Fatigue-related decrease in Piper rhythm frequency of the abductor pollicis brevis muscle during isometric contractions

The purpose of this study was to analyze how the frequency of the Piper rhythm of the abductor pollicis brevis muscle (APB) and thus of the rhythmic synchronization of motor units changes with fatigue. Fourteen subjects participated in the study. The EMG signals were measured during maximum voluntary contractions, and a mimicked motor unit action potential was used to simulate an EMG signal containing no rhythmicity. The simulated EMG was used as a reference. The Piper rhythm was extracted from the high frequency power (170–271 Hz) of the wavelet transformed real and simulated EMG data using the difference of the autocorrelation functions of the power. The study shows that the Piper rhythm of the APB muscle, its pacing frequency and pacing amplitude can be extracted from the EMG signal recorded during a fatiguing task. One can conclude that the pacing frequencies observed in various hands covered the whole frequency range of the Piper band which includes the beta and the gamma band frequencies observed in brain activity (17–60 Hz). While the pacing frequency decreased with fatigue the pacing amplitude did not change significantly. The Piper rhythm is a result of a changing central drive and its measurement thus allows observing changes of central drive to the muscle. The ability to better resolve the Piper rhythm in the EMG without using the coherence with the brain activity opens the possibility to study the behavior of central control in the peripheral signal.