It is generally thought that intracellular pH (pH_i) of skeletal muscle falls at least 0.5 units during intense activity, but evidence on natural (i.e., voluntary, two-legged (2L)) locomotor activity in man has exclusively come from invasive studies of upper leg muscle. Here, noninvasive ³¹P nuclear magnetic resonance spectroscopy (³¹P NMRS) was used to study human quadriceps muscle energetics and pH_i during incremental bicycling exercise to exhaustion in six normally active subjects. Cellular energy charge (CEC; [PCr]/([PCr]+[P_i])) linearly (r 0.90) dropped 83 ± 3% during ramp exercise to exhaustion from 0.92 ± 0.01 at rest to 0.16 ± 0.03 at maximal sustained work rate (WR) (166±17 W; range: 108−223 W). Surprisingly, pH_i likewise dropped linearly (r 0.82) no more than 0.2 units over the entire range of WR between rest and maximal (pH_i 7.08±0.01 and 6.84±0.02, respectively). But after termination of exercise pH_i dropped rapidly to textbook acidic values of 6.6 explaining classic biopsy results. Comparative coresponse analysis of pH_i and CEC changes during 2L- vs. 1L-cycling showed that homeostatic control of quadriceps pH_i during bicycling is robust and unique to natural locomotor exercise. These results highlight the robustness of the integrative set of physicochemical and physiological control mechanisms in acid−base balance during natural locomotor activity in man.

Key words: acid−base balance • exercise • skeletal muscle • cellular energetics • ³¹P NMR spectroscopy • coresponse analysis