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The aim of this study was to investigate whether the median frequencies (MF) of the electromyogram (EMG) and the integrated EMG (IEMG) of histochemically differentiated ankle plantarflexors, the gastrocnemius and soleus, were force dependent. Bipolar intramuscular wire electrodes were used to measure EMG of the soleus (SO), medial head of gastrocnemius (GM), and lateral head of gastrocnemius (GL) during ramp (single ongoing contractions) with the force increasing linearly from 0 to 100% of maximum voluntary contraction (MVC) and stepwise (steady force levels) ankle plantarflexion at 10, 20, 30, 40, 60, and 80% MVC. EMG and force were measured simultaneously. Power spectral analysis of these signals was performed to calculate MF on 1024-point by fast Fourier transform (FFT) technique. IEMG value of each muscle was also obtained at the same levels of force. While IEMG of three heads of triceps surae in both stepwise and ramp contractions increased significantly with increasing force, MF values of GL during stepwise contraction increased significantly (20, 40, 60, 80% MVC). These results suggest that the sensitivity of EMG power spectrum might be influenced by the proportion of fast twitch muscle fibers, which histochemically corresponds to type II fibers.

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