Variability of human biceps brachii spinal stretch reflexes: Control conditions

The variability of the human biceps brachii spinal stretch reflex (SSR) elicited by rotational stretches is not known. Data obtained by others using tendon taps suggests considerable variability of biceps stretch reflex. The purpose of this study was to determine the variability of the biceps SSR across and within sessions in able-bodied subjects. In addition, we wanted to determine if four baseline sessions was a sufficient number against which to compare an intervening variable. Ten able-bodied subjects participated in this study, which included four baseline sessions and eight additional baseline sessions among five of these subjects. Five bins of 50 stretches were elicited at each session (i.e., a total of 250 stretches per session). Subjects were provided no feedback of SSR magnitude during any session. Means, standard deviations, and coefficients of variation (standard deviation/mean) were calculated. A nested analysis of variance was used in assessing relative contributions to the total variance. Within-session variance contributed at least five times the amount to the total variance as day-to-day variability in both baseline and extended baseline phases. During the baseline phase within-session coefficients of variation were approximately 60%, whereas across-session coefficients of variation were approximately 30%. The within-session coefficients of variation increased during the extended baseline phase, whereas the across-session coefficients were similar to the baseline phase. In addition, the mean SSR decreased during the extended baseline phase. We conclude that biceps SSRs elicited by rotational stretches are highly variable, particularly within sessions. Thus, multiple trials and sessions are necessary if meaningful data are to be gathered.