Context: Training and assessment of the abdominal and trunk muscles are widely used in the clinical setting. However, it is unknown what types of exercises are most effective in activation of both the global and local stabilizers in these regions. Objective: The purpose of this study was to establish the reliability of a novel clinical screening tool (sling screen) to assess the muscles of the abdomen and trunk. The second aim was to use the clinical screening tool and musculoskeletal ultrasound to compare the effects of a rotary-based exercise program that targets both the global and local muscles to the effects of a traditional exercise program on the activation of the abdominal and trunk muscles. Design: Double-blind, randomized controlled trial. Setting: Sports medicine facility. Participants and Interventions: Thirty-one healthy participants were randomly allocated to receive a single-session rotary-based or traditional “core” exercise program. Main Outcome Measures: The participants were assessed at the baseline and immediately postintervention. The primary outcome measures were muscle thickness examined by musculoskeletal ultrasound and clinical examination of muscle activation using a screening tool. The data were collected by blind assessors. Reliability and validity of a clinical screening tool (sling screen) were also assessed. Results: The analysis of the covariance tests showed a significant increase in oblique thickness for the rotary exercise group. All participants displayed a significant increase in multifidus thickness. The Wilcoxon signed-rank tests revealed a significant increase in clinical assessment scores in the rotary exercise group but not the traditional exercise group. Reliability of the sling screen ranged from moderate to good. Conclusion: This clinical trial provides evidence that a rotary-based exercise program may be more effective in producing increases in oblique muscle thickness than traditional “core” exercises in young, healthy adults. The sling screen tool was able to identify these muscle thickness changes. Future studies should investigate how these results correlate to injury risk, other populations, and also how to implement the sling screen into clinical practice.