The direction of the endpoint motion producing maximum response for each muscle was calculated from the linearized state equations for the model. The direction of the endpoint motion producing maximum response for each muscle was calculated from the linearized state equations for the model. The mean response directions across all 10,000 activation patterns is shown in the transverse (A) and sagittal (B) planes. Abbreviations in A and B are in Table 1. The hemisphere of the response direction is indicated by the symbol (circle, downward; square, upward; solid, rightward; outline, leftward). The angular standard deviation of each muscle for the 10,000 patterns is represented by circles around each muscle. Mean pulling direction for individual muscle groups in the transverse plane (C) and in the sagittal plane (D) are concentrated at ±90 deg. HA, hip adductors; HE, hip extensors; HF, hip flexors; IPS, iliopsoas; KE, knee extensors; DF, ankle dorsiflexors; PF, ankle plantarflexors; AE, ankle everters; AI, ankle inverters. Nathan E. Bunderson et al. J Exp Biol 2010;213:2131-2141 © 2010.