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Arnold, E.M. and Delp, S.L. Fibre operating lengths of human lower limb muscles during walking. Philosophical Transactions of the Royal Society B, vol 366, pp 1530-1539. doi: 10.1098/rstb.2010.0345 (2011)
Abstract    View

Muscles actuate movement by generating forces. The forces generated by muscles are highly dependent on their fibre lengths, yet it is difficult to measure the lengths over which muscle fibres operate during movement. We combined experimental measurements of joint angles and muscle activation patterns during walking with a musculoskeletal model that captures the relationships between muscle fibre lengths, joint angles and muscle activations for muscles of the lower limb. We used this musculoskeletal model to produce a simulation of muscle– tendon dynamics during walking and calculated fibre operating lengths (i.e. the length of muscle fibres relative to their optimal fibre length) for 17 lower limb muscles. Our results indicate that when musculotendon compliance is low, the muscle fibre operating length is determined predominantly by the joint angles and muscle moment arms. If musculotendon compliance is high, muscle fibre operating length is more dependent on activation level and force– length– velocity effects. We found that muscles operate on multiple limbs of the force– length curve (i.e. ascending, plateau and descending limbs) during the gait cycle, but are active within a smaller portion of their total operating range.

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The study associated with this project used a musculoskeletal model that describes the relationship between fiber lenght and joint angle to determine the fiber operating lengths of lower limb muscles during a typical gait cycle for a single subject, using literature values of muscle activation. This project contains the setup files, input data, simulation results, and intermediate programs used to find the results presented in the attached publications. These files include motion files for a single, averaged gait cycle, a scaled model, input muscle controls, OpenSim setup files, and several custom API programs used to add necessary functionality to the OpenSim workflow.