Relationship between electrical and vibratory output of muscle during voluntary contraction and fatigue

Measurements were done on the biceps muscles of 6 healthy volunteers to record simultaneously the surface electromyogram (EMG) and vibromyogram (VMG) by means of a piezoelectric device (accelerometer). The VMG is generated by mechanical waves due to the contraction mechanism and often measured as sound. The frequency spectrum and integrated value (IEMG and IVMG) of both signals were calculated. Both IEMG and IVMG showed a clear linear correlation with force, although at high forces, the variability of the VMG became rather high. Two series of experiments were performed to study the EMG and VMG changes in relation to changes induced by fatigue: (1) during constant force at 50% of the maximal voluntary contraction (MVC) and recovery; (2) for one 1 minute during declining force at MVC. The main finding was that the IVMG was related to the absolute force, irrespective of the fatigue state of the muscle. In contrast, the IEMG showed the well-known changes during fatigue, such as an increase during endurance. The spectral changes of the two measurements also showed a divergence. The spectra of the EMG shifted to lower frequencies in both fatigue protocols. In contrast, the spectra of the vibratory signal did not shift, except for several measurements at MVC. However, the shape did change to a somewhat flatter spectrum with less pronounced peaks. Possible explanations for this different behavior are discussed. It is concluded that the vibratory energy generated by the contraction mechanism is linearly related to force. Changes induced by fatigue do not alter this relationship: this contrasts with the behavior of the EMG.(ABSTRACT TRUNCATED AT 250 WORDS)