An automated method for describing muscle fatigue

This paper reports on the development of a method to extract features related to fatigue from the isometric force (or moment) records of the quadriceps muscle during cyclic contractions. The force record is modelled as a three segment piece-wise linear curve consisting of an initial region of high force (the plateau), followed by a second segment of rapidly declining force production (transition) and a final region of little or no decline in force (steady state). Repeated maximal isometric contractions lasting 2.5 seconds and separated by 0.5 seconds of rest were elicited by applying 20 Hz bursts of tetanizing stimulation at an amplitude of 20 mA and a fixed pulse duration to individuals with complete spinal cord injuries via chronically indwelling percutaneous intramuscular electrodes. Raw data from a load-cell dynamometer were sampled at 100 Hz, reduced by averaging over the constant force portions of each contraction and low pass filtered at 0.25 Hz. The intersections of the three linear segments are first estimated by computing the inflection points of the resulting smooth record. The linearization is optimized by varying the positions of the intersection points until the mean squared error between the processed data and the piece-wise linear model was minimized. The slopes of the three segments and the locations of the two intersection points were recorded and used as features to characterize the fatiguability of the stimulated muscle. The algorithm also computes widely accepted measures such as the time to 50% reduction in initial force (T50), or extrapolates the values from incomplete data records. The procedure may be applicable to continuous contractions and may be valuable for determining the effects of various exercise programs designed to enhance muscular endurance, or to infer muscle fiber composition indirectly from the force record