How the optimal design of an ergometer changes when the muscles were weakened locally

Abstract

Rotating a crank by legs is the popular way to transform manpower to rotative power, as used for bicycles. Thus ergometers or pedaling wheelchairs are used for the elderly or patients who have weakened muscles because it is useful for their training or rehabilitations. Then, how to design the crank or saddle lengths for these users? We confirmed it by musculoskeletal model simulations. The lower limb and crank were modeled as a four-bar linkage, fixing an ankle joint. Hip and knee joints were driven by the six muscles around thigh. We compared four types of subjects; the healthy people, quadriceps weakened, hamstrings weakened and all the muscles weakened. As a result, in all the cases, the high saddle was the best to extend the knee at bottom dead center. The optimal crank length was longer by weakened quadriceps and was shorter by weakened hamstrings.