Importance of sarcomere length when determining muscle physiological cross-sectional area: a spine example

Muscle physiological cross-sectional area predicts the maximum capability of a muscle to generate isometric force. Biomechanical models often use estimates of individual muscle physiological cross-sectional area to partition internal forces among different muscles and predict joint forces and stability. In the spine literature, these physiological cross-sectional area values are generally obtained from imaging or cadaveric studies that have not accounted for a potential lengthened or shortened (and thus thinned or thickened, respectively) state of the muscles in question. Sarcomere length measurements can be used to normalize muscle lengths and correct for these length discrepancies. This article was designed to demonstrate potential effects of not accounting for instantaneous sarcomere length when calculating the physiological cross-sectional area of muscles of the spine region. Because some muscles of the spine region appear to be shortened and others lengthened in the neutral spine posture, both over- and under-estimations of physiological cross-sectional area are possible. Specifically, it is shown that the muscle physiological cross-sectional area could be over-estimated or under-estimated by as much as + 36% (multifidus) and -21% (rectus abdominis), respectively. This differential error effect poses difficulties in accurately estimating individual muscle forces and subsequent spine forces and stability that result from biomechanical models incorporating physiological cross-sectional area data obtained in the absence of sarcomere length measurements. Future work is needed to measure the dynamic range of sarcomere lengths of all spinal muscles to ensure correct inputs to biomechanical models.