MUSCLE COORDINATION OF MAXIMUM-SPEED PEDALING

A simulation based on a forward dynamical musculoskeletal model was co mputed from an optimal control algorithm to understand uni- and bi-art icular muscle coordination of maximum-speed startup pedaling. The musc le excitations, pedal reaction forces, and crank and pedal kinematics of the simulation agreed with measurements from subjects. Over the cra nk cycle, uniarticular hip and knee extensor muscles provide 55% of th e propulsive energy, even though 27% of the amount they produce in the downstroke is absorbed in the upstroke. Only 44% of the energy produc ed by these muscles during downstroke is delivered to the crank direct ly. The other 56% is delivered to the limb segments, and then transfer red to the crank by the ankle plantarflexors. The plantarflexors, espe cially soleus, also prevent knee hyperextension, by slowing the knee e xtension being produced during downstroke by the other muscles, includ ing hamstrings. Hamstrings and rectus femoris make smooth pedaling pos sible by propelling the crank through the stroke transitions. Other si mulations showed that pedaling can be performed well by partitioning a ll the muscles in a leg into two pairs of phase-controlled alternating functional groups. with each group also alternating with its contrala teral counterpart. In this scheme, the uniarticular hip/knee extensor muscles (one group) are excited during downstroke, and the uniarticula r hip/knee flexor muscles (the alternating group) during upstorke. The ankle dorsiflexor and rectus femoris muscles (one group of the other pair) are excited near the transition from upstroke to downstroke, and the ankle plantarflexors and hamstrings muscles (the alternating grou p) during the downstroke to upstroke transition. We conclude that thes e alternating functional muscle groups might represent a centrally gen erated primitive for not only pedaling but also other locomotor tasks as well.