Load-induced inflexion of the surface electromyographic signal during isometric fatiguing activity of normal human paraspinal muscle.

In twelve normal subjects (aged 21-57 years) changes in surface electromyographic (EMG) activity of the paraspinal muscles were examined during "unloaded" and "weight-loaded" fatigue tests. These comprised 60 second isometric holds against gravity with the upper body unsupported in a horizontal and prone position while the lower body was firmly secured to a couch. During one of the tests sand bags, representing 10% of subjects' body weight, were placed between the scapulae. Integrated EMG (IEMG) and the frequency power spectrum were monitored throughout each test. During both tests IEMG activity initially increased and then plateaued in the unloaded test but decreased in the loaded test. Since frequency changes in the power spectrum were similar during both fatigue tests, IEMG reductions in the loaded test appeared due to reductions in motor unit recruitment. Despite these IEMG reductions the horizontal position was maintained, suggesting the operation of intrinsic peripheral fatigue resistance mechanisms which were utilised to a greater extent during the loaded test.