All forces applied to the human hand are transmitted through the arm to the trunk. Hence, the arm represents an important mechanical interface between the trunk and the environment. By regulating this interface in a task-appropriate manner, humans are able to interact with a wide range of tools and objects, precisely controlling posture and force in the face of unexpected disturbances. The success of these interactions is largely determined by the ability to control the mechanical properties of the arm relative to those of the environment. The goal of this thesis was to examine how interactions with different mechanical environments influence the regulation of arm mechanics and the mechanisms responsible for that regulation. Both neural and biomechanical factors were considered. During the present studies, subjects attempted to maintain a stable arm posture while interacting with a 3DOF (three degrees of freedom) robotic manipulator, programmed to compromise arm stability in different directions. During these interactions we evaluated, first, how subjects self-select postures to orient arm mechanics relative to the mechanics of the environment, second, how the excitability of spinal reflex pathways is adapted at a constrained posture, and, finally, the extent to which limb mechanics can be adapted at a constrained posture. We also present methodological considerations relating to two techniques used to evaluate arm mechanics. We propose that when the arm is unconstrained, shifts in posture are used to match limb mechanics to the functional constraints of a task. At a fixed posture, limb mechanics can be adapted to the environment, but to a limited extent. The adaptation that does occur appears related to the task-appropriate modulation of stretch-sensitive reflex responses, which exhibit changes in gain that are tuned to the orientation of limb mechanics relative to the environment. Stretch-sensitive reflexes are the earliest neural response to an unexpected perturbation. The observed reflex modulation may contribute to the involuntary stabilization of the arm before the execution of a more appropriate response, such as a postural shift. A number of mechanisms can adapt limb mechanics; we propose that each serves a unique role in the overall behaviors that ensure arm stability.

Last modified

• 09/20/2018

Creator

• Matthew Krutky

DOI

• https://doi.org/10.21985/N23F19

Subject

• Biomedical Engineering

Keyword

• Biomedical Engineering

Date created

• 2008-11-25

Resource type

• Dissertation

Rights statement

• In Copyright