The relations between the torsional loading applied at the foot and the axial rotations of the three leg joints (that is, ankle/boot, knee, hip) have been determined experimentally for three male test subjects of comparable height, age, and weight (1.83 m, 25 years, 90 kg). Experiments examined these relations under both quasi-static and dynamic loadings. The quasistatic loading was applied manually while the dynamic loading was provided by means of a computer-controlled pneumatic system, which applied single haversine axial moment pulses of variable amplitude (0 to 100 Nm) and duration (60 to 600 ms). Potentiometers measured the absolute rotations of the three leg segments.
Quasi-static loading experiments explored the effects of weight bearing, flexion, and muscle resistance on stiffness, particularly of the knee joint. Experiments were conducted up to the torque limit that subjects felt they could withstand. The results indicate nonlinear stiffness for all three joints with the direction of rotation having a marked effect on overall stiffness especially for the ankle/boot and knee. Both weight bearing and muscle resistance lead to increased overall knee stiffness with muscle resistance having a more pronounced effect. Stiffness increases are accompanied both by a decrease in laxity and by an increase in the torque limit. Although the torque limit changes by as much as a factor of two depending on the test conditions, both the ankle and knee maximum rotations at the torque limit are relatively constant.
Dynamic loading experiments recorded the transient rotational response under long (∼ 600-ms), medium (∼ 250-ms), and short (∼ 60-ms) duration pulses. The leg responds quasi-statically to the long duration pulse, transitionally to the medium duration pulse, and impulsively to the short duration pulse. These results emphasize the potential importance of system dynamics as a mechanism of lower limb injury.