Small Lateral Displacement of the Muscle Represents Low-Frequency Force Fluctuations During Voluntary Contractions in Humans

Abstract

Small lateral vibration of the muscle surface, termed as muscle sound or mechanomyogram, has been recorded with accelerometers or displacement sensors. The amplitude of this vibration depends on the development of fusion of motor units during electrically stimulated contractions in humans (Yoshitake et al. J. Appl. Physiol. 93:1744–1752,2002). PURPOSE: To examine if there is a temporal correlation between force fluctuations and small lateral displacement of the muscle surface during voluntary contractions of a muscle in humans. METHODS: Seven healthy young adults (20–33 yrs) performed steady isometric contractions with the first dorsal interosseus muscle. Target force ranged from 2.5 to 80% of the maximal voluntary contraction force. Small lateral displacement of the muscle surface was measured with a high-resolution laser displacement sensor (LK-G155, Keyence, Japan; 0.4 urn resolution). Abduction force of the index finger and small lateral displacement of the muscle surface were recorded simultaneously. The standard deviations (SD) of force and muscle displacement were calculated after the linear trend was removed. To examine the extent of temporal correlation between force and muscle displacement, cross-correlation function (CCF) between these signals was calculated. RESULTS: The major frequency range in signals was <5 Hz for both force and muscle displacement across target forces. The SD of force increased with target force from 47.0 ± 5.7mN to 606.6 ± 77.1 mN. Similarly, the SD of muscle displacement increased with target force from 21.3 ± 2.1 um to 57.7 ± 9.3 urn. A significant linear correlation was found between the SD of force and the SD of muscle displacement (r = 0.60, P <0.0001). There was a significant temporal correlation between the force and muscle displacement as shown by a positive peak in the CCF at all target forces (P <0.01). The peak value of the CCF and its time lag ranged from 0.49 to 0.59 and from −0.60 to −0.34 ms, respectively. These values were not influenced by target force (P > 0.05). CONCLUSION: Temporal characteristics of low-frequency force fluctuations in a muscle were represented by small lateral displacement of the muscle surface across a wide range of contraction intensity.