Can increased intra-abdominal pressure in humans be decoupled from trunk muscle co-contraction during steady state isometric exertions?

The purpose of the present study was to investigate whether increased intra-abdominal pressure (IAP) can be achieved without elevating the overall trunk muscle co-contraction that causes increased spine compression force. Ten subjects performed isometric trunk flexion, extension, and lateral bending exertions while generating 0%, 40% and 80% of their maximal IAP or while co-contracting trunk muscles without consciously raising IAP. An additional three subjects performed a variety of ramp IAP, co-contraction and isometric exertion tasks while holding their breaths and while exhaling. An 18 degree-of-freedom, electromyogram (EMG)-assisted biomechanical model was used to quantify trunk muscle co-contraction with calculations of spine compression force and stability. Spine stability and compression force increased proportionally with increased IAP regardless of whether the subjects intentionally generated IAP or consciously avoided it. This increase was accomplished with significantly greater co-contraction of 12 major trunk muscles. The EMG activation of all muscles was highly correlated with IAP and intra-thoracic pressure (ITP) (r from 0.59 to 0.95). Activity of the thoracic erector spinae correlated the best with ITP (r=0.81), which in turn was correlated with IAP (r=0.91). It was not possible to co-contract trunk muscles without generating IAP and ITP, or conversely to generate IAP without trunk muscle co-contraction and increased ITP.