Biomechanical constraints on the feedforward regulation of endpoint stiffness

Abstract

Although many daily tasks tend to destabilize arm posture, it is still possible to have stable interactions with the environment by regulating the multijoint mechanics of the arm in a task-appropriate manner. For postural tasks, this regulation involves the appropriate control of endpoint stiffness, which represents the stiffness of the arm at the hand. Although experimental studies have been used to evaluate endpoint stiffness control, including the orientation of maximal stiffness, the underlying neural strategies remain unknown. Specifically, the relative importance of feedforward and feedback mechanisms has yet to be determined due to the difficulty separately identifying the contributions of these mechanisms in human experiments. This study used a previously validated three-dimensional musculoskeletal model of the arm to quantify the degree to which the orientation of maximal endpoint stiffness could be changed using only steady-state muscle activations, used to represent feedforward motor commands. Our hypothesis was that the feedforward control of endpoint stiffness orientation would be significantly constrained by the biomechanical properties of the musculoskeletal system. Our results supported this hypothesis, demonstrating substantial biomechanical constraints on the ability to regulate endpoint stiffness throughout the workspace. The ability to regulate stiffness orientation was further constrained by additional task requirements, such as the need to support the arm against gravity or exert forces on the environment. Together, these results bound the degree to which slowly varying feedforward motor commands can be used to regulate the orientation of maximum arm stiffness and provide a context for better understanding conditions in which feedback control may be needed.

A multi-level approach to investigate the control of an input device: application to a realistic pointing task

“…Thus, forearm supination would require more complex neuromotor adjustments from the sensory and motor systems (Borges et al 2007). The results of the present work are in line with recent studies that suggested the influence of biomechanical constraints on muscle control of end-point position regarding the direction of movement (Hu, Murray, and Perreault 2012;Tee et al 2010). These last authors used a musculoskeletal model of the human arm for investigating the control of end-point stiffness (i.e.…”

Grip forces during fast point-to-point and continuous hand movements

“…This yielded an estimated equivalent stiffness K = 5384 N/m and an estimated equivalent viscosity C = 808 N s/m. Though approximate, stiffness estimates are well in keeping with those reported by Hu et al (2012) for maximally stiffened arms in the horizontal plane.…”

“…Finally, the U-grip condition is different from both B-grip conditions in that different muscles are involved in maintaining the required high grip baseline and in stabilizing the limb at one end-point. There is evidence, however, that the stiffening of the arm and that of the fingers are actually correlated (Friedman and Flash 2007;Hu et al 2012). One can then assume that the upsurge of arm agonist activity and the concomitant release of the antagonist activity marking the brisk onset of the movement is accompanied by a similar alteration of the force balance at the finger level.…”

Musculoskeletal simulation can help explain selective muscle degeneration in Duchenne muscular dystrophy

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