LAUSR

The grip force applied to maintain grasp of a hand-held object has been typically reported as tightly coupled to the load force exerted by the object as it is actively manipulated, occurring proportionally and consistently in-phase with changes in load force. However, continuous grip force-load force coupling breaks down and grip force is instead only intermittently coupled to load force when overall load force levels and oscillation amplitudes are lower (Grover, Lamb, Bonnette, Silva, Lorenz, & Riley, 2018) or more predictable (Grover, Nalepka, Silva, Lorenz, & Riley, 2019). The current study investigated the nature of the transition between continuous and intermittent modes of grip force control by scaling load force oscillation amplitude continuously in time via scaling the required frequency of movement oscillations. Participants grasped a cylindrical object between the thumb and forefinger and oscillated their arm about the shoulder in the sagittal plane in time with a metronome that scaled through an ascending or descending frequency progression. Faster frequencies produced greater overall load force levels and more prominent load oscillations. We observed smooth but nonlinear transitions between clear regimes of intermittent and continuous grip force-load force coordination, for both scaling directions, indicating that grip force control can flexibly reorganize as parameters affecting grasp (e.g., variations in load force) change over time.