Title: the Mechanical Actions of Muscles Predict the Direction of Muscle Activation During

35 Humans and cats respond to balance challenges, delivered via horizontal support surface 36 perturbations, with directionally selective muscle recruitment and constrained ground reaction 37 forces. It has been suggested that this postural strategy arises from an interaction of limb 38 biomechanics and proprioceptive networks in the spinal cord. A critical experimental validation 39 of this hypothesis is to test the prediction that the principal directions of muscular activation 40 oppose the directions responding muscles exert their forces on the environment. Therefore, our 41 objective was to quantify the endpoint forces of a diverse set of cat hindlimb muscles and 42 compare them to the directionally sensitive muscle activation patterns generated in the intact 43 and decerebrate cat. We hypothesized that muscles are activated based upon their mechanical 44 advantage. Our primary expectation was that the principal direction of muscle activation during 45 postural perturbations will be directed oppositely (180 degrees) from the muscle endpoint 46 ground reaction force. We found that muscle activation during postural perturbations was indeed 47 directed oppositely to that muscle’s endpoint reaction forces. These observations indicate that 48 muscle recruitment during balance challenges is driven, at least in part, by limb architecture. 49 This suggests that sensory sources that provide feedback about the mechanical environment of 50 the limb are likely important to appropriate and effective responses during balance challenges. 51 Finally, we extended the analysis to three dimensions and different stance widths, laying the 52 groundwork for a more comprehensive study of postural regulation than was possible with 53 measurements confined to the horizontal plane and a single stance configuration. 54 55