Assessing the Impact of Unilateral and Bilateral Resistance Training on Strength Development and Bilateral Index

Purpose: The purpose of this study was to investigate the effects of unilateral (UL) and bilateral (BL) resistance training on muscular strength, bilateral index (BLI), and body composition. Methods: Fourteen recreationally resistance-trained individuals (mean ± SD; age = 21.8 ± 2.6 years; BMI = 25.3 ± 3.2 kg/m²) were randomized into UL (n=8) and BL (n=6) groups. The training protocol consisted of two lower-body and two upper-body compound exercises, followed by two lower-body and two upper-body isolation exercises, performed for 3 sets of 6-8 repetitions for compound exercises and 10-12 repetitions for isolation exercises, 3 times per week for 8 weeks. A 2×2 mixed ANOVA was conducted to analyze differences between time points and training groups. Results: Pre- and post-intervention assessments included one-repetition maximum (1RM) for UL and BL leg and chest press exercises, BLI, and body composition. Both UL and BL training modalities resulted in significant increases in lower-body muscular strength (p < 0.01) and BLI for the leg press (p < 0.05), with no significant differences between groups. Upper-body strength improvements were not statistically significant. Body composition changes were similar across groups, though a significant interaction was found for percent body fat reduction (p < 0.05). Conclusions: These findings suggest that both UL and BL training are effective for enhancing lower body strength and BLI, with comparable effects on body composition. Integrating both modalities into resistance training programs may enhance strength development, address muscle imbalances, and improve overall fitness and performance.