Background and aims: Skeletal muscle plays an important role in maintaining the stability of the lumbar region and reducing injury risk. Exercise training may increase strength and cross sectional area of trunk muscles, however the most effective exercise intervention for training the complex trunk musculature is currently not known. Therefore, we systematically reviewed the literature to 1) identify the effects of exercise training on trunk muscle morphology and 2) identify the exercise strategy with the largest effect. Method: A systematic search strategy was conducted in the following databases from inception to April 30, 2012: PubMed, SPORTDiscus, CINAHL, The Cochrane Library and PEDro. We assessed bias using the Cochrane risk of bias tool and estimated treatment effects with standardised mean differences (SMD) using Review Manager v5.1 (software). Results: The systematic search identified 1910 studies; SportDiscus (n = 597), PubMed (n = 595), CINAHL (n = 495), The Cochrane Library (n = 143) and PEDro (n = 80). Ultimately, 28 studies were included for qualitative synthesis. The most common source of bias involved random sequence generation, allocation concealment and blinding. We found the largest effect for motor control exercise (SMD - 0.02; 95% CI - 0.24, 0.19 to SMD 2.93; 95% CI 2.34, 3.52), and no effect for cardiovascular exercise (SMD - 0.03; 95% CI - 0.33, 0.26 to SMD - 0.02; 95% CI -0.97, 0.92). Discussion: Few exercise interventions altered trunk muscle morphology while 80% demonstrated no change. Motor control exercise had the largest effect, whereas, cardiovascular exercise had no effect. However, these results should be interpreted with caution owing to the potential for bias and suboptimal reporting of exercise details in the included studies. Conclusion: Given that most studies to date have potentially important methodological limitations and lack explicit exercise descriptions, additional high quality research is needed to identify the effects of exercise training on trunk muscle morphology

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