Speed-dependent variation in the Piper rhythm

Rhythmic fluctuations (Piper rhythm) have been resolved in surface electromyography (sEMG) recordings. For a long time this periodic signal could only be detected in isometric contractions, where the innervation time is much longer than the oscillation period. However, recently a periodic signal could also be resolved in cyclic movements like running, using non-linear scaled wavelets. The purpose of this study was to characterise the rhythm for different running velocities. A rhythmic signal (the Piper rhythm) was found in the EMG signal of both bellies of the gastrocnemius muscle with a frequency range of 20-35 Hz. A decrease of the Piper frequency could be observed for all of the 13 subjects when increasing the running speed from 1.3 to 4.9 m/s. The time and frequency analysis indicated that the origin of the periodic signal is from the activation of the muscle and is not likely resulting from muscle vibrations. No correlation between the medial and the lateral part of the muscle bellies could be found indicating that the two muscle compartments are fine tuned by two distinct processes. According to the literature, the signal might be the manifestation of a cortical rhythm, which has been shown for isokinetic movements. (C) 2013 Elsevier Ltd. All rights reserved.