Grip responses to object load perturbations are stimulus and phase sensitive.

Responses to load changes of a held object that challenge grasp stability are known to be adept and fast, but the responses to changes in load where grasp stability is not challenged are not well understood. In order to compare responses to these functionally opposite perturbations, the grasp response to increases and decreases in the load of a held object was examined. A pulling force used to create object load was abruptly altered so that it felt lighter (decreased load) or felt heavier (increased load). The perturbation occurred either during movement of the object (lift) or when the object was held steady (hold). Grip force modulation was earlier, larger, had a faster maximum rate and a smaller change in relative safety margin when load increased. Also, the grip force modulation was earlier, larger, had a faster maximum grip force rate and a smaller change in relative safety margin when the perturbation occurred during active lift. In the decreased loading condition, participants were not required to make a grip force adjustment to maintain grip. Interestingly, participants chose to make the adjustment (decreasing grasp force), albeit more slowly. During the lift phase, the nature of the task is more dynamic and the resulting additional mechanical stimulation may have lead to a facilitated response. The results point to the greater functional significance of increasing load for grip force modulation and the potential for greater sensory or motor facilitation during dynamic lifting.