ELECTROMYOGRAPHY RELIABILITY IN MAXIMAL AND SUBMAXIMAL ISOMETRIC CONTRACTIONS

Abstract

The reliability of average surface electromyography (EMG) within and between days was studied. Nine subjects were tested on 3 different days. Five maximum and 10 submaximum (5 at each of 2 different levels) isometric contractions were performed each day. The submaximum levels were determined by the 30% and 50% maximum voluntary contraction (MVC) on day 1 and kept constant over days 2 and 3. The myoelectric signal from the triceps was full wave rectified and low-pass filtered at 1 Hz to yield a relatively noise-free average EMG that corresponded in time to the force signal. The scores were normalized to the force level. The reliability was estimated with intraclass correlation coefficients (R). The magnitude of score variation was also expressed as a ratio, coefficient of variation, CV = SD/mean .times. 100%. R values were significantly greater for the submaximal levels than the 100% MVC; no significant difference existed between the 2 submaximal levels. The within-day CV values were similar for all 3 levels, ranging from 8%-10%. The between-days variability was higher than that within days, ranging from 12%-16%. Submaximal isometric contractions are more reliable. Measurement error is substantial in this technique and should be included in statistical designs.