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Why are Antagonist Muscles Co-activated in My Simulation? A Musculoskeletal Model for Analysing Human Locomotor Tasks


Existing “off-the-shelf” musculoskeletal models are problematic when simulating movements that involve substantial hip and knee flexion, such as the upstroke of pedalling, because they tend to generate excessive passive fibre force. The goal of this study was to develop a refined musculoskeletal model capable of simulating pedalling and fast running, in addition to walking, which predicts the activation patterns of muscles better than existing models. Specifically, we tested whether the anomalous co-activation of antagonist muscles, commonly observed in simulations, could be resolved if the passive forces generated by the underlying model were diminished. We refined the OpenSim™ model published by Rajagopal et al. (IEEE Trans Biomed Eng 63:1–1, 2016) by increasing the model’s range of knee flexion, updating the paths of the knee muscles, and modifying the force-generating properties of eleven muscles. Simulations of pedalling, running and walking based on this model reproduced measured EMG activity better than simulations based on the existing model—even when both models tracked the same subject-specific kinematics. Improvements in the predicted activations were associated with decreases in the net passive moments; for example, the net passive knee moment during the upstroke of pedalling decreased from 36.9 N m (existing model) to 6.3 N m (refined model), resulting in a dramatic decrease in the co-activation of knee flexors. The refined model is available from and is suitable for analysing movements with up to 120° of hip flexion and 140° of knee flexion.

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We thank Taylor Dick, Sidney Morrison and Glen Lichtwark for their assistance in collecting and post-processing the experimental data used in this study, and we are grateful to Andy Biewener and Carolyn Eng for helpful discussions. Funding for this work was provided by the National Institutes of Health Grant 2R01AR055648.

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Corresponding author

Correspondence to Adrian K. M. Lai.

Additional information

Associate Editor Estefanía Peña oversaw the review of this article.

Electronic supplementary material

Below is the link to the electronic supplementary material. Supplementary Figure S1 Knee moment arms of MTUs as predicted by Rajagopal et al.’s model25 and by our intermediate model after updating the tibiofemoral translations and MTU paths. The MTUs include vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), rectus femoris (RF), semimembranosus (SM), semitendinosus (ST), biceps femoris long head (BFLH), biceps femoris short head (BFSH), medial gastrocnemius (MG), lateral gastrocnemius (LG), sartorius (SA) and gracilis (GR). Moment arms predicted by the models are compared with measured moment arms reported by Buford et al.4 from tendon excursion experiments. Note that the moment arms for Rajagopal et al.’s model are extrapolated for knee angles greater than 120° (lighter dotted lines). Supplementary Figure S2 Net total moments (dotted lines) and net passive and active moments (shaded regions) generated by MTUs crossing the hip, knee, and ankle as estimated from muscle-driven simulations of pedalling (left), walking (middle), and running (right). Net moments estimated using our refined model are compared to the moments estimated using our intermediate model.

Supplementary material 1 (PDF 113 kb)

Supplementary material 2 (PDF 89 kb)

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Lai, A.K.M., Arnold, A.S. & Wakeling, J.M. Why are Antagonist Muscles Co-activated in My Simulation? A Musculoskeletal Model for Analysing Human Locomotor Tasks. Ann Biomed Eng 45, 2762–2774 (2017).

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  • Musculoskeletal model
  • Hill-type muscle model
  • Simulation
  • Passive force
  • Running
  • Pedalling