Locomotor strategy for pedaling: Muscle groups and biomechanical functions

A group of coexcited muscles alternating with another group is a common ele ment of motor control, including locomotor pattern generation. This study u sed computer simulation to investigate human pedaling with each muscle assi gned at times to a group. Simulations were generated by applying patterns o f muscle excitations to a musculoskeletal model that includes the dynamic p roperties of the muscles, the limb segments, and the crank load. Raasch et al. showed that electromyograms, pedal reaction forces, and limb and crank kinematics recorded during maximum-speed start-up pedaling could be replica ted with two signals controlling the excitation of four muscle groups (1 gr oup alternating with another to form a pair). Here a four-muscle-group cont rol also is shown to replicate steady pedaling. However, simulations show t hat three signals controlling six muscle groups (i.e.. 3 pairs) is much mor e biomechanically robust, such that a wide variety of forward and backward pedaling tasks can be executed well. We found the biomechanical functions n ecessary for pedaling, and how these functions can be executed by the muscl e groups. Specifically, the phasing of two pairs with respect to limb exten sion and flexion and the transitions between extension and flexion do not c hange with pedaling direction. One pair of groups (uniarticular hip and kne e extensors alternating with their anatomic antagonists) generates the ener gy required for limb and crank propulsion during limb extension and flexion , respectively. In the second pair, the ankle plantarflexors transfer the e nergy from the limb inertia to the crank during the latter part of limb ext ension and the subsequent limb extension-to-flexion transition. The dorsifl exors alternate with the plantarflexors. The phasing of the third pair (the biarticular thigh muscles) reverses with pedaling direction. In forward pe daling. the hamstring is excited during the extension-to-flexion transition and in backward pedaling Juring the opposite transition. In both cases ham strings propel the crank posteriorly through the transition. Rectus femoris alternates with hamstrings and propels the crank anteriorly through the tr ansitions. With three control signals, one for each pair of groups. differe nt cadences (or power outputs) can be achieved by adjusting the overall exc itatory drive to the pattern generating elements, and different pedaling go als (e.g., smooth, or energy-efficient pedaling: 1- or 2-legged pedaling) b y adjusting the relative excitation levels among the muscle groups. These s ix muscle groups are suggested to be elements of a general strategy for ped aling control, which may be generally applicable to other human locomotor t asks.