Slope walking causes short-term changes in soleus H-reflex excitability

Abstract

The purpose of this study was to test the hypothesis that downslope treadmill walking decreases spinal excitability. Soleus H-reflexes were measured in sixteen adults on 3 days. Measurements were taken before and twice after 20 min of treadmill walking at 2.5 mph (starting at 10 and 45 min post). Participants walked on a different slope each day [level (Lv), upslope (Us) or downslope (Ds)]. The tibial nerve was electrically stimulated with a range of intensities to construct the M-response and H-reflex curves. Maximum evoked responses (Hmax and Mmax) and slopes of the ascending limbs (Hslp and Mslp) of the curves were evaluated. Rate-dependent depression (RDD) was measured as the % depression of the H-reflex when measured at a rate of 1.0 Hz versus 0.1 Hz. Heart rate (HR), blood pressure (BP), and ratings of perceived exertion (RPE) were measured during walking. Ds and Lv walking reduced the Hmax/Mmax ratio (P = 0.001 & P = 0.02), although the reduction was larger for Ds walking (29.3 ± 6.2% vs. 6.8 ± 5.2%, P = 0.02). The reduction associated with Ds walking was correlated with physical activity level as measured via questionnaire (r = -0.52, P = 0.04). Us walking caused an increase in the Hslp/Mslp ratio (P = 0.03) and a decrease in RDD (P = 0.04). These changes recovered by 45 min. Exercise HR and BP were highest during Us walking. RPE was greater during Ds and Us walking compared to Lv walking, but did not exceed "Fairly light" for Ds walking. In conclusion, in healthy adults treadmill walking has a short-term effect on soleus H-reflex excitability that is determined by the slope of the treadmill surface.

Keywords: Homosynaptic depression; locomotion; rate‐dependent depression; spinal excitability; treadmill.

Figure 1

The depressive effect of Ds walking (ordinate) as a function of physical activity level (7-d PAR, abscissa) is illustrated in this scatterplot. The post-pre H_max/M_max difference is expressed as a percentage of pre on the ordinate. A horizontal dashed line is aligned at zero on the ordinate, or an absence of change in H_max/M_max. Values below zero reflect a reduction in H_max/M_max after Ds walking. There was a significant negative correlation between the percent change in H_max/M_max with Ds walking and physical activity (r = −0.52, P = 0.04).

Figure 2

Representative H-reflex curves collected before (Pre, black trace) and after (Post, gray trace) 20 min of downslope (Ds), level (Lv), and upslope (Us) treadmill walking. Stimulus intensities are expressed as a percentage of the first stimulus intensity at which the M-response appeared (x-axis). H-reflexes are expressed as a percentage of the maximum M-response (y-axis). Solid lines represent the polynomial fit and dots represent raw data. R-squared values that represent the fit of the polynomial transformation with the raw data are shown. For this subject the peaks of the Pre curves were similar across days. The peak of the curves were virtually identical before and after Us walking. For this subject effects emerge for Ds and Lv walking, after which there was a reduction in the maximal amplitude of the curve.

Figure 3

H-reflex results before and after 20 min of downslope (Ds), level (Lv), and upslope (Us) treadmill walking. (A) H_max/M_max ratio, mean + SE, *P < 0.05 versus pre. There was a significant reduction after Ds walking and after Lv walking. (B) Hslp/Mslp ratio, mean + SE, *P < 0.05versus pre. There was a significant increase after Us walking. Prestimulus background EMG activity (100 msec prior to electrical stimulation) (C). There was no change in prestimulus background EMG activity across time.

Figure 4

Rate-dependent depression (RDD) of the H-reflex before and after downslope (Ds), level (Lv), and upslope (Us) treadmill walking. (A) The average of 10 sweeps at 0.1 Hz (dotted line) and at 1.0 Hz (solid line), pre and post walking for one representative subject. Average H-reflex size was similar for 0.1 Hz, but depression with 1.0 Hz stimulation frequency was less at Post. (B) Average RDD results expressed as percent depression (i.e., % depression when H-reflexes were elicited at 1 Hz vs. when they were elicited at 0.1 Hz), mean + SE, *P < 0.05versus pre. There was a reduction in RDD after Us walking.