BACKGROUND: Limited evidence exists to inform how strength and endurance exercises commonly used to prevent low back pain affect muscle morphometry and endurance. OBJECTIVE: The purpose of this study was to analyze the effects of two exercise regimens on the morphometry and endurance of key trunk musculature in a healthy population. DESIGN: Randomized controlled trial. SETTING: Military training setting. PARTICIPANTS: A random subsample (n = 340, 72% male, 21.9 +/- 4.2 years, 24.8 +/- 2.8 kg/m2) from the larger Prevention of Low Back Pain in the Military trial (n = 4,325) were included. INTERVENTION: The core stabilization exercise program (CSEP) included low load/low repetition motor control exercises while the traditional exercise program (TEP) included exercises conducted fast using a high load/high repetition trunk strengthening exercises. MEASUREMENTS: Baseline and follow-up examination included ultrasound imaging of the trunk muscles and endurance tests. Linear mixed models were fitted to study the group and time effect and their interactions, accounting for the clustering effect. RESULTS: Symmetry generally improved in the rest and contracted states, but there were no differences suggestive of muscle hypertrophy or improved ability to contract the trunk muscles between Soldiers receiving CSEP or TEP. Total trunk endurance time decreased over the 12-week period; but endurance performance favored those Soldiers in the CSEP group (p <= 0.05). Endurance time was not associated with future episodes of LBP. LIMITATIONS: The lack of morphological changes may not be detectable in an already active cohort or a more intensive dose was needed. CONCLUSIONS: Although improved symmetry was noted, neither the CSEP or TEP resulted in muscle hypertrophy. Longer endurance times were noted in individuals who completed CSEP but were not strongly predictive of future low back pain episodes.

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