Precision Grip Control while Walking Down a Stair Step

Abstract

The aim of this study was to determine whether the internal model regulating grip force (GF)/load force (LF) coordination during a brisk load increase is preserved when the lower extremities produce a perturbation during a single step-down task. We observed the coordination of the vertical ground reaction force (vGRF), GF and LF while holding a handheld object during a single step-down task. The 3 forces (vGRF, GF and LF) decreased during the start of the task. While the subject was descending, LF and GF became dissociated from vGRF and increased in value, probably to anticipate the first foot contact. Coordination of LF and GF was maintained until the maximal vGRF (knee extension). LF peaked in the same time window as vGRF, whereas GF peaked about 70 ms later. This desynchronization, which was previously observed in direct load increase on a handheld object, was interpreted to be a predictive action to ensure the smooth management of the brisk increase in load induced by the lower extremities. Incidentally, in this group, kinematic and dynamic differences were observed between men and women, which may highlight a gender-specific strategy to perform the step-down task. In conclusion, these results suggest that the internal model of precision grip is able to integrate a brisk load change, whatever its origin, and regulate the forces to provide an ideal GF to dampen a brisk load increase and secure the object.

Precision grip control while walking down a step in children with unilateral cerebral palsy

“…In this study, the TD children presented an early coupling of the GF and LF while walking down the step. Similarly to adults [ 25 ], during the initial downward movement of the body from the step, children first showed a parallel decrease in the forces to their minima. Subsequently, an increase in the LF was followed shortly by an increase in the GF, which corresponded to an upward movement of the arm [ 25 ].…”

“…Similarly to adults [ 25 ], during the initial downward movement of the body from the step, children first showed a parallel decrease in the forces to their minima. Subsequently, an increase in the LF was followed shortly by an increase in the GF, which corresponded to an upward movement of the arm [ 25 ]. This early coupling was present but displayed a shift in the more affected hand of children with UCP; the GF increase was observed before the LF increase.…”

“…The instruments included a wooden step (16.5×61×30 cm), and a grip-lift Manipulandum [ 25 ] (Arsalis) with a mass of 220 g and width of 30 mm ( Fig 1 ). The Manipulandum was used to measure the grip force (GF, perpendicular to the object in hand) and the load force (LF, tangential to the object in hand).…”

“…In children with UCP they observed a difference between hands, suggesting the presence of impairments in the coordination of the forces associated with the grasp and the locomotion in the more affected hand. In a discrete task of walking down a step, healthy adults are able to adapt their precision grip to a brisk load change induced by the lower extremities by regulating the forces providing an ideal GF to dampen the LF increase and secure the object in hand [ 25 ]. It is unknown whether such adaptation of fingertip forces is present in the more or less affected hand of children with UCP while a brisk load increase is induced on a handheld object by walking down a step.…”

Smart switching in feedforward control of grip force during manipulation of elastic objects

“…However, in many natural object manipulation tasks, the central nervous system must also adjust grip forces to deal with impulse-like destabilizing forces induced by the nearly instantaneous contact between an object and a hard surface. Several studies also addressed the control of grip force in impactlike tasks: When participants had to anticipate a sudden increase of weight after dropping a ball in a hand-held receptacle (Johansson and Westling, 1988; Bleyenheuft et al, 2009), when opening a drawer to its mechanical stop (Serrien et al, 1999), when hitting an object against a pendulum (Turrell et al, 1999) or a surface (White et al, 2011, 2012) or in a step-down task (Ebner-karestinos et al, 2016). A common observation was the occurrence of a maximum of grip force approximately 60 ms after peak load force that signed the impact.…”