Effects of sex and walking speed on the dynamic stiffness of lower limb joints.

Fast walking may require a non-uniform change of dynamic stiffness among lower limb joints to deal with this daily task's demands. The change of dynamic joint stiffness may be distinct between females and males. This study aimed to test for differences in dynamic stiffness among lower limb joints in response to increased walking speed in males and females. Thirty-five participants walked in two randomized conditions: self-selected speed and fast speed (25% greater than the self-selected speed). Dynamic stiffnesses of the ankle, knee, and hip were calculated as the linear slope of the moment-angle curve's regression line during their major power absorption phase of the walking cycle. The comparison between conditions showed that the knee (p < 0.001) and hip (p = 0.031) increased their stiffness at the fast compared to self-selected speed. Ankle stiffness was not different between conditions (p = 0.818). The comparison among joints across speeds showed that the knee had a greater increase than the ankle (p = 0.001) and hip (p < 0.001), with no difference between ankle and hip (p = 0.081). The sex of the participant influenced only the ankle stiffness, in which males had greater stiffness than females (p = 0.008). These findings demonstrated that the lower limb joints changed their dynamic stiffness differently, and only the ankle stiffness was influenced by sex. The non-uniform adjustments of stiffness may provide the necessary stability and allow the individual to deal with greater demand for walking fast.