Spectral parameters of trunk muscles during fatiguing isometric axial rotation in neutral posture.

Axial rotation of the trunk is commonly associated with back injury and pain. However, the behaviour of trunk muscles in axial rotation is poorly understood. The objective of this study was to measure spectral parameters from the EMG of erector spinae at T10 and L3 levels, latissimus dorsi, external and internal oblique, rectus abdominis and pectoralis major muscles bilaterally in a standardized repeatable axial rotation at 60% MVC to fatigue. Twelve young and healthy subjects were recruited after screening for musculoskeletal disorders. Surface electrodes were applied to the named seven trunk muscles bilaterally. Subjects were seated in the device called Axial Rotation Tester and stabilized such that they could rotate only their thoracolumbar spine. Other motions were prevented. Subjects held 60% of their MVC for a period of 2 min. Samples (2.1 s) were obtained at every 10 s interval at a sampling frequency of 1 KHz. Samples were subjected to FFT analysis. The total power and the median frequencies were analyzed. The median frequency for different muscles were different (p < 0.001). The slopes of decline of the median frequencies of the agonists were different for different muscles (p < 0.001). This differential fatiguing rate could conceivably create a force imbalance potentiating back injury.