The effect of sustained spinal load on intra-abdominal pressure and EMG characteristics of trunk muscles.

Fourteen young male students (mean age 21 years, mean weight 69.4 kg, and mean height 175.4 cm) and 12 young female students (mean age 22.2 years, mean weight 60.6 kg, and mean height 169.3 cm) held 9.07 kg and 6.8 kg, respectively, at their three-quarters horizontal reach distance in upright and stooping postures for a period of 5 min. During these periods the external torque on the lumbosacral disc, intra-abdominal pressure, and electromyographic signals from erectores spinae at T12 and L3 levels, latissimus dorsi and external obliques were recorded at 1 kHz for 2.1 s every 15 s for a period of 5 min. The EMGs were processed in magnitude and time domains to determine muscle fatigue. Through the data obtained it was seen that the intra-abdominal pressure did not follow or reflect either the spinal load or the muscle activity. Based on the arguments presented, it was concluded that the intra-abdominal pressure does not appear to have a role of relieving the spine of some of its load. Instead, it is suggested that it is a dependent variable manifesting itself when mechanisms for spinal stability are evoked to overcome large voluntary and inertial loads.