Longer Duration of Downslope Treadmill Walking Induces Depression of H-Reflexes Measured during Standing and Walking

Abstract

Objectives: The Hoffman-reflex (H-reflex) is an electrophysiological technique used to evaluate the excitability of the monosynaptic spinal reflex arc. In individuals with upper motor neuron lesions who show elevated spinal excitability, a depression of spinal excitability may indicate adaptive spinal plasticity. Downslope walking (DSW), an exercise intervention comprising repetitive eccentric muscle activity, has been shown to induce depression of soleus H-reflex amplitudes while seated, however, the dose-response time-course of H-reflex modulation during DSW has not been characterized. The objectives of this study were twofold: (1) to evaluate DSW-induced soleus H-reflex depression in the standing posture and during walking, and (2) to investigate the effect of walking duration (20 minutes and 40 minutes) of DSW (-15% decline) on soleus H-reflexes, (with level walking (LW) as a control intervention).

Methods: Soleus H-reflexes were collected Pre, Post-20 minutes, and Post-40 minutes of walking in the standing position; and H-reflexes were also measured at 4 different time points during the terminal stance phase of walking.

Results: Our results showed that soleus H-reflexes evaluated in standing showed a greater % depression after DSW compared to LW, with a statistical trend for greater depression with longer durations (40-minutes). H-reflexes measured during walking showed greater depression after 40 minutes of walking compared to 20- or 30-minutes for both DSW and LW.

Conclusions: Longer duration treadmill walking (40-minutes) may induce a greater acute depressive effect on soleus H-reflex excitability compared to shorter durations (20-minutes) of treadmill walking. Future work will investigate the potential for DSW as a gait training intervention in people with upper motor neuron lesions such as multiple sclerosis and stroke.

Keywords: Hoffman’s Reflex; Neuroplasticity; Spinal Cord Plasticity; Spinal Reflex; Treadmill Training.

Figure 1:

Overview of experimental protocol. A schematic shows the time points during the experimental sessions.

Figure 2:

Raw data from a representative subject demonstrating the timing of delivery of peripheral nerve stimulation to elicit H-reflexes with respect to the gait cycle. Graphs show raw data from (A) the accelerometer, (B) Soleus EMG, and (C) timing of delivery of the peripheral nerve stimulation. The accelerometer signal was used to identify heel strike, demarcating the beginning and end of a gait cycle (marked with dashed vertical lines). Stimulation was delivered at 50% of the gait cycle, which corresponds to the terminal stance phase of gait. The H-reflex elicited from the soleus muscle during gait is shown enlarged in the inset.

Figure 3:

Representative Soleus H-reflex recruitment curves collected in the standing position during the DSW and LW. The graphs show soleus H-reflex recruitment curves collected in the standing position for a representative participant for the DSW (A) and LW (B) sessions at 3 time points - Pre, Post 20 minutes of walking (Post-20min), and Post 40-minutes of walking (Post-40min). The H-reflex amplitudes have been normalized to the corresponding Mmax. Note the marked H-reflex depression induced after 20- and 40-minutes of DSW.

Figure 4:

H-reflex data measured during standing and walking. (A) Group average (N = 9) and standard deviation for Hmax/Mmax values for the 2 walking conditions (DSW, LW) at three different time points (Pre, Post 20-min, and Post-40min of walking). (B) Average % depression of soleus Hmax/Mmax post 20-minutes and post 40-minutes (compared to the Hmax/Mmax value at Pre) for each of the 2 walking conditions (DSW, LW) measured in the standing position. (*p > 0.05). (C) Group average (N = 9) and standard deviation for H-reflex amplitudes values for the 2 walking conditions (DSW, LW) collected during the terminal stance phase of gait at 4 different time epochs (Pre, Post 10-min, Post 20-min, and Post-40min of walking). The H-reflex amplitudes measured during walking were normalized to the Mmax obtained during the same phase of the walking cycle, and then normalized to Hmax/Mmax obtained in the “Pre” standing position. Thus, these are H-responses as a percentage of the standing baseline Hmax/Mmax. (D) Average % depression of the H-responses measured during walking compared to the H-reflexes collected during the first 10-minute walking block.