CHANGES IN INTRAABDOMINAL PRESSURE, TRUNK MUSCLE ACTIVATION AND FORCE DURING ISOKINETIC LIFTING AND LOWERING

Intra-abdominal pressure (IAP), force and electromyographic (EMG) activity from the abdominal (intra-muscular) and trunk extensor (surface) muscles were measured in seven male subjects during maximal and sub-maximal sagittal lifting and lowering with straight arms and legs. An isokinetic dynamometer was used to provide five constant velocities (0.12-0.96 m.s(-1)) of lifting (pulling against the resistance of the motor) and lowering (resisting the downward pull of the motor). For the maximal efforts, position-specific lowering force was greater than lifting force at each respective velocity. In contrast, corresponding IAPs during lowering were less than those during lifting. Highest mean force occurred during slow lowering (1547 N at 0.24 m.s(-1)) while highest IAP occurred during the fastest lifts (17.8 kPa at 0.48-0.96 m.s(-1)). Among the abdominal muscles, the highest level of activity and the best correlation to variations in IAP (r=0.970 over velocities) was demonstrated by the transversus abdominis muscle. At each velocity the EMG activity of the primary trunk and hip extensors was less during lowering (eccentric muscle action) than lifting (concentric muscle action) despite higher levels of force (r between -0.896 and -0.851). Sub-maximal efforts resulted in IAP increasing linearly with increasing lifting or lowering force (r=0.918 and 0.882, respectively). However, at any given force IAP was less during lowering than lifting. This difference was negated if force and IAP were expressed relative to their respective lifting and lowering maxima. It appears that the IAP increase primarily accomplished by the activation of the transversus abdominis muscle can have the dual function of stabilising the trunk and reducing compression forces in the lumbar spine via its extensor moment. The neural mechanisms involved in sensing and regulating both IAP and trunk extensor activity in relation to the type of muscle action, velocity and effort during the maximal and sub-maximal loading tasks are unknown.