Introduction: Despite the high incidence of stroke, the mortality following stroke is decreasing [Schmidt et al., 2014]. Increasing number of stroke survivors is causing an increased demand for new rehabilitation techniques improving their quality of life [Bushnell et al., 2014]. One of the most limiting impairments of stroke survivors are gait impairments. Regaining the ability to walk is therefore one of the main goals of the rehabilitation [Olney and Richards, 1996]. New techniques are being proposed and one of them is electrical stimulation on the sole of the foot, which elicits the withdrawal reflex, to initiate and facilitate the swing phase [Spaich et al., 2014]. This technique was found to be effective in rehabilitating gait in stroke patients [Spaich et al., 2014]. However, the effect of the activation of the withdrawal reflex on the cortical and subcortical pathways is unknown. Better understanding of the underlying mechanism could lead to improvements of the treatment. In the present study the effect of walking with electrical activation of the withdrawal reflex on the corticospinal and spinal pathways in healthy subjects was investigated.

Methods: A total of 17 healthy participants participated in the experiments. 9 subjects were placed in the intervention group and walked 30 minutes on a treadmill with electrical stimulation on the sole of the foot at heel-off, which activated the withdrawal reflex. 8 subjects were placed in the control group and walked 30 minutes on a treadmill without stimulation. Measurements of corticospinal and spinal excitability were carried out before, immediately after and 30 minutes after treadmill walking. All measurements were recorded from the tibialis anterior (TA). Single pulse TMS (transcranial magnetic stimulation) was used to asses conrticospinal pathways which led to I/O (input/output) curves. The different parameters of the I/O curves were analyzed. Stretch reflexes were mechanically induced to asses spinal excitability. The amplitude and latency of the first and second components of the stretch reflex were analyzed. Two-way repeated measures ANOVA was used to analyze the effect of time and group on the parameters of the I/O curves and on the amplitude and latency of the first two components of the stretch reflex.

Results: The rMT (resting motor threshold) showed a significant difference between measurements immediately post and 30 minutes post intervention (3.7 ± 4.4 %). The MEPmax-value of the I/O curve showed a significant decrease immediately post (127,2 ± 206.1 μV) and 30 minutes post (110.6 ± 120.9 μV) intervention compared to the baseline measurements. Other parameters (Slope K, S50) of the I/O curves did not show any significant differences over time. No significant effect of group on any of the outcome measures was found. Analysis of the first component of the stretch reflex showed a significant decrease of the peak amplitude immediately (85,2 ± 93.3 μV) and 30 minutes post (74 ± 70.4 μV) intervention compared to baseline. Analysis of the second component of the stretch reflex showed a significant decrease in peak amplitude between pre and 30 minutes post intervention (108.4 ± 185.3 μV). The latencies of both analyzed stretch reflex components did not show any significant differences over time and no significant difference was found between the groups for any of the outcome measures of the stretch reflex.

Conclusion: The study did not found any difference between the two groups for any of the measured outcomes, but did found changes over time for the MEPmax-value of the Boltzmann fit and the first and second component of the stretch reflex. This suggests that treadmill walking itself can modify (decrease) the excitability of corticospinal/spinal pathways. However, due to high inter-subject variability further research on broader sample size is needed to verify or disprove these results.