Introduction<br />Inverse dynamics joint kinetics are often used to infer contributions<br />from underlying groups of muscle-tendon units (MTUs). However, such<br />interpretations are confounded by multiarticular (multi-joint)<br />musculature, which can cause inverse dynamics to over-or under-estimate<br />net MTU power. Misestimation of MTU power could lead to incorrect<br />scientific conclusions, or to empirical estimates that misguide<br />musculoskeletal simulations, assistive device designs, or clinical<br />interventions. The objective of this study was to investigate the degree<br />to which ankle joint power overestimates net plantarflexor MTU power<br />during the Push-off phase of walking, due to the behavior of the flexor<br />digitorum and hallucis longus (FDHL)-multiarticular MTUs crossing the<br />ankle and metatarsophalangeal (toe) joints.<br />Methods<br />We performed a gait analysis study on six healthy participants,<br />recording ground reaction forces, kinematics, and electromyography<br />(EMG). Empirical data were input into an EMG-driven musculoskeletal<br />model to estimate ankle power. This model enabled us to parse<br />contributions from mono-and multi-articular MTUs, and required only one<br />scaling and one time delay factor for each subject and speed, which were<br />solved for based on empirical data. Net plantarflexing MTU power was<br />computed by the model and quantitatively compared to inverse dynamics<br />ankle power.<br />Results<br />The EMG-driven model was able to reproduce inverse dynamics ankle power<br />across a range of gait speeds (R-2 >= 0.97), while also providing<br />MTU-specific power estimates. We found that FDHL dynamics caused ankle<br />power to slightly overestimate net plantarflexor MTU power, but only by<br />similar to 2-7%.<br />Conclusions<br />During Push-off, FDHL MTU dynamics do not substantially confound the<br />inference of net plantarflexor MTU power from inverse dynamics ankle<br />power. However, other methodological limitations may cause inverse<br />dynamics to overestimate net MTU power; for instance, due to rigid-body<br />foot assumptions. Moving forward, the EMG-driven modeling approach<br />presented could be applied to understand other tasks or larger<br />multiarticular MTUs.
Honert, E. C., & Zelik, K. E. (2016). Inferring muscle-tendon unit power from ankle joint power during the push-off phase of human walking: Insights from a multiarticular EMG-Driven model. PLoS ONE, 11(10). https://doi.org/10.1371/journal.pone.0163169