GRABINER, M. D. and V. JAQUE. Activation patterns of the triceps brachii muscle during sub-maximal elbow extension.Med. Sci. Sports Exerc., Vol. 19, No. 6, pp. 616–620, 1987. This study was conducted to assess the effects of practice/learning upon the electromechanical delays (EMD) of each head of the triceps brachii, the “true” EMD, “activation lag” periods, and the activation order associated with this multi-pennate muscle. Eight subjects participated in the study. Bipolar surface electrodes detected electromyographic signals (digitized at 1,000 Hz) from each of the three heads of the triceps brachii muscle. The task used was elbow extension with the forearm attached to, and supported by, a dynamometer. Each subject performed 10O trials (analyzed in groups of 10 trials) at approximately 70% the previously established maximum peak acceleration. Analysis consisted of calculating the EMD of each head of the triceps, “true” EMD (time from the first head's activation to movement) activation lag time (difference between the first and last activated heads EMD), and activation order. Correlations between EMD and peak acceleration for the three heads were found to be low, consistent with previous research, and the relationship was not improved by use of the true EMD. Activation lag times were found to decrease (non-significantly) over the 100 trials. Activation order was found to vary widely between subjects but as a group favored the medial head. Results of this study appear to support the contention that the “product” nature of EMD measure is of less merit than generally considered and that a more “process”-related parameter would provide better information regarding peripheral processes.