Abstract
The anterior (ADELT) and posterior (PDELT) deltoid muscles present a stereotypical activation pattern during upright walking and primarily function to dampen arm swing. If this damping activity is based on online sensory feedback, applying a force perturbation should lead to immediate adaptations that do not persist when the force is removed. If the muscle activity is preprogrammed based on past experiences of arm mechanics, we would expect a gradual adaptation in response to a force field, and persistent activity changes once the force is removed. The current study investigated the responses of the deltoid muscles to a posterior elastic perturbation at the right wrist during treadmill walking in order to explore the motor control mechanisms governing arm swing. Thirteen healthy young adults walked on a treadmill for a baseline walking condition for 2 minutes, followed by 10 minutes of perturbed walking adaptation, finishing with 5 minutes of unperturbed walking readaptation. Previous results indicated that range of motion of the perturbed arm did not change during the adaptation period. During the perturbation, ADELT activation acted to accelerate the arm forwards against the elastic force, while the PDELT activation decreased when the arm was decelerating forwards. Moreover, increased activation was present once the perturbation was removed to accelerate the arm backwards. In conclusion, the decreased activity in the PDELT during the perturbation supports the deceleration function of the deltoids during locomotion, and the increased ADELT activity following removal of the perturbation suggests a feedforward control mechanism to adapt to the elastic force.
