DALEY, M.A.; CARD, G.; BIEWENER, A.A.: Effective Mechanical Advantage of the guinea fowl hindlimb during steady and variable speed running

Animals live in an unpredictable environment and rarely move at constant speeds. Thus, muscle function under non-steady conditions (acceleration, deceleration) is a critical component of locomotor system design. Previous studies have suggested that, during steady speed locomotion, muscles function primarily to generate the force required to support body weight. However, it is not understood how these force requirements change during acceleration and deceleration. In order to address this, we examined changes in the Effective Mechanical Advantage (EMA) of the guinea fowl hindlimb using force platform and high-speed video recordings of the bird running at both steady and varying speeds. Limb EMA, defined as the total muscle force required per unit of ground reaction force, allows comparison of muscle force generation requirements during steady running, acceleration, and deceleration. We found that under all conditions, the majority of muscle force required to support the body is generated by the knee and ankle extensors, while the hip extensors produce less than 10% of the total force. EMA distribution among the joints changes during acceleration and deceleration, showing the greatest variation at the knee and ankle. During acceleration, knee extensors produce a greater percentage of the total muscle force, while during deceleration ankle extensors generate the majority of total muscle force. These changes in EMA likely reflect redistribution of force generation to muscles best suited for the prevailing locomotor requirements, allowing more efficient, rapid force generation during acceleration, and less costly absorption of energy during deceleration. (Supported by NSF IBN 9923699, and a Howard Hughes Medical Institute Predoctoral Fellowship)