To obtain more insight into the changes in mean muscle fiber conduction velocity (MFCV) during sustained isometric exercise at relatively low contraction levels, we performed an in-depth study of the human tibialis anterior muscle by using multichannel surface electromyogram. The results show an increase in MFCV after an initial decrease of MFCV at 30 or 40% maximum voluntary contraction in all of the five subjects studied. With a peak velocity analysis, we calculated the distribution of conduction velocities of action potentials in the bipolar electromyogram signal. It shows two populations of peak velocities occurring simultaneously halfway through the exercise. The MFCV pattern implies the recruitment of two different populations of motor units. Because of the lowering of MFCV of the first activated population of motor units, the newly recruited second population of motor units becomes visible. It is most likely that the MFCV pattern can be ascribed to the fatiguing of already recruited predominantly type I motor units, followed by the recruitment of fresh, predominantly type II, motor units.

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.

fax +31 24 3540 866, email a.heerschap@rdiag.azn.nl 31 P magnetic resonance spectroscopy (MRS) offers a unique non-invasive window on energy metabolism in skeletal muscle, with possibilities for longitudinal studies and of obtaining important bioenergetic data continuously and with sufficient time resolution during muscle exercise. The present paper provides an introductory overview of the current status of in vivo 31 P MRS of skeletal muscle, focusing on human applications, but with some illustrative examples from studies on transgenic mice. Topics which are described in the present paper are the information content of the 31 P magnetic resonance spectrum of skeletal muscle, some practical issues in the performance of this MRS methodology, related muscle biochemistry and the validity of interpreting results in terms of biochemical processes, the possibility of investigating reaction kinetics in vivo and some indications for fibre-type heterogeneity as seen in spectra obtained during exercise. P magnetic resonance spectroscopy: Skeletal muscle: High-energy phosphates: Energy metabolism: ExerciseFollowing initial experiments on animal tissue (Hoult et al. 1974;Ackerman et al. 1980), magnetic resonance (MR) spectroscopy (MRS) was first applied to human subjects in the early 1980s, using the 31 P nucleus to monitor the levels and fate of high-energy phosphates in skeletal muscle (Chance et al. 1981;Cresshull et al. 1981;Ross et al. 1981). From these first experiments it was clear that 31 P MRS offers a unique non-invasive window on energy metabolism in skeletal muscle. Of particular interest is the possibility of obtaining important bioenergetic data continuously and with sufficient time resolution during muscle exercise. Another important aspect is that longitudinal monitoring is possible. Numerous studies applying this technique to human subjects have been published, and several reviews are available addressing specific results obtained in this way (for example, see Barbiroli, 1992;Cozzone & Bendahan, 1994;Kemp & Radda, 1994;McCully et al. 1994;Radda et al. 1995).The present paper provides an introduction to 31 P MRS as applied to skeletal muscle of human subjects, and also gives some illustrative examples from our recent studies on skeletal muscle of transgenic mouse models lacking creatine kinase (EC 2.7.3.2). Information content of the 31 P magnetic resonance spectrum of skeletal muscleTo appreciate the potential of the method, first, the information content of a spectrum obtained from human skeletal muscle at rest should be examined (see Fig. 1(A)).The most dominant signals in the spectrum are from phosphocreatine (PCr) and the three non-equivalent phosphate groups of ATP. Usually also a signal for inorganic phosphate (Pi) can be observed, and under favourable conditions signals for phosphomonoesters and phosphodiesters are observable as well. What is the origin of the distinct resonance frequencies of the 31 P nuclei in these compounds? In a first approximation the resonance frequency of the nuclear spins is...

The speed of propagation of an action potential along a muscle fiber, its conduction velocity (CV), can be used as an indication of the physiological or pathological state of the muscle fiber membrane. The motor unit action potential (MUAP), the waveform resulting from the spatial and temporal summation of the individual muscle fiber action potentials of that motor unit (MU), propagates with a speed referred to as the motor unit conduction velocity (MUCV). This paper introduces a new algorithm, the MU tracking algorithm, which estimates MUCVs and MUAP amplitudes for individual MUs in a localized MU population using SEMG signals. By tracking these values across time, the electrical activity of the localized MU pool can be monitored. An assessment of the performance of the algorithm has been achieved using simulated SEMG signals. It is concluded that this analysis technique enhances the suitability of SEMG for clinical applications and points toward a future of noninvasive diagnosis and assessment of neuromuscular disorders.

The occurrence of an abrupt acceleration in phosphocreatine hydrolysis in the tibial anterior muscle during the last part of a sustained isometric exercise at 30% maximal voluntary contraction until fatigue is demonstrated in seven out of eight healthy subjects by applying in vivo 31P NMR spectroscopy at 1.5 T field strength. This additional third phase in PCr hydrolysis, is preceded by a common biphasic pattern (first fast then slow) in PCr use. The NMR spectra, as localized by a surface coil and improved by proton irradiation, were collected at a time resolution of 16 s. Mean rates of PCr hydrolysis during exercise were -0.44 +/- 0.19% s(-1), -0.07 +/- 0.04% s(-1), and -0.29 +/- 0.10% s(-1) for the three successive phases. The increased rate of PCr hydrolysis, and also the loss of fine force control evident in the force records are consistent with increased involvement of large, fast-fatiguable units later in the contraction.