The reliability of H-reflex recordings in standing subjects

Several studies have used the H-reflex to investigate the effect of upright stances and locomotion on spinal reflex excitability. The reliability of eliciting this reflex during weight-bearing has however yet to be addressed. This study was undertaken to determine the reliability of individual differences in the H-reflex recorded from healthy subjects during quiet standing. Secondary aims of the study were to evaluate individual reliability during prolonged standing, and to establish the minimum number of trials required to provide reliable measurements of the H-reflex.

Twenty neurologically healthy volunteers participated in a repeated measures design consisting of 8 blocks of 20 trials evenly distributed over two testing sessions. H-reflex recordings were elicited from the subject's dominant side soleus muscle by percutaneously stimulating the posterior tibial nerve. Stimuli were presented every 10 s, with 2 min seated rest provided between blocks of trials. Peak-to-peak amplitude of H-reflexes and m-responses determined for individual trials were used for subsequent analysis.

It was found that the reliability of measuring the H-reflex and m-response during quiet standing was extremely robust (r = .97 for both measures). This pattern of individual differences remained consistent over 80 trials confirming the stability of the measures. High reliabilities (r = .96 and .87 for the H-reflex and m-responses respectively) were also observed when as few as four trials were analysed. When measures obtained during the first session of testing were compared with those obtained for session two, the correlation coefficients were generally of a lower order (r = .54 to .90).

The results demonstrate that the H-reflex in quiet standing provides high intra-individual reliability, suggestive of a stable reflex resistant to potentially confounding postural influences, or other sources of biological variation. The between-session reliability underscores the difficulty in reproducing conditions between sessions, and emphasises the need for within-session comparisons of H-reflex amplitudes. Given the functional challenge of maintaining an upright posture, the H-reflex appears to be a well maintained and stable phenomenon.