Coordination of human grasp and manipulation forces

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PROJECT SUMMARY The hand’s sophisticated neuromuscular system enables us to interact with the world in a myriad of ways, one of which is dexterous manipulation. The hand’s incredible versatility, however, comes at a cost: when peripheral neuropathies, neurological disorders or traumatic injury occur, our ability to improve sensorimotor function is severely challenged by our limited understanding of the hand’s sensorimotor control mechanisms. Therefore, improving our understanding of these mechanisms could enhance the impact of clinical intervention. This notion has driven extensive research over the past four decades aiming at unravelling how the hand’s sensorimotor control mechanisms operate. Previous research has provided significant insights into the coordination of digit forces required to prevent object slip. Surprisingly, however, this previous work neglected to address another important component of manipulation: the ability to control object position and orientation, i.e., pose. This is a critical gap because dexterous manipulation often requires both object slip prevention and dexterous control of object pose. To address this gap, in the past decade our laboratory has developed an experimental paradigm that allows subjects to choose where to grasp the object and contains a dexterity component, i.e., lifting an object straight while preventing it from tilting. Nevertheless, the major limitation of this work is that the modulation of load and grip forces cannot be decoupled to identify their relative contribution to object slip prevention and pose control. To fill this gap, we propose an approach that will allow us, for the first time, to identify the control mechanisms underlying grasp and manipulation. Our approach combines a task with a dexterity component, surface electromyography of digit muscles, and a novel application of digit force analysis developed for robotic manipulation. Our task requires subjects to coordinate digit forces to simultaneously prevent object slip and minimize object tilt. Our analysis can mathematically decouple digit forces into grasp force (object slip prevention) and manipulation force (object pose control). We will pursue two aims: (1) To determine the causal relation between grasp and manipulation forces, and (2) To quantify the effect of predictability of object properties on the coordination between grasp and manipulation forces. If successful, this new knowledge will enable clinicians to extract information about the state of the hand’s sensorimotor function that cannot otherwise be extracted by clinical assessment tools. Our long-term objective is to improve the theoretical constructs of dexterous manipulation control and their translation to clinical applications, including tools for quantifying recovery of hand function following clinical intervention, as well as design of assistive and/or rehabilitation devices.