntary activation, V/M-wave in the soleus and the potentiated resting twitch amplitudes were evident throughout the task (p < 0.05). Central fatigue appeared to be muscle specific, as reductions in central drive (V/M-wave and rate of sEMG rise) persisted in the soleus but not the medial gastrocnemius. Conclusions These data suggest that calf fatigue associated with intermittent, isometric contractions to task failure is unaffected by alterations in pH; however, central drive reductions may be muscle specific.