Time course of trunk extensor muscle fatigue as measured using dynamometry and electromyography

Poor trunk extensor endurance is implicated in low back pain, less is known regarding contributions of different regions of the erector spinae to maximum torque production particularly after fatigue. This study examines torque and electromyographic (EMG) activity following a maximal voluntary contraction (MVQ hold in these muscles. 16 students (9 male, 7 female, mean age 21.3 +/- 0.6 years) participated. Testing was performed using a Cybex isokinetic dynamometer with synchronous bilateral EMG recordings (during MVCs of 5 second duration) from the left and right the erector spinae (ES) muscles at vertebral levels T12 and L4, prior to and immediately after the effort and then at 1, 5 and 10 min following a 60 s MVC. A small decrease in maximum torque was observed during 60 s MVC, followed by a non significant step-wise increase. The highest torque value was recorded 10 min post MVC. There was a trend for the ratios of left:right EMG RMS amplitudes to be higher following the fatigue in the lower back and to decrease in the upper back, suggesting the task involved differential use of left and right sides in addition to upper and lower back muscles. This study suggests that 60 s MVC induces differential activation of left and right sides and upper and lower parts of the trunk extensors. The apparent potentiation in force and asymmetry of activation following the 60 s MVC task requires further investigation.