Spine update - Difficulties in estimating muscle forces from muscle cross-sectional area - An example using the psoas major muscle

Most biomechanical models use muscle cross-sectional area (CSA) as an indic ator of maximum isometric muscle force. In general, there are multiple esti mates of CSA for the same muscle. For example, numerous studies have estima ted the CSA of the psoas major muscle using different subject populations a nd positions. However, few studies have combined the available information to obtain an overall estimate of CSA or investigated the effect different s ubject characteristics may have on CSA. In the present update, nine studies that reported psoas major CSA or physiologic CSA were compared with respec t to subject characteristics, methodology, and results. Corrections to cada veric data were made to adjust physiologic CSA to CSA. Comparison of report ed values for living subjects indicated that females have smaller mean CSA than males for the psoas major muscle and that body size does not significa ntly influence muscle CSA in males. Areas derived from cadaveric data were smaller than similar studies on living subjects, possibly because of subjec t age, removal of tendinous and fatty components of fascicles, and lack of detailed data for fascicle angles in the supine position. Results indicate that researchers who use muscle CSA in biomechanical models should carefull y assess the appropriateness of the data used, particularly in relation to potential sex differences and the influence of postural changes on CSA.