The aim of this study was to investigate whether the mean power frequency of the electromyogram of the knee extensors was force and/or muscle fibre‐type dependent. Ten female subjects performed a gradually increasing static knee extension (5 seconds duration) using an isokinetic dynamometer. Electromyogram‐signals were obtained from the vastus lateralis, vastus medialis and the rectus femoris muscles. The torque signal and the three electromyogram signals were recorded on a tape recorder. From the electromyogram recordings the mean power frequency and the signal amplitude were determined. Muscle biopsies were later obtained from the right vastus lateralis and stained for alkaline and acid mATPase for the determination of fibre‐type proportion and areas. Both the mean power frequencies and the signal amplitudes of the three knee extensors were positively torque dependent. Furthermore it was found that the fibre type proportion and the regression coefficient of the torque (%)‐mean power frequency relationship were positively correlated. Also a negative correlation existed between the type‐1 (%) proportion and the intercept of the individual torque (%)‐mean power frequency relationships. In contrast to proposed models of the electromyogram signal no correlation was found between the mean power frequency and the fibre area.

The dependence of the mean-power frequency and the signal amplitude of the electromyogram (EMG) on the angular velocity and the fiber-type proportion were investigated in nine female volunteers. The subjects were required to perform maximum knee extensions using an isokinetic dynamometer at different angular velocities; 0.57, 1.05, 1.57, 2.09 and 3.14 rad s-1. Electromyographic signals were obtained from the vastus lateralis, vastus medialis and the rectus femoris muscles. The angle and the torque signals were recorded simultaneously with the three EMG signals on a tape-recorder. From the EMG recordings the mean power frequency (MPF) and the signal amplitude were determined. Muscle biopsies were later obtained from the right vastus lateralis and stained for alkaline and acid mATPase for the determination of fibre-type proportions and areas. Neither the signal amplitude nor the MPF of the EMG of the three muscles were dependent on the angular velocity. The MPF of the vastus lateralis correlated significantly (r = -0.93) with the type 1 fibre proportion at 1.57 rad s-1. However, there was no significant correlation between the areas of the fibre types, alone or together, and the MPF. In conclusion the fibre-type proportion was the major factor behind the MPF irrespective of angular velocity.

Several studies have reported estimations of the total number of fibres in a muscle, e.g. before and after training or before and after inactivity. In those investigations a combination of computed tomographic estimations of muscle size and morphological studies of fibre size has most often been used. There have been doubts about the reliability of those studies on pennate muscles, since changes in muscle fibre size have been said to alter fibre angulation and thus the number of fibres that will cross a section. If such an alteration in fibre angulation takes place with an increase in fibre size, there ought to be some correlation between fibre size and fibre angulation. The present study was designed to test whether repetitive estimations of muscle fibre angulation could be performed in vivo and if any such correlation could be found between fibre size and fibre angulation. A group of 15 women volunteered to take part in the study. Repeated ultrasonographic recordings were made on five subjects on 3 consecutive days to test the repeatability of ultrasonographic measurement of fibre angulation. Both muscle morphological analyses and ultrasonographic measurements of fibre angulation were performed on the other 10 subjects. Ultrasonographic measurement of fibre angulation was found to be reproducible since no variation between measurements made on different days was found. When trying to correlate muscle fibre size to the muscle fibre angulation, measured ultrasonographically, no significant correlation was found.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the study was to determine the distribution, total number and size of different types of fibres in whole cross-sections of female m tibialis anterior and to compare these data with previous results obtained from male skeletal muscles as well as with data on female muscles obtained by biopsy technique. The distribution of fibre types varied within the muscle cross-sections, and the pattern was slightly different from that earlier described for male muscles (Henriksson-Larsén et al. 1983). No difference in mean fibre type occurrence was found between the left and right muscles of the females. The type 2 fibres varied in size in different muscle regions and were usually largest at the deep surface of the muscle. Both the total number and size of the muscle fibres were smaller in the female muscles than in the corresponding male muscles (Henriksson-Larsén et al. 1985a, b). This resulted in a 30-40% smaller value for the total muscle fibre area in the female muscles compared to the male muscles.

Changes in mechanical performance and electromyographic activity during fatiguing dynamic knee-extensions were evaluated with respect to muscle fibre type composition of the vastus lateralis muscle in nine sedentary female [23 (3) years] volunteers. The subjects performed 150 repetitive maximum knee-extensions using a Cybex dynamometer at 1.57 rad x s(-1). EMG activity was recorded from the vastus lateralis, the vastus medialis and the rectus femoris muscles. For each contraction, mean power frequency (MPF) and the root mean square (RMS) of the EMG were calculated, simultaneously with the peak torque (PT), contractional work (CW) and the mean power (MP). The MPF showed an initial decrease followed by a stable phase. The RMS increased during the initial seven contractions, after which a period of variability was displayed until about the 60th contraction. At the plateau level (last 50 contractions) the relative RMS values were not significantly different from the initial values. The PT, CW and MP increased during the initial five to ten contractions, after which a two-phase pattern was displayed, with a gradual decline followed by a stable phase. The absolute plateau level of MPF for the vastus lateralis muscle showed a significant negative correlation with the area percentage of type-1 fibres (r = -0.71). Significant correlations were also demonstrated to occur between the absolute plateau levels of PT, CW and MP and the relative proportion of type-1 fibres (r = 0.80, r = 0.82 and r = 0.82 respectively). Thus, in female subjects the mechanical performance and the MPF during fatigue are at least partly determined by muscle morphology.