Biomechanics of the squat exercise using a modified center of mass bar.

The purpose of the study was to investigate the effects of load height on selected performance characteristics of a squat exercise. A lower center of mass bar was designed that allowed the integrity of the squat exercise to be maintained while possibly reducing the chances of injury. Five trials were performed with the center of mass of the bar was set at shoulder height (C1) and lowered 18% (C2) and 36% (C3) of the subject's height below the normal bar position using the inverted "U" bar. All trials were filmed as the subjects lifted on a force platform. A balloon catheter was inserted into the subject's recta to monitor intra-abdominal pressure (IAP). High correlations were found between IAP, joint moment, and force data. Many of the critical parameters occurred just after the lowest squat position. Significant differences (P less than 0.05) in trunk angle excursion and trunk angular velocity indicated a greater ease of hip extension for the center of mass bar conditions. No differences were observed between conditions for thigh and knee angles and joint moments indicating kinematic similarity for the lower extremity. IAP was always least for C2 and C3, while compression, shear, and back muscle forces did not differ. It was estimated that the greater IAP was responsible for relieving back muscle forces and compression by up to 15 and 21%, respectively, and increased stress with the weight at shoulder height stimulated a response for greater IAP to help alleviate the stresses on the spine.