Withdrawal reflexes examined during human gait by ground reaction forces: site and gait phase dependency

The objective of this study was to investigate the modulation of the nociceptive withdrawal reflex during gait measured using Force Sensitive Resistors (FSR). Electrical stimulation was delivered to four locations on the sole of the foot at three different time points between heel-off and toe-off. Peak force changes were measured by FSRs attached to the big toe, distal to the first and fourth metatarsophalangeal joints, and the medial process of the calcaneus on both feet. Force changes were assessed in five gait sub-phases. The painful stimulation led to increased ipsilateral unloading (10 +/- 1 N) and contralateral loading (12 +/- 1 N), which were dependent on stimulation site and phase. In contrast, the hallux of the ipsilateral foot plantar flexed, thus facilitating the push-off. The highest degree of plantar flexion (23 +/- 10 N; range, 8-44 N) was seen in the second double support phase following the stimulation. Site and phase modulation of the reflex were detected in the force signals from all selected anatomical landmarks. In the kinematic responses, both site and phase modulation were observed. For stimulations near toe-off, withdrawal was primarily accomplished by ankle dorsiflexion, while the strategy for stimulations at heel-off was flexion of the knee and hip joints.