Submitted on October 3, 2007
Accepted on February 17, 2008
Interactions with Compliant Loads Alter Stretch Reflex Gains but not Intermuscular Coordination
Eric Jon Perreault1*, Kuifu Chen2, Randy D. Trumbower3, and Gwyn N Lewis4
1 Biomedical Engineering, Northwestern University, Chicago, Illinois, United States
2 Mechanics, China Agricultural University, Beijing, China
3 Sensory Motor Performance Program, Rehabilitation Institute of Technology, Chicago, Illinois, United States
4 Health and Rehabilitation Research Center, AUT University, Aukland, New Zealand
* To whom correspondence should be addressed. E-mail: e-perreault{at}northwestern.edu.
The human motor system regulates arm mechanics to produce stable postures during interactions with different physical environments. This occurs partly via involuntary mechanisms, including stretch reflexes. Previous single joint studies demonstrated enhanced reflex sensitivity during interactions with compliant environments, suggesting reflex gain increases to enhance limb stability when that stability is not provided by the environment. This study examined if similar changes in reflex gain are present throughout the limb following perturbations that influence multiple joints simultaneously. Furthermore, we investigated whether any observed modulation was accompanied by task-specific changes in reflex coordination across muscles, a question that cannot be addressed using single joint perturbations. Reflexes were elicited during the maintenance of posture by perturbing the arm with a 3 degrees of freedom robot, configured to have isotropic stiffness of either 10 N/m (compliant) or 10kN/m (stiff). Perturbation characteristics were matched in both environments. Reflex magnitude was quantified by the average rectified electromyogram, recorded from 8 muscles crossing the elbow and shoulder. Reflex coordination was assessed using independent components analysis to compare reflex activation patterns during interactions with stiff and compliant environments. Stretch reflex sensitivity increased significantly in all muscles during interactions with the compliant environment, and these changes were not due to changes in background muscle activity. However, there was no significant difference in the reflex coordination patterns observed during interactions with the stiff and compliant environments. These results suggest that reflex modulation occurred through altered use of fixed muscle coordination patterns rather than through a change in reflex coordination.
