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

The occurrence of pH heterogeneity in human tibial anterior muscle during sustained isometric exercise is demonstrated by applying P-31-nuclear magnetic resonance (NMR) spectroscopy in a study of seven healthy subjects. Exercise was performed at 30 and 60% of maximal voluntary contraction (MVC) until 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 revealed the simultaneous presence of two pH pools during most experiments. Maximum difference in the two pH levels during exercise was 0.40 +/- 0.07 (30% MVC, n = 7) and 0.41 +/- 0.03 (60% MVC, n = 3). Complementary two-dimensional P-31 spectroscopic imaging experiments in one subject supported the supposition that the distinct pH pools reflect the metabolic status of the main muscle fiber types. The relative size of the P-i peak in the spectrum attributed to the type II fiber pool increases with decreasing pH levels. This phenomenon is discussed in the context of the size principle stating that the smaller (type I) motor units are recruited first.