Reproducibility of electromyographic patterns in stretch-shortening type contractions.

Within a set of repeated measurements, 12 male subjects performed the following types of muscle contraction: (1) running at constant velocity (12 km.h-1); (2) one-leg hopping at the frequency preferred; (3) both-legs hopping at the frequency preferred; (4) drop jumps from a height of 0.40 m. The surface electromyographs (EMG) from the left and right legs were recorded, together with the averaged signals from vertical ground reaction forces and angular displacement of the ankle joint. Reliability coefficients for the day to day and the week to week comparison were calculated. In addition, on any one test day the positions of the electrodes were systematically changed. The analysis of various EMG parameters (integrated-EMG) revealed high coefficients for both comparisons as well as for the total set of measurements. Qualitative comparisons of the EMG showed a high reproducibility of the shape of the patterns. If the relative position of the electrodes with respect to the belly of the muscle is changed, the amplitudes of the EMG recordings are markedly reduced. It is concluded that surface EMG is a reliable method for studies of the neuromuscular system. On the basis of the data presented, it is suggested that the term "reliability" should be replaced by the term "reproducibility" in order to cover both the aspect of the relative stability plus the aspects of linear changes and the scattering of the data in repeated measurements.