Spatial and temporal changes of upper trapezius muscle fiber conduction velocity are not predicted by surface EMG spectral analysis during a dynamic upper limb task.
The purpose of the study was to examine the temporal and spatial correlation between estimates of trapezius muscle fiber conduction velocity (CV) and surface EMG instantaneous mean power spectral frequency (iMPF) during dynamic movement of the upper limb. Surface EMG signals were detected from the upper division of the trapezius muscle in 13 healthy volunteers using linear arrays of eight electrodes at three locations in the cephalad-caudal direction. Subjects were asked to tap with their hands in a cyclic manner between targets positioned mid thigh and 120 degrees of shoulder flexion, to the beat of a metronome set at 88 beats per minute for 5 min. Muscle fiber CV and iMPF were estimated for each cycle at the time instant corresponding to 90 degrees of shoulder flexion. Non-significant correlations were identified between CV and iMPF initial values (R(2)=0.03-0.01), rate of change over time (R(2)=0.10-0.004) and normalized rate of change (R(2)=0.12-0.01) at all three locations on the upper trapezius muscle. These results demonstrate that both spatial and temporal variations in trapezius muscle fiber CV are not predicted by EMG spectral analysis during dynamic movement of the upper limb. This finding suggests that spectral analysis cannot be used to infer changes in the spatial and temporal behavior of muscle fiber CV during dynamic tasks.