Assessment of Muscle Fatigue during Load Carrying

Abstract

This paper presents preliminary results of a study to assess localized muscle fatigue during load carrying. The assessment procedure is based on parameters of the surface electromyographic (EMG) signal that allow one to measure changes in the amplitude and frequency content of the EMG data. Subjects were instructed to walk on a treadmill at 1.3 mph for approximately 10 minutes, carrying a standard box containing a weight equal to 10% of the subject's body weight. Foot switches and a two-camera motion analysis system were used to record walking cadence and kinematics of the task. EMG data were recorded bilaterally from the multifidus muscle at L5, the latissimus dorsii muscle at T10, the upper trapezius muscle, and the biceps brachii muscle. Preliminary analysis of the mechanics of movement indicated that only minor changes took place during the task. These changes appear to be mainly related to the observed decrease in stride length. Analysis of the EMG data showed that a considerable load was applied to the multifidus and the latissimus dorsii muscle as indicated by the amplitude of the EMG data that reached about 20 % of the EMG amplitude recorded during a maximum voluntary extension of the torso. The time-course of the root mean square (RMS) value of the EMG signal demonstrated an increase in the activation of the upper trapezius muscle during the task, indicating either increased force production or compensation for localized fatigue. Alternate phases of increase and decrease of the instantaneous median frequency (IMDF) was observed. This phasic behavior may reflect a load-sharing mechanism among muscle groups to minimize fatigue that is characterized by alternate phases of fatigue and recovery.