## 4 Discussion  
To the authors' knowledge, this systematic review with meta-analyses is the first screening of RCTs for the effects of all types of exercises on the architecture of upper extremity muscles. This systematic review with meta-analyses aimed to overview the effects of exercise interventions on improving the architecture of upper extremity muscles. The meta-analyses of this systematic review revealed that most exercise interventions with at least 4-weeks of exercise duration showed large effect sizes for increasing the size of individual upper extremity muscle or muscle groups (Figures 25-28). In summary, the following exercises showed large effects on increasing the size of the targeted muscles: high-intensity concentrically-biased bench press training for the pectoralis major; lying concentrically-biased triceps extension, isometric maximal voluntary co-contraction training, and nonlinear periodised resistance training for the triceps brachii; traditional concentric elbow flexion exercise, low-load concentric forearm flexion-extension training without and with blood-flow restriction, isometric maximal voluntary co-contraction training, and nonlinear periodised resistance training for the biceps brachii; and isometric ulnar deviation training for the flexor carpi ulnaris and radialis. Figure 23: The effect size of isometric maximal voluntary co-contraction training on the elbow flexors MT. References for the studies in the table with the same publication year are [PERSON] et al. (2014) [83], [PERSON] et al. (2014) [84]. Figure 24: The effect size of 6-weeks of isometric ulnar deviation training [78] on flexor carpi ulnaris and radialis MT of the trained right and left hands. The first line represents the right hand (dominant) trained group, and the second line represents the left hand (non-dominant) trained group. Figure 26: The effect size of the exercise interventions on the elbow extensor muscle thickness, cross-sectional muscle area and muscle volume. The red colour indicates a small or trivial effect size, the yellow colour indicates a medium effect size, and the green colour indicates a large effect size. Abbreviations: CSA, cross-sectional area; MT, muscle thickness; MV, muscle volume. Figure 25: The effect size of the exercise interventions on the pectoralis major muscle cross-sectional area and volume. The yellow colour indicates a medium effect size, and the green colour indicates a large effect size. Abbreviations: CSA, cross-sectional area, MV, muscle volume. In addition to the training modalities included in the meta-analyses, six RCTs [75; 76; 80; 81; 82; 87] were not included in the meta-analyses due to missing outcome data. The findings and intervention characteristics of the RCTs are presented in Supplementary Table S3. Among these RCTs, [PERSON] et al. [85] investigated the effects of a nonlinear periodised strength training program on biceps brachii and triceps brachii MT and the triceps brachii long head PA and reported significant alterations in the outcome measures depending on the arm sites. The study of [PERSON] et al. [85] was the only RCT that measured the PA of a muscle, which is a fascicle geometry component, among the eligible RCTs. The triceps brachii long head PA was significantly correlated with the strength parameters of the elbow extensors [45]. By comparison, the triceps brachii long head FL was one of the best predictors of a better swimming performance [36] and significantly correlated with lifting performance [17]. However, there was no RCT that investigated the effects of an exercise intervention on the FL of triceps brachii long head.