This study was designed to assess the contribution of the lumbar posterior ligaments in generating trunk extensor moment during extremely heavy lifts performed by national class powerlifters. Lumbar vertebrae kinematics were obtained from videofluoroscopy utilizing both image and signal processing techniques to enhance accuracy. This method included multiple digitizing (4 times per frame), correction for optical distortions and digital filtering (Butterworth filter with a cutoff frequency of 3 Hz). Four vertebral corners in the sagittal plane were used as markers and were digitized at a sampling rate of 30 Hz.

Two separate experiments were conducted. The first consisted of the quantification of error in the processed images obtained from prepared physical models of vertebrae. The mean absolute measurement error of the data collecting and processing system was estimated to be 0.69° (SD = 0.43°) for rotational and 0.33 mm (SD = 0.25 mm) for linear displacements.

In a second experiment, images of the lumbar spines were collected from four experienced powerlifters. Three trials were performed, approximately 6 seconds in duration each, resulting in about 72 mA.s of total radiation exposure (equal approximately to one conventional X-ray exposure). In the first trial, subjects fully flexed their spines to create myoelectric silence in the extensor musculature to facilitate measurement of the joint angles at which passive tissues are responsible for supporting the extensor moment. Next, two trials 183.7 kg to 210.9 kg. The relative intervertebral joint angles, distance between the ligament attachment points, shear and compressive displacements and instantaneous centres of rotation were calculated from the rigid body motion approach, for the full flexion and lifting trials.

Analysis revealed that except for one trial of one subject, they accomplished their lifts with the amount of lumbar flexion between 1.5° to 13° less than they demonstrated in the full flexion trial. The ligament lengths at the beginning of the lifts were on average between 78% and 98% of their fully flexed length. Therefore, it was concluded that ligaments did not strain sufficiently to contribute substantially to the trunk extensor moment and it would appear that the posterior ligament sprain injury is not likely to occur in powerlifting. Shear or compressive translations were not detected. Instantaneous centres of rotation exhibited the large scatter due to measurement error, preventing meaningful conclusions to be drawn.