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.