Background Skeletal muscle plays an important role in maintaining the stability of the lumbar region. However, there is conflicting evidence regarding the effects of exercise on trunk muscle morphology. Objective To systematically review the literature on the effects of exercise training on lower trunk muscle morphology to determine the comparative effectiveness of different exercise interventions. Data Source and Study Selection A systematic search strategy was conducted in the following databases: Pub-Med, SportDiscus, CINAHL, the Cochrane Library and PEDro. We included full, peer-reviewed, prospective longitudinal studies, including randomized controlled trials and single-group designs, such as pre- to post-intervention and crossover studies, reporting on the effect of exercise training on trunk muscle morphology. Study Appraisal and Synthesis Study quality was assessed with the Cochrane risk-of-bias tool. We classified each exercise intervention into four categories, based on the primary exercise approach: motor control, machine-based resistance, non-machine-based resistance or cardiovascular. Treatment effects were estimated using within-group standardized mean differences (SMDs). Results The systematic search identified 1,911 studies; of which 29 met our selection criteria: motor control (n = 12), machine-based resistance (n = 10), non- machine-based resistance (n = 5) and cardiovascular (n = 2). Fourteen studies (48 %) reported an increase in trunk muscle size following exercise training. Among positive trials, the largest effects were reported by studies testing combined motor control and non-machine-based resistance exercise (SMD [95 % CI] = 0.66 [0.06 to 1.27] to 3.39 [2.80 to 3.98]) and machine-based resistance exercise programmes (SMD [95 % CI] = 0.52 [0.01 to 1.03] to 1.79 [0.87 to 2.72]). Most studies investigating the effects of non-machine-based resistance exercise reported no change in trunk muscle morphology, with one study reporting a medium effect on trunk muscle size (SMD [95 % CI] = 0.60 [0.03 to 1.16]). Cardiovascular exercise interventions demonstrated no effect on trunk muscle morphology (SMD [95 % CI] = -0.16 [-1.14 to 0.81] to 0.09 [-0.83 to 1.01]). Limitations We excluded studies published in languages other than English, and therefore it is possible that the results of relevant studies are not represented in this review. There was large clinical heterogeneity between the included studies, which prevented data synthesis. Among the studies included in this review, common sources of potential bias were random sequence generation, allocation concealment and blinding. Finally, the details of the exercise parameters were poorly reported in most studies. Conclusion Approximately half of the included studies reported an increase in lower trunk muscle size following participation in an exercise programme. Among positive trials, studies involving motor control exercises combined with non-machine-based resistance exercise, as well as machine-based resistance exercises, demonstrated medium to large effects on trunk muscle size. Most studies examining the effect of non-machine-based resistance exercise and all studies investigating cardiovascular exercise reported no effect on trunk muscle morphology. However, these results should be interpreted with caution because of the substantial risk of bias and suboptimal reporting of exercise details in the included studies. Additional research, using methods ensuring a low risk of bias, are required to further elucidate the effects of exercise on trunk muscle morphology.

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