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First published online June 7, 2004
Journal of Experimental Biology 207, 2519-2528 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01042

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Spectral properties of myoelectric signals from different motor units in the leg extensor muscles

James M. Wakeling^1,2,* and Antra I. Rozitis¹

¹ Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, T2N 1N4, Canada
² Royal Veterinary College, North Mymms, Herts, AL9 7TA, UK

View larger version (21K): [in a new window]   Fig. 1. Myoelectric signals and principle components from the electrical stimulation and tendon tap recordings from the soleus. (A) Myoelectric signals for the electrical stimulation (black lines) and tendon taps (gray lines) for the ten trials for one subject. The artefact for the electrical stimulation is shown by the arrow. (B) The weightings for the first two principle components (PC) are shown that describe the intensity spectra for both the electrical stimulation and tendon tap trials for all subjects (N=100), with the relative proportion of the total signal that they describe. The principle component weightings are shown by solid circles and line. The mean ± S.E.M. intensity spectra for the data (N=100) are shown by the open diamonds. (C) Mean ± S.E.M. scores for PC I and PC II for the electrical stimulation (black symbols) and tendon taps (gray symbols) for all subjects (N=50). The arrows mark vectors in the PC I–PC II scoring plane, which form the reconstructed spectra.

View larger version (24K): [in a new window]   Fig. 2. Force (A) and EMG from vastus medialis (B,C) during a 4 s ramped isometric contraction. Increasing EMG intensity is shown by darker regions in time–frequency space (D).

View larger version (25K): [in a new window]   Fig. 3. Principle component weightings from the intensity spectra from the ramped isometric contractions. The components were calculated from 2160 intensity spectra from the rectus femoris, vastus lateralis, vastus medialis, lateral gastrocnemius, medial gastrocnemius and soleus muscles from 10 subjects and over a force range from 5 to 95% max. voluntary contraction (MVC). The first four principle components (PC) are shown, with the relative proportion of the total signal that they describe.

View larger version (21K): [in a new window]   Fig. 4. Mean scores for the first four principle components. For each muscle the mean scores across all forces are shown for PC I in the left bar through to PC IV in the right bar. Bars show the mean ± S.E.M. (270N450).

View larger version (21K): [in a new window]   Fig. 5. Principle component scores from the intensity spectra from the ramped isometric contractions. The points denote the mean ± S.E.M. scores (30<N<50) for each force bin for the rectus femoris (open triangles), vastus lateralis (solid diamonds), vastus medialis (open diamonds), lateral gastrocnemius (solid circles), medial gastrocnemius (open circles) and soleus (open squares). The asterisks mark the 5–15% force bins, and the lines join sequentially higher force bins to the 85–95% bin.

View larger version (12K): [in a new window]   Fig. 6. Intensity peaks for the contraction in Fig. 2. Symbols show the peaks in EMG intensity at the frequency and contraction force at which they occurred. The initial peaks in each frequency band are shown by open squares; these peaks were used for further analysis.

View larger version (19K): [in a new window]   Fig. 7. Contraction force (% max. voluntary contraction; MVC) at which the initial intensity peak occurred during each graded contraction as a function of the center frequency for each wavelet domain. Rectus femoris, open triangles; vastus lateralis, solid diamonds; vastus medialis, open diamonds; lateral gastrocnemius, solid circles; medial gastrocnemius, open circles; soleus, open squares. Values are the mean ± S.E.M. (30N50 for fc<250 Hz and 6N50 for 250fc400 Hz). The third order polynomial regression lines (Table 3; P<0.001) were calculated from the mean points for each muscle and for 1k13 (19fc542 Hz).

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