Postural adaptation to unilateral hip muscle fatigue during human bipedal standing

Although previous studies have investigated the effects of bilateral muscle fatigue on bipedal postural control, whether and how the central nervous system could adapt to unilateral muscle fatigue for controlling bipedal stance remains to be investigated. The purpose of the present experiment was designed to address this issue by assessing the effect of unilateral muscle fatigue induced on the hip's abductors of the dominant leg on bipedal standing. Twenty-four young healthy adults stood barefoot, feet together, with their eyes closed and were asked to sway as little as possible. The experimental group (n=12) executed this postural task in two conditions, pre- and post-fatigue. In the post-fatigue condition, the measurements were performed immediately after a designated fatiguing exercise for the hip abductors of their dominant leg. For the control group (n=12), this fatiguing exercise was replaced by a laying rest period corresponding to the fatiguing exercise. The distribution of the body weight and plantar centre of foot pressure (CoP) displacements from the non-fatigued and fatigued leg were recorded along the mediolateral and anteroposterior axes using a plantar pressure data acquisition system. Results of the experimental group showed that unilateral muscle fatigue induced on the hip's abductors of the dominant leg had different effects on the plantar CoP displacements (1) under the non-fatigued and fatigued legs, yielding larger displacements under the non-fatigued leg only, and (2) in the anteroposterior and mediolateral axes, yielding larger displacements along the mediolateral axis only. These observations could not be accounted for by any asymmetrical distribution of the body weight on both legs which were similar for both pre- and post-fatigue conditions. The observed postural responses could be viewed as an adaptive process to cope with an unilateral alteration in the hip neuromuscular function induced by the fatiguing exercise for controlling bipedal stance. The increase in CoP displacements observed under the non-fatigued leg in the fatigue condition could reflect enhanced exploratory "testing of the ground" movements with sensors of the non-fatigued leg's feet, providing supplementary somatosensory inputs to the central nervous system to preserve/facilitate postural control in condition of altered neuromuscular function of the dominant leg's hip abductors induced by the fatiguing exercise.