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The target article presents a framework for coordination of one- and two-joint muscles in a variety of tasks. Static optimization analyses were performed that minimize muscle fatigue, and it is claimed that the predicted muscle forces account for essential features of EMG activity “qualitatively” well. However, static optimization analyses use the observed joint moments, which implicitly assumes that they minimize the total muscle fatigue of the task. We use a forward dynamics (i.e., relationship between muscle forces and the kinematics and kinetics of task performance) modeling approach to show that this assumption does not appear to be true in cycling (which was used as an example task in the target article). Our results challenge the hypothesized coordination framework and the underlying concept that general coordination principles for dynamic tasks can be elucidated using inverse-dynamics-based analyses.
S.A. Kautz is with the Rehabilitation R&D Center (153) VA Palo Alto Health Care Systems, 3801 Miranda Ave., Palo Alto, CA 94304-1200, and the Department of Functional Restoration at Stanford University, Stanford, CA 94305-3030. R.R. Neptune is with the Rehabilitation R&D Center. FE. Zajac is with the Rehabilitation R&D Center, the Department of Functional Rehabilitation, and the Mechanical Engineering Department at Stanford University.