A study on the effect of 12-week unstable bench press barbell training on improving upper limb muscle strength of...

This study aimed to explore the effect of 12-week unstable bench press barbell training on improving upper limb muscle strength in college students and analyze the potential mechanisms of strength changes induced by training. Eighteen healthy male college student volunteers with no upper limb strength training experience were randomly divided into an unstable bench press training group (IBP group, n=9) and a Smith machine bench press training group (SBP group, n=9). The two groups received 3 sessions of bench press training per week for 12 consecutive weeks under unstable and stable conditions, respectively. The IBP group trained with 55%→65%→75% 1RM across three 4-week phases, while the SBP group trained with 65%→75%→85% 1RM, reflecting the greater coordinative and postural demands imposed by the unstable condition. The rationale for using lower nominal loads in the unstable condition was that instability inherently increases neuromuscular and postural control demands, thereby equating the effective physiological training stimulus between conditions. Before and after the training, indicators including upper arm muscle circumference, 1 repetition maximum (1RM), maximal voluntary isometric contraction (MVIC), unstable bench press movement stability were tested, and surface electromyography (sEMG) signals of biceps brachii (BB), triceps brachii (TB), anterior deltoid (AD), posterior deltoid (PD) and pectoralis major (PM) were collected during the unstable bench press test. The results showed that only the SBP group had a significant increase in relaxed upper arm muscle circumference after training (P<0.05). Both groups achieved significant improvements in 1RM and MVIC after 12 weeks of training (P<0.05). Group × time interaction was significant for 1RM (F = 4.36, p=0.033, d=0.82, 95% CI = 1.24 to 18.67), indicating that IBP improved significantly more than SBP. For MVIC, the group × time interaction was not significant (F = 0.06, p=0.809), suggesting comparable static strength gains between groups. The acceleration integral value of the IBP group on the X-axis in the unstable bench press test was significantly smaller than that of the SBP group, with significant group × time interactions on the X-axis (p=0.037) and Y-axis (p=0.049). In the post-training unstable bench press test, the integrated electromyography (iEMG) of TB, AD, PD and PM in the IBP group and TB, AD in the SBP group decreased significantly (P<0.05), and only the PD-PM co-activation ratio in the IBP group decreased significantly after training (P<0.05). In the Smith machine bench press fatigue test, the IBP group completed significantly more repetitions than the SBP group after training (p<0.05). It is concluded that both 12-week unstable and stable bench press barbell training can significantly improve the upper limb muscle strength of college students, and unstable bench press training demonstrates advantages over stable Smith machine training for dynamic strength and movement stability. Unstable bench press training may improve agonist contraction ability and muscle work efficiency, accompanied by plausible neuromuscular adaptations in coordination and antagonist muscle regulation. However, interpretations should be tempered by the task-specific testing design, which may favor the unstable training group.