In addition to the training modalities included in the meta-analyses, six RCTs
[50][51][52][53][54][55] were not included due to missing outcome data. Among these RCTs, Matta et al.
[56] 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 Matta et al.
[56] 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 parameters
[17]. However, there was no RCT that investigated the effects of an exercise intervention on the FL of triceps brachii long head. A
lthough reit did not meet the inclusion criteria of this systematic review, a recent uncontrolled trial
[57] compared the effects of concentrically-biased cable push-down and cable overhead extension exercises, and Stasinaki and colleagues
[57] did not report significant alterations in the FL of the triceps brachii long head even when the concentric elbow extension starts from a fascicle lengthened position. This may be due to the effects of concentric training. A future RCT should examine the impacts of eccentric training on the FL of triceps brachii long head.
In terms of the muscle size parameters, the triceps brachii MT has been found to be strongly correlated with elbow extension strength
[41]. Additionally, the triceps brachii MT was stated as being significantly correlated with better swimming performance (r = −0.56)
[36]. Moreover, elbow extensors’ and flexors’ muscle size parameters (ACSA, PCSA and MV) showed significant strong correlations with elbow joint torque (r = 0.705–0.945)
[39]. Furthermore, the elbow extensors’ cross-sectional muscle area (CSA) was correlated with rowing performance, and was the significant best predictor of arm pull during the rowing activity in rowers (r
2 = 0.195)
[37]. Elbow flexors CSA showed a strong correlation with elbow flexion maximal power (r = 0.81)
[40]. Arm muscles CSA was significantly correlated with shot put performance (r = 0.68)
[38]. The pectoralis major muscle CSA was strongly correlated with bench press strength (r = 0.866), and muscle volume was strongly correlated with bench throw peak power (r = 0.821)
[43]. Either concentric, isometric, eccentric or blood-flow restricted resistance training modalities led to significant muscle hypertrophies. Based on these findings, athletes, healthy individuals aiming to increase their related performance or muscle strength parameters, astronauts after a space mission
[58] and patients experiencing muscle atrophies after bedrest
[59][60], which were mentioned above, may refer to the training regimens that showed large effects sizes on increasing the pectoralis major, arm and forearm muscles’ size parameters. However, exercise selection should cautiously be made due to the small numbers of studies included in each meta-analysis.
Additionally, the infraspinatus MT was significantly correlated with shoulder external rotation strength in professional baseball pitchers (r = 0.287)
[44]. The subscapular MT was the best single predictor for powerlifting performance in professional powerlifters
[17]. However,
th
ereinis systematic review did not detect any RCTs focusing on exercise-induced alterations in these muscle architectural parameters. Future RCTs may be conducted to investigate exercise-induced alteration in these muscle architectural parameters in the relevant samples, such as exercise-induced alterations in the infraspinatus MT in baseball pitchers, in the subscapular MT in powerlifters, and in the fascicle geometry of the triceps brachii in swimmers.
3. Conclusions
Regarding the pectoralis major muscle size, 6-weeks of high-intensity bench press training
[61] and 10 weeks of 12 RM bench press exercises
[54] can be applied for hypertrophy in this muscle. To achieve hypertrophy in elbow extensors, 6-weeks of lying triceps extension exercise
[51], isometric maximal voluntary co-contraction training
[62][63], and 12-weeks of nonlinear periodised resistance training
[64] may be a suitable intervention. From the perspectives of elbow flexors, 6-weeks of traditional elbow flexion exercises
[65], 4-weeks of concentric low-load forearm flexion-extension training
[66], isometric maximal voluntary co-contraction training
[62][63], 4-weeks of concentric low-load forearm flexion-extension training with vBFR
[66], or 12-weeks of nonlinear periodised resistance training
[67][68] can be applied to gain hypertrophies in the elbow extensors. Finally, 6-weeks of isometric ulnar deviation training can be used to increase the flexor carpi ulnaris and radialis muscle size
[69].
However, these results should be cautiously interpreted due to the small numbers of the RCTs included in each meta-analysis. More RCTs are needed to provide more precise and more robust conclusions about the effects of exercise on the architecture of the upper extremity muscles. Additionally, all the eligible studies of this systematic review were restricted to muscle size measurements, and not did not expand towards the fascicle geometry such as the FL of the triceps brachii long head. Future RCTs can examine the effects of exercise on the triceps brachii FL and PA, the infraspinatus MT and the subscapular MT, due to their associations with sports performance.