The multibody nature of the musculoskeletal system makes each applied force potentially accelerate all body segments. Hence, muscles` actions on the kinematics of crossed and non-crossed joints should be estimated based on multibody dynamics. The objective of this study was to systematically investigate the actions of main lower limb muscles on the sagittal-plane angular kinematics of the hip, knee, and ankle joints, during upright standing and gait. Subject-specific simulations were performed to compute the muscle-tendon forces based on three-dimensional kinematic data collected from 10 able-bodied subjects during walking at preferred speed and during relaxed standing posture. A subject-scaled model consisting of the lower limb segments, 19 degrees of freedom and 92 Hill-type muscle-tendon units was used. Muscle-induced joint angular accelerations were estimated by Induced Acceleration Analysis in OpenSim. A comprehensive description of the estimated joint accelerations induced by lower limb muscles was presented, for upright standing and for the whole gait cycle. The observed muscle actions on crossed and non-crossed joints were phase- and task-specific. The main flexors and extensors for each joint were reported. Particular biarticular muscles presented actions opposite to their anatomical classification for specific joints. Antagonist muscle actions were revealed, such as the hitherto unknown opposite actions of the soleus and gastrocnemius at the ankle, and of the iliopsoas and soleus at the knee and ankle, during upright standing. Agonist actions among remote muscles were also identified. The presented muscle actions and their roles in joint kinematics of bipedal standing and walking contribute to understanding task-specific coordination.
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