A quantitative electromyographic index that is independent of the force of contraction

Quantitative EMG has been used to improve the accuracy of the diagnosis of myopathy. Equipment is available commercially that utilizes the Willison method of quantification, but the technique has not been widely used because of the necessity of measuring muscle contraction force. An analysis of the basic parameters of the Willison method of quantification was performed for forces of 10, 20, 40 and 60% of maximum for 16 normal subjects for the deltoid muscle. An index was derived from the basic parameters that is independent of the force of contraction. The index is a standardized value for turns/sec expressed as a logarithm. The index has a mean of 1.171 with a standard error of 0.059 for a sample size of 10 muscle sites. Quantitative EMG can be utilized in routine clinical examinations by applying mild to moderate resisting forces to patients, and the proposed index can be determined graphically. The force applied to patients can be maintained within the range of the forces investigated by estimating the force of contraction, evaluating patient effort and fatigue, and monitoring the parameter of total amplitude per second.