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.

Establishing between-session reliability of TMS-conditioned soleus H-reflexes

“…However, reliability for unconditioned H-reflexes in our study was lower than previous reports [3, 17, 22, 31]. One major difference between the current study and previous investigations is the number of experimental sessions.…”

“…Prior to testing, soleus EMG activity was measured during 30 seconds of quiet standing, and the average rectified activity was recorded. During testing, participants were instructed to contract their soleus muscle by pushing against a foot plate, and visual feedback was provided to maintain their average rectified soleus EMG activity at a target level equal to the EMG amplitude obtained during quiet standing [31]. …”

“…Sixty single 1-ms duration rectangular pulses were delivered at pseudo-random intervals (5–8 sec inter-pulse interval) using a constant-current electrical stimulator (STMISOLA, Biopac Systems Inc.) that was controlled with custom-written scripts in AcqKnowledge software (Biopac Systems Inc.). To acquire the H-reflex recruitment curve, electrical stimulation intensity was increased in increments of 0.5–1.0mA until maximal H-reflexes (H max ) and muscle responses (M max ) were obtained, as measured by the peak-to-peak amplitude of the raw EMG signal [31]. An example of an H-reflex/M-wave recruitment curve is illustrated in Figure 1.…”

“…Consistency and accuracy in coil placement over the hot spot within and across sessions was maintained using stereotaxic neuronavigation (Brainsight v. 2.2.14, Rogue Research Inc., Canada). Procedures for obtaining AMT were modified from typical protocols [30]; tonic EMG activity during sustained low-level isometric contraction (10–20% maximum voluntary contraction (MVC) [23] of the soleus was based on the H-reflex procedures [31]. In this study, AMT was determined as the lowest stimulator intensity needed to evoke a soleus MEP of ≥100μV peak-to-peak amplitude in at least 3 out of 5 trials during tonic plantar flexion at a level equal to EMG activity measured during quiet standing.…”

“…Excellent reliability has also been reported for soleus H-reflex measures (H max , M max , and H max /M max ratio) [6, 17, 22, 31, 35]. Each of these measures is commonly used to evaluate cortical and spinal excitability changes over time or in response to a given intervention in clinical populations.…”