Intra-abdominal pressure during trunk extension motions.

OBJECTIVE: This study was designed to help interpret the biomechanical role of intra-abdominal pressure during lifting type motions of the trunk.
DESIGN: An in vivo study was performed in which intra-abdominal pressure was observed as subject trunks were subjected to different dynamic trunk loading conditions common during industrial lifting.
BACKGROUND: There is a little consensus as to the biomechanical role of intra-abdominal pressure during lifting. Previous studies have suggested that: it may assist in load relief when lifting, may be involved in trunk stability, and/or may be used as a measure fo spine loading. Thus, in general, our understanding of intra-abdominal pressure is rather poor.
METHODS: In this study intra-abdominal pressure was monitored using a radio pill in 114 subjects over a series of four experiments. Subject's trunks were subjected to different dynamic trunk symmetric and asymmetric trunk loading conditions that are common during industrial lifting tasks.
RESULTS: The results indicated that (1) intra-abdominal pressure increased to significant levels (above 10 mmHg) only when more than 54 Nm of trunk torque were supported; (2) intra-abdominal pressure increases monotonically (up to 150 mmHg) as a function of trunk velocity; and (3) under concentric conditions intra-abdominal pressure increases as a function of greater asymmetry, whereas, under eccentric conditions the response changes to a much lesser extent as asymmetry changes.
CONCLUSIONS: These findings suggest that intra-abdominal pressure appears to be more a by-product of trunk muscle coactivation. Any mechanical advantage gained from intra-abdominal pressure might be in the form of a preparatory action resulting from muscle coactivation that stiffens the trunk just prior to a rapid trunk extension exertion. This function may reinforce previous hypotheses regarding the stability role of intra-abdominal pressure. RELEVANCE: Intra-abdominal pressure has been observed during lifting for several decades, yet the biomechanical role of intra-abdominal pressure is poorly understood. This study has attempted to describe how intra-abdominal pressure behaves during lifting motions as the components of lifting are changed. The findings place in doubt biomechanical significance of intra-abdominal pressure. Thus, based upon this study, clinicians need not worry about interpreting intra-abdominal pressure, since it appears to be a by-product of muscle contraction and cocontraction.