Between-day variability of soleus H reflex recruitment curve parameters: how many stimulations are required to maximise the reproducibility of the measurements?

Abstract

Purpose: The H reflex recruitment curve represents the gold standard for quantifying changes in spinal circuitries. However, there is no agreement on how many stimulations should be applied for each parameter. Thus, we explored the impact of varying the number of stimulations (3, 6, 9, 12 and 15 stimuli per intensity) on between-day reliability of soleus H reflex.

Materials and methods: Twenty healthy participants (11 males, 9 females; age: 22.4 ± 2.3 years) visited the laboratory on two days for H-recruitment curves construction, using a 3 s inter-stimuli interval. To explore whether H reflex parameters differed between days, without varying the number of stimulations, paired-sample t tests were performed. Relative and absolute reliability were calculated using the intraclass correlation coefficients (ICCs) and the coefficients of variation (CVs), respectively.

Results: M_max, H_max/M_max, H slope, H_thresh, current at 50% of H_max and current at H_max were not significantly different between days (all p > 0.05). ICCs of M_max, H_max/M_max and H_thresh were all good (0.79-0.89). H slope ICCs were moderate (0.56-0.73) between 3 and 12, but good (0.75) with 15 stimulations. Current at H_max ICCs were moderate (0.55-0.0.73) with all stimulations, except with 9 (good: 0.76). CVs of the current at H_max were all moderate (between 5-to-10%). H_thresh CV was poor (>10%) with three, but moderate for other stimulations. M_max, H_max/M_max and H slope CVs were generally poor.

Conclusions: 9 stimulations are required to ensure higher between-day reliability of H reflex parameters, except for H slope that requires 15 stimulations.