Reproducibility of trunk isoinertial dynamic performance in patients with low back pain.

The purpose of this study was to assess consistency of response in repeated measurements of isoinertial trunk performance among patients with low back pain. An underlying assumption was that inconsistency can detect lack of effort. Ninety-two patients, 53 men and 39 women, were evaluated using a multiaxis isoinertial dynamometer. The patients were asked to perform with maximum effort against 25 and 50% maximum isometric torque in each plane of motion. Torque, velocity, and range of motion were measured in repeated trials. Correlations between first and second trials were extremely high in all cases even through the range of performance was diverse. Percentage differences between first and second trials were found to be approximately normally (Gaussian) distributed. On the basis of normality, we calculated limits for the percentage difference between first and second trials, within which patients with low back pain would be expected to fall 95% of the time. These limits, presented separately for men and women, are given for each combination of measurements of torque, velocity, and range of motion; plane of motion; and two levels of resistance. Resistance was measured at 25 and 50% of maximum isometric. The limits may be used as guidelines to assess maximum performance for patients with low back pain.