Caruso JF, Coday MA, Taylor ST, Mason ML, Lutz BM, Ford JL, Kraemer WJ. Prediction of resultant testosterone concentrations from flywheel-based resistive exercise. Aviat Space Environ Med 2010; 81:825–32.Introduction: Numerous variables impact resultant testosterone concentrations (TC) that foretell the efficacy of workouts. Identifying variables may aid the development of in-flight exercise prescription. Methods: To identify variables that predict the variance in TC from flywheel ergometer exercise, 17 subjects did 3 workouts in a randomized order. Comprised of 10-repetition leg press sets, workouts entailed either: 1) 3 sets of both concentric and eccentric muscle actions (CE3), and concentric-only actions done for 2) three (CO3), or 3) six (CO6) sets. Venous plasma TC were collected before and at 1 and 30 min post-exercise. The last two collection points served as criterion measures. Body mass, delta blood lactate levels, peak angular velocity, average power, and total work from workouts were used to predict the variance in TC. Results: Predictor variables accounted for significant levels of variance at both 1 and 30 min post-exercise for both the CE3 and the concentric-only (CO3 and CO6 bouts combined) workouts using multivariate regression. Inclusion of eccentric variables (only collected from the CE3 bout; r² = 0.90) predicted nearly twice the variance than the concentric-only (r² = 0.54) workouts. Conclusions: Body mass and average power indices were the best predictors of the variance in post-workout TC. Since a flywheel-based device is used to abate in-flight muscle atrophy and strength losses, exercise prescriptions may wish to monitor these indices as they impacted post-workout TC to the greatest extent. Future research should assess why eccentric variables increased the amount of explained variance from flywheel ergometer workouts.