engleski

The potential role of muscle synergies and skin sensory receptors in force coordination in static manipulation

Object manipulation is one of the most important motor activities and its control has been object of study for many years. For a proper object manipulation the grip force (GF ; normal component of force acting at the finger- object contact area) should be applied by the digits against the object?s surface in order to prevent the slippage. A number of studies have shown a close coordination between GF and the external forces, also referred to as load force (LF ; tangential component which tends to cause slippage), in several dynamic and static manipulation tasks. However, our previous studies have shown that a high coordination of GF and LF in static manipulation tasks becomes disrupted when LF changes direction, regardless of the difference in the magnitude of LF exerted in either of the directions. We hypothesized that this phenomenon could have either a muscle control (i.e. switching synergies between GF and two antagonistic LF muscle groups) or a sensory activity origin (i.e. changes in the pattern of the afferent firing of the skin receptors associated with the changes in LF direction). Therefore, the aim of our study was to discern possible roles of muscle synergies and activity of skin receptors in this phenomenon. Fourteen participants exerted a sinusoidal LF against an externally fixed handle in trials that gradually changed from unidirectional (LF exerted only in one direction) to fully bidirectional (consecutive LF peaks equal in both directions). Three forearm supports (i.e., no support, single support, and double support, where the single support counteracted the arm weight) were also applied. This affected the action of arm muscles exerting LF, but not the force applied against handles. As expected, the results revealed an abrupt decrease in all indices of GF-LF coordination when the task switched from uni- to bidirectional. However, the change in external support demonstrated no effect on the studied force coordination. Since the external support alters muscle synergies, but not the forces acting at the finger-handle contact area, we concluded that the switching in sensory afferent activity rather than switching of muscle synergies is likely to play a role in the studied phenomenon. Further research is needed to reveal whether our findings regarding the distinction between the force coordination in uni- and bidirectional tasks could be extended to manipulation tasks in general.

sensory input ; grip force ; grasping

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