Forces And Moments Generated By The Human Arm: Variability And Control

Open Access
Xu, Yang
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
October 17, 2014
Committee Members:
  • Vladimir M Zatsiorsky, Dissertation Advisor
  • Vladimir M Zatsiorsky, Committee Chair
  • Mark Latash, Committee Chair
  • Mark Latash, Committee Member
  • Stephen Jacob Piazza, Committee Member
  • Andris Freivalds, Committee Member
  • Biomechanics
  • Motor Control
  • Ergonomics
  • Force Production
  • Arm Control
  • Force Variability
To move and manipulate objects people exert forces and moments of force (further addressed as simply “moments”) on the environment. The five studies in this dissertation explored the accurate endpoint force vector production by the human arm in isometric conditions. In the first study, three common-sense hypotheses were proposed and falsified. The subjects exerted static forces on the handle in eight directions. The forces were of different magnitude levels. The torsion moment on the handle (grasp moment) was not specified in the instruction. The two force components and the grasp moment were recorded, and the shoulder, elbow, and wrist joint torques were computed. The following main facts were observed: (a) While the grasp moment was not prescribed by the instruction, it was always produced. The moment magnitude increased with force magnitude and moment direction changed with the instructed force direction. (b) The within-trial angular variability of the exerted force vector (angular precision) did not depend on the target force magnitude. (c) The time profiles of joint torques in the trials were always positively correlated, even for the force directions where flexion torque was produced at one joint and extension torque was produced at the other joint. (d) In contrast to the previously studied tasks, the analytical inverse optimization (ANIO) method failed to determine the optimization cost function. A hypothesis was formulated that this is a general property of the static tasks performed by the serial kinematic chains. In the second study, the preceding findings in the first study were confirmed in tasks where arm postures were systematically varied. Further, it was observed that the distribution pattern of endpoint force variability is dependent on the arm posture instead of the orientation of trunk. A following-up simulation was conducted to examine why the pattern of the joint torques distribution could not be explained by an optimization cost function additive with respect to the torques. The results suggested that the grasp capability might serve as the limit to a more optimal pattern of the joint torques distribution. In the third study, handle size was varied to test its effect on performance. Major results indicated: (a) There existed effects of handle size on the magnitude, but not on the distribution pattern of MVC end-point force. (b) Changing the handle diameters in the range between 4.5 and 9.0 cm does not affect the maximal torque production. (c) Although systematic change in the coefficients of ANIO was observed, the method still failed to reconstruct an optimal cost function additive with respect to joint torques with large handle size. In the fourth study, two handles with different surface friction were used to characterize its effects on grasp moment and further on the endpoint force. It was found that: (a) No significant differences in the MVC forces existed between the low- and high- surface friction handles; (b) A higher surface friction led to a higher grasp moment magnitudes; (c) Higher surface friction resulted in lower variability of force direction and higher enslaved moment at a specific direction. In the fifth study, grasp moments were prescribed or canceled out by using rotatable handle, and the results showed that: (a) Rotatable handle did not change the MVC forces significantly; (b) At the force level of 30% of MVC, using the rotatable handle didn’t lead to different variability of force magnitude and directions at all prescribed directions; (c) Exerting of the prescribed grasp moment reduced the MVC force production; and (d) Exerting of the prescribed grasp moment led to higher variability of both force magnitude and direction. In conclusion, this dissertation demonstrated several attractive mechanisms and properties of arm control in isometric condition. Discovered unintentional grasp moment production is important for explaining mechanisms and ergonomics of arm control.