EMG differences between concentric and eccentric maximum voluntary contractions are evident prior to movement onset.

This investigation addressed the question of whether the muscle activation signal prior to movement onset, as measured by surface EMG, differs if the contraction to be performed is concentric (shortening) or eccentric (lengthening). Specifically, the purpose was to determine if differences in knee extensor muscle EMG prior to voluntary maximum concentric and eccentric contractions and initiated from the same knee joint angle are evident at a time before muscle length changes could be influential. A protocol was designed using isokinetic knee extensions. The EMG of the vastus lateralis, vastus medialis, rectus femoris, and hamstrings muscles and the associated knee extension moment were measured during the isometric phase preceding the onset of dynamometer motion. During this isometric phase the muscles initially contracted under identical conditions, irrespective of whether the contraction was to be concentric or eccentric. The EMG of the eccentric contractions was significantly smaller than that of the concentric contractions. However, the rate of change of knee extension moment generally did not differ between the two conditions. This was found for both the monoarticular and multiarticular knee extensor muscles. The results suggest that initial differences between the EMG of maximum voluntary concentric and eccentric knee extensor contractions are selected a priori and support the contention that the central nervous system distinguishes between maximum eccentric and concentric contractions. The emergence of differences in activation prior to muscle length changes suggests supraspinal influences.