knee and ankle joint torque–angle relationships of multi-joint leg extension

the force–length-relation (f–l-r) is an important property of skeletal muscle to characterise its function, whereas for in vivo human muscles, torque–angle relationships (t–a-r) represent the maximum muscular capacity as a function of joint angle. however, since in vivo force/torque–length data is only available for rotational single-joint movements the purpose of the present study was to identify torque–angle-relationships for multi-joint leg extension. therefore, inverse dynamics served for calculation of ankle and knee joint torques of 18 male subjects when performing maximum voluntary isometric contractions in a seated leg press. measurements in increments of 10° knee angle from 30° to 100° knee flexion resulted in eight discrete angle configurations of hip, knee and ankle joints. for the knee joint we found an ascending–descending t–a-r with a maximum torque of 289.5°±43.3 n m, which closely matches literature data from rotational knee extension. in comparison to literature we observed a shift of optimum knee angle towards knee extension. in contrast, the t–a-r of the ankle joint vastly differed from relationships obtained for isolated plantar flexion. for the ankle t–a-r derived from multi-joint leg extension subjects operated over different sections of the force–length curve, but the ankle t–a-r derived from isolated joint efforts was over the ascending limb for all subjects. moreover, mean maximum torque of 234.7±56.6 n m exceeded maximal strength of isolated plantar flexion (185.7±27.8 n m). from these findings we conclude that muscle function between isolated and more physiological multi-joint tasks differs. this should be considered for ergonomic and sports optimisation as well as for modelling and simulation of human movement.