Normalisation of EMG amplitude: an evaluation and comparison of old and new methods

The purpose of this study was to evaluate and compare four different methods of normalising the amplitude of electromyograms (EMGs), from the biceps brachii. Five males performed isotonic contractions of the elbow flexors with an external force of 50 N, 100 N, 150 N and 200 N. These were followed by a single isometric maximal voluntary contraction (MVC) and ten isokinetic MVCs at 0.35 rad s(-1) intervals between 0.35 rad s(-1) and 3.50 rad s(-1). The processed EMGs recorded from the isotonic contractions were normalised by expressing them as a percentage of: (i) the mean (Dynamic Mean Method) and (ii) the peak EMG from the same contraction (Dynamic Peak Method), (iii) the EMG from the isometric MVC (Isometric MVC Method), and (iv) the EMG from an isokinetic MVC at the same elbow angle and angular velocity (Isokinetic MVC Method). The root mean square difference (RMSD) between the outputs of the Isokinetic MVC and Dynamic Mean methods was significantly greater (P<0.05) than between the Isokinetic MVC method and the Dynamic Peak and the Isometric MVC methods. The small (10%) difference between the Isokinetic MVC and the Isometric MVC Methods was a consequence, firstly, of the lack of difference in EMG recorded from the isometric and isokinetic MVCs and, secondly, the consistency in EMG over the range of motion and at different angular velocities of isokinetic MVC. We conclude that only the Isometric and Isokinetic MVC methods should be used to normalise the amplitude of EMGs from the biceps brachii.