pH heterogeneity in tibial anterior muscle during isometric activity studied by P-31-NMR spectroscopy

The occurrence of pH heterogeneity in human tibial anterior muscle during s ustained isometric exercise is demonstrated by applying P-31-nuclear magnet ic resonance (NMR) spectroscopy in a study of seven healthy subjects. Exerc ise was performed at 30 and 60% of maximal voluntary contraction (MVC) unti l fatigue. The NMR spectra, as localized by a surface coil and improved by proton irradiation, were obtained at a high time resolution (16 s). They re vealed the simultaneous presence of two pH pools during most experiments. M aximum difference in the two pH levels during exercise was 0.40 +/- 0.07 (3 0% MVC, n = 7) and 0.41 +/- 0.03 (60% MVC, n = 3). Complementary two-dimens ional P-31 spectroscopic imaging experiments in one subject supported the s upposition that the distinct pH pools reflect the metabolic status of the m ain muscle fiber types. The relative size of the P-i peak in the spectrum a ttributed to the type II fiber pool increases with decreasing pH levels. Th is phenomenon is discussed in the context of the size principle stating tha t the smaller (type I) motor units are recruited first.