Recent studies have described muscle synergies as overlapping, multimuscle groups defined by synchronous covariation in activation intensity. A different approach regards a synergy as a fixed temporal sequence of bursts of activity across groups of motoneurons. To pursue this latter definition, the present study used a principal component (PC) analysis tailored to reveal the across-muscle temporal synergies of human hand movement. Electromyographic (EMG) activity was recorded as subjects used a manual alphabet to spell a list of words. The analysis was applied to the EMG waveforms from 27 letter-to-letter transitions of equal duration. The first PC (of 27) represented the main temporal synergy; after practice, it began to account for more of the EMG variance (up to 40%). This main synergy began with a burst in the 4-finger extensor and a silent period in the flexors. There were then progressively later and shorter bursts in the thumb abductor, thumb flexor, little finger abductor, and finally the finger flexors. The results suggest that hand movements may be generated by activity waves unfolding in time. Because finger muscles are under relatively direct cortical control, this suggests a specific form of cortical pattern generation.
Bibliographical noteFunding Information:
This work was supported by National Institutes of Health R01 NS027484. The authors thank Professor John F. Soechting for his many helpful suggestions. We also thank Philip Barbosa for assisting with our cadaver study and an anonymous reviewer for suggesting the sine wave analysis shown in Figure 9. Conflict of Interest: None declared.
Funding to pay the Open Access publication charges for this article was provided by the National Institute of Neurological Disorders and Stroke.
- Muscle synergy
- Temporal synergy