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

Intra-abdominal pressure (IAP), force and electromyographic (EMG) acti vity from the abdominal (intra-muscular) and trunk extensor (surface) muscles were measured in seven male subjects during maximal and sub-ma ximal sagittal lifting and lowering with straight arms and legs. An is okinetic dynamometer was used to provide five constant velocities (0.1 2-0.96 m.s(-1)) of lifting (pulling against the resistance of the moto r) and lowering (resisting the downward pull of the motor). For the ma ximal efforts, position-specific lowering force was greater than lifti ng force at each respective velocity. In contrast, corresponding IAPs during lowering were less than those during lifting. Highest mean forc e 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 b est correlation to variations in IAP (r=0.970 over velocities) was dem onstrated by the transversus abdominis muscle. At each velocity the EM G activity of the primary trunk and hip extensors was less during lowe ring (eccentric muscle action) than lifting (concentric muscle action) despite higher levels of force (r between -0.896 and -0.851). Sub-max imal efforts resulted in IAP increasing linearly with increasing lifti ng or lowering force (r=0.918 and 0.882, respectively). However, at an y given force IAP was less during lowering than lifting. This differen ce was negated if force and IAP were expressed relative to their respe ctive lifting and lowering maxima. It appears that the IAP increase pr imarily accomplished by the activation of the transversus abdominis mu scle can have the dual function of stabilising the trunk and reducing compression forces in the lumbar spine via its extensor moment. The ne ural mechanisms involved in sensing and regulating both IAP and trunk extensor activity in relation to the type of muscle action, velocity a nd effort during the maximal and sub-maximal loading tasks are unknown .