The purpose of this study was 1) to provide normative data for isokinetic trunk strength as measured by peak torque, best work repetition, and a new composite variable, “muscle performance index” and 2) to examine the effects of protocol velocity, subject age, gender, height, and weight on these measurements. A significant decrease in peak torque of extension for females (P < 0.0001) was seen as protocol velocity increased, but no differences were seen in males. A statistically significant amount (0.5-0.7) of the variance in isokinetic measurements was explained by subject age, height, and weight. Multivariant analysis of covariance showed the effects of these variances to be significant on all measurements except for the muscle performance index, which was unaffected by protocol velocity and remained stable without adjustments for subject age, weight, or height.