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.

The modulation of the lower limb nociceptive withdrawal reflex elicited during late stance by a stimulus train with frequencies of 15 and 30 Hz delivered to the mid-forefoot, arch of the foot, and heel was investigated. Stimulation was delivered at four moments of the gait cycle between heel-off and toe-off. Stimulation at 15 Hz induced larger kinematic responses at the knee and hip. Reduced plantarflexion and increased dorsiflexion, compared to control steps, were evoked at the ankle; these kinematic responses were site dependent with minimum responses evoked by stimulation at the heel. The dorsiflexion response was largest when stimulating at toe-off and was larger for stimulation at 15 Hz than at 30 Hz. The muscle reflex responses were site modulated in tibialis anterior with largest responses evoked by stimulation at the arch of the foot and mid-forefoot, and phase and frequency modulated in soleus. This study presents a detailed assessment of the lower limb nociceptive reflex modulation and provides results, which might have application in the rehabilitation of the hemiparetic gait.

A novel closed-loop system for improving gait in hemiparetic patients by supporting the production of the swing phase using electrical stimulations evoking the nociceptive withdrawal reflex was designed. The system exploits the modular organization of the nociceptive withdrawal reflex and its stimulation site- and gait-phase modulation in order to evoke movements of the hip, knee, and ankle joints during the swing phase. A modified model-reference adaptive controller (MRAC) was designed to select the best stimulation parameters from a set of 12 combinations of four electrode locations on the sole of the foot and three different stimulation onset times between heel-off and toe-off. It was hypothesized that the MRAC system would result in a better walking pattern compared with an open-loop preprogrammed fixed pattern of stimulation (FPS) controller. Thirteen chronic or subacute hemiparetic subjects participated in a study to compare the performance of the two control schemes. Both control schemes resulted in a more functional gait compared to no stimulation (P < 0.05) with a weighted joint angle peak change of 4.0 ± 1.6 (mean ± Standard deviation) degrees and 3.1 ± 1.4 degrees for the MRAC and FPS schemes, respectively. This indicates that the MRAC scheme performed better than the FPS scheme (P < 0.001) in terms of reaching the control target.

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