INTRODUCTION: To now it has been primarily amplitude and timing information that is extracted from electromyography (EMG) in alpine skiing. However, the EMG frequency also contains information about the pattern of activation of different muscle fiber types (fast/slow twitch). This additional information can help to describe the muscle recruitment in a more in-depth manner. The analysis of the frequency content was performed previously in alpine skiing by calculating the median power frequency (Kroll 2005). This method, however, resulted in the loss of timing information, which is essential in alpine skiing research. The aim of the current project was to perform a time-frequency analysis of the EMG intensities during alpine skiing using wavelet transformation. Our primarily interests were to determine functional differences in recruitment and timing within the quadriceps muscle and the influence of geographic variations over a group of skiers. METHOD: Ten healthy female subjects participated in this study, and were selected by their skiing ability and amount of skiing days per year. The skiers performed two runs through a standardized corridor on a slope with three different sections of inclination (middle; steep; flat). Knee angle (Goniometer) and EMG of m. vastus lateralis (VL) & m. rectus femoris (RF) of the right leg was measured during the runs. The Signals were recorded at 2000Hz with a mobile EMG measurement device (Biovision™) carried in a backpack. The knee angle and raw EMG were used to determine the start and end of each double turn [1st turn right leg = inside leg (IL); 2nd turn right leg= outside leg (OL)]. The recorded EMG Signal was resolved with a wavelet transformation into intensities for each point in time calculated for a set of 10 center frequencies between 19.3 and 395.4 Hz (von Tschamer 2000). For data visualization and statistical analysis the intensity spectra were calculated for the first and second half and also for ten evenly spaced Windows of each double turn. Descriptive analysis was completed using 3D intensity plots for the overall group and individual skiers. Statistical analysis was done for mean intensity and mean frequency on both muscles using a 2(IL / OL) x 3(Inclination) MANOVA for repeated measures. RESULTS and DISCUSSION: Differences in timing, intensity and frequency content of RF and VL can be observed in the 3D plots. Regarding the timing, a more pronounced inactivity phase can be observed during edge change in RF than VL. In RF significantly higher frequencies and similar intensities were detected on IL compared to OL. Otherwise the frequencies and intensities of VL were significant lower on IL compared to OL. The influence of the inclination was observable in the intensities of both muscles (high in steep, low in flat) but not in the frequencies. The principal differences of RF and VL between IL and ÖL seem to be due to the functional and structural differences of the two-pint RF and the one-joint VL. For further considerations it could make sense to focus on the high activity of RF on both turn sides and how this is affected by factors such as fatigue. In conclusion the used method can be a powerful tool in describing muscular activity for alpine skiing in a more complex way.
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