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The frequency content of the surface electromyography (SEMG) signal, expressed as median frequency (MF), is often assumed to reflect the decline of muscle fiber conduction velocity in fatigue. MF also decreases when motor unit firings synchronize, and we hypothesized that this effect can explain the electrode-dependent pattern in our previous recordings from the trapezius muscle. An existing motoneuron (MN) model describes the afterhyperpolarization following a spike as an exponential function on which membrane noise is superimposed. Splitting the noise into a common and an individual component extended the model to a MN pool with a tunable level of firing synchrony. An analytical volume conduction model was used to generate motor unit action potentials to simulate SEMG. A realistic level of synchrony decreased the MF of the simulated bipolar SEMG by approximately 30% midway between endplate position and tendon but not above the endplate. This is in accordance with experimental data from the biceps brachii muscle. It was concluded that the pattern of decrease of MF during sustained contractions indeed reflects MN synchronization.

The occurrence of pH heterogeneity in human tibial anterior muscle during sustained isometric exercise is demonstrated by applying (31)P-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 (31)P 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.