Correcting trunk muscle geometry obtained from MRI and CT scans of supine postures for use in standing postures

Many studies have reported the geometry of the trunk muscles for use in models to predict spine loads. However, all have reported data collected from subjects positioned in a prone or supine posture which is required by CT or MRI scan techniques. This study compared the abdominal and extensor muscle moment arms obtained with muscles relaxed and activated and in both supine and upright postures. Both internal ultrasound measurements and external anthropometer measurements were collected. The anterior abdominal wall increased its flexor moment arm by approximately 30% when in a standing posture compared to the supine posture. The effect of an upright posture on the moment arm of erector spinae at the level of L3 was much smaller (3% increase in males and 12% increase in females). There was no moment arm change due to activating the muscles when compared to the relaxed condition for either posture. Modelers should be aware of the need to scale muscle moment arms obtained from MRI/CT scans in order to more accurately predict spine loads in upright postures.