Use of Self-Selected Postures to Regulate Multi-Joint Stiffness During Unconstrained Tasks

Abstract

BackgroundThe human motor system is highly redundant, having more kinematic degrees of freedom than necessary to complete a given task. Understanding how kinematic redundancies are utilized in different tasks remains a fundamental question in motor control. One possibility is that they can be used to tune the mechanical properties of a limb to the specific requirements of a task. For example, many tasks such as tool usage compromise arm stability along specific directions. These tasks only can be completed if the nervous system adapts the mechanical properties of the arm such that the arm, coupled to the tool, remains stable. The purpose of this study was to determine if posture selection is a critical component of endpoint stiffness regulation during unconstrained tasks.Methodology/Principal FindingsThree-dimensional (3D) estimates of endpoint stiffness were used to quantify limb mechanics. Most previous studies examining endpoint stiffness adaptation were completed in 2D using constrained postures to maintain a non-redundant mapping between joint angles and hand location. Our hypothesis was that during unconstrained conditions, subjects would select arm postures that matched endpoint stiffness to the functional requirements of the task. The hypothesis was tested during endpoint tracking tasks in which subjects interacted with unstable haptic environments, simulated using a 3D robotic manipulator. We found that arm posture had a significant effect on endpoint tracking accuracy and that subjects selected postures that improved tracking performance. For environments in which arm posture had a large effect on tracking accuracy, the self-selected postures oriented the direction of maximal endpoint stiffness towards the direction of the unstable haptic environment.Conclusions/SignificanceThese results demonstrate how changes in arm posture can have a dramatic effect on task performance and suggest that postural selection is a fundamental mechanism by which kinematic redundancies can be exploited to regulate arm stiffness in unconstrained tasks.

Biomechanical constraints on the feedforward regulation of endpoint stiffness

“…Our results demonstrate that arm posture has a dramatic effect on the ability to regulate endpoint stiffness orientation. These findings help to explain the influence of arm posture on the ability to control unstable loads (Milner 2002) and the discovery that subjects self-select postures that match the orientation of maximal arm stiffness to the stability requirements of the task being performed (Trumbower et al 2009). The influence of posture on endpoint stiffness has been documented in numerous experiments, including passive (Mussa-Ivaldi et al 1985) and active (Gomi and Osu 1998;Perreault et al 2001) conditions.…”

“…Although the central nervous system (CNS) is responsible for the regulation of arm mechanics, we do not yet have a complete understanding of the limits on this regulation and the mechanisms by which it occurs. During unconstrained tasks, posture selection plays a critical role in the regulation of limb mechanics (Milner 2002;Mussa-Ivaldi et al 1985;Trumbower et al 2009). When posture is constrained, however, the nervous system can regulate mechanics only by changing muscle activation through feedforward mechanisms such as cocontraction or feedback mechanisms including the many types of stretch-evoked reflexes Gomi and Osu 1998;Krutky et al 2010;Perreault et al 2002).…”

Posture and hand load alter muscular response to sudden elbow perturbations

“…The initial arm postures in our study did not affect forearm muscle length, so the changes in activity were partly attributed to another mechanism. Trumbower et al (2009) investigated the effects of arm posture on endpoint stiffness using a 3D robotic manipulator and found that arm posture influenced task performance and is a fundamental mechanism to regulate arm stiffness. Therefore, it is likely that our changes in body orientation influenced the participants' ability to compensate for load type.…”

Anisotropy in the Haptic Perception of Force Direction and Magnitude

“…To provide a natural posture for task execution, the arm of our subjects was not positioned in the horizontal plane. One other study investigating arm stiffness outside of the horizontal plane used a posture with the elbow below the shoulder and hand [35]. The shoulder and hand were both positioned in the horizontal plane.…”