Turns and averaging as static and dynamic measures of masseter EMG activity

Surface EMG records from human masseter muscles were analyzed at various fractions of the maximum voluntary contraction (MVC). Frequencies of turns, or reversals of signal direction, and integrated or average rectified values were compared as quantitative measures of this type of data. Averaged signals were useful for within-study comparisons or for revealing dynamic changes in EMG during excitation or inhibition. However, averaged values depend strongly on signal amplitude (and thus on electrode characteristics and muscle size), and are not easily transferable between studies. Turns frequencies increased with contraction up to 75% MVC, and were less dependent on signal amplitude than were averages. Turns frequencies may thus be useful for quantitative comparisons between studies or between different muscles.