VARIABILITY OF HUMAN BICEPS BRACHII SPINAL STRETCH REFLEXES - CONTROLCONDITIONS

The variability of the human biceps brachii spinal stretch reflex (SSR ) elicited by rotational stretches is not known. Data obtained by othe rs using tendon taps suggests considerable variability of biceps stret ch 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 su fficient number against which to compare an intervening variable. Ten able-bodied subjects participated in this study, which included four b aseline sessions and eight additional baseline sessions among five of these subjects. Five bins of 50 stretches were elicited at each sessio n (i.e., a total of 250 stretches per session). Subjects were provided no feedback of SSR magnitude during any session. Means, standard devi ations, and coefficients of variation (standard deviation/mean) were c alculated. A nested analysis of variance was used in assessing relativ e contributions to the total variance. Within-session variance contrib uted at least five times the amount to the total variance as day-to-da y variability in both baseline and extended baseline phases. During th e baseline phase within-session coefficients of variation were approxi mately 60%, whereas across-session coefficients of variation were appr oximately 30%. The within-session coefficients of variation increased during the extended baseline phase, whereas the across-session coeffic ients were similar to the baseline phase. In addition, the mean SSR de creased during the extended baseline phase. We conclude that biceps SS Rs elicited by rotational stretches are highly variable, particularly within sessions. Thus, multiple trials and sessions are necessary if m eaningful data are to be gathered.