Variable and intermittent grip force control in response to differing load force dynamics.

A recent study (Grover et al. Exp Brain Res 236(10):2531-2544, 2018) found that the grip force applied to maintain grasp of a hand-held object exhibited intermittent coupling to the changing load forces exerted by the object as it was oscillated. In particular, the strength and consistency of grip force response to load force oscillations was tied to overall load force levels and the prominence of load force oscillations. This contrasts with previous reports of grip force-load force coupling as generally continuous and stable and, therefore, has implications for theoretical accounts of grip force control that are predicated on these prior understandings of the coupling. The finding of intermittency additionally raises questions about the consistency of the temporal relation (i.e., lead/lag) between grip force and load force over time. The objective of the current study was, therefore, to investigate how the time-varying pattern (i.e., the regularity vs. complexity) of load force variations contribute to shifts between more intermittent and more continuous grip force control, and to determine the temporal consistency of the coupling. It was found that grip force became more tightly and continuously responsive to load force as load force changes became less predictable. Additionally, we report strong evidence that the temporal (i.e., lead/lag) relation between grip force and load force and the strength of their coupling vary substantially over time.