Intention to be force efficient improves high-level anticipatory coordination of finger positions and forces in young and elderly adults

Successful object manipulation requires anticipatory high-level control of finger positions and forces to prevent object slip and tilt. Unlike young adults, who efficiently scale grip forces (GFs) according to surface conditions, old adults were reported to exert excessive grip forces. In this study, we theoretically show how grip force economy depends on the modulation of the centers of pressure on opposing grip surfaces (DCoP) according to object properties. In a grasp-to-lift study with young and elderly participants, we investigated how the instruction to lift the object with efficient GF influences the anticipation of torques, DCoP and GF control during complex variations of mass distributions and surface properties. Provision of the explicit instruction to strive for force efficiency prompted both age groups to optimize their DCoP modulation, although to a lesser degree in the elderly, and also led to a refinement of torque anticipation for a right-sided weight distribution in the young, but not the elderly participants. Consequently, marked drops in GF levels resulted. Furthermore, participants enhanced DCoP modulation and lowered GF safety ratios in challenging surface conditions. Higher GF in the elderly was due to decreased skin-surface friction but also worse DCoP modulation for lateralized mass distributions when trying to be force efficient. In contrast, safety margins were not elevated in the elderly, suggesting preserved GF control. Our findings demonstrate how task goals influence high-level motor control of object manipulation differentially in young and elderly participants and highlight the necessity to control for both instructions and friction when investigating GF control. NEW & NOTEWORTHY Previous studies have shown that forces are covaried as a function of centers of pressure (CoPs) to exert adequate torques. Here, we demonstrate that force-efficient object manipulation requires the modulation of CoPs and show that providing the instruction to be force efficient and challenging surface conditions elicits a GF safety ratio reduction as well as an optimization of anticipatory CoP modulation and torques in the young and, to a lesser degree, in the elderly.