Hypertrophic response of lower extremity muscles to different resistance training intensities: a meta-analysis and meta-regression

This research aims to detect the effectiveness of different resistance training intensities (high, moderate, and low) for quadriceps femoris muscle group hypertrophic response in healthy adults. The literature search was conducted during January 2023 using the Web of Science, SPORTDiscus, Embase, PubMed, Google Scholar, ClinicalTrials website, and grey literature databases. Using the TESTEX scale, we assessed the studies’ methodological quality. This meta-analysis was comprised of 22 studies (with 519 participants). Statistical analysis was performed using ReviewManager 5.2 (RevMan) software. This analysis showed potential benefits for all resistance training intensities on quadriceps femoris muscle group hypertrophy (muscle thickness (mm) for quadriceps muscle) compared with control (or pre-test values) groups. The data obtained are as follows, high-intensity resistance training: [Raw mean difference (RMD)= [2.3; confidence interval (CI) 95%: [2.21-2.38]]; Moderate-intensity (60-79% of 1RM) resistance training: [RMD = 1.88; (CI) 95%: [1.74- 2.02]] and low-intensity (30-59 % of 1 RM) resistance training: [RMD = 10.92; CI 95%: [10.77- 11.08]] with p<0.00001. Meta-regression analysis revealed that the non-significant direct relationship between resistance training intensity (%) and change in RF (mm) (β=0.03; p = 0.417; R2 = 0.04) and VM muscle thickness (β = 0.003; p = 0.895; R2 = 0.002). In addition, a significant direct relationship between resistance training intensity and change in VI (β=0.01; p = 0.05; R2 = 0.56) and VL muscle thickness (β = 0.01; p = 0.007; R2 = 0.34) was detected. In conclusion, studies employing different resistance training intensities showed similar hypertrophic responses in all muscles. The meta-regression analysis found that for every 10% greater difference in resistance exercise intensity between studies, the hypertrophic response in the VI muscle was determined to be 0.1 mm greater. No significant hypertrophic response was observed for VL, VM, and RF muscles.