Influence of sex and strength differences on the load-velocity relationship of the Smith-machine back squat

Purpose We aimed at determining whether the load-velocity relationship of the Smith machine concentric-back squat differs between sexes and persons with different levels of strength. Methods Thirty-two participants (16 males: 23.3 +/- 3.8 years and 16 females: 26.1 +/- 2.7 years) were included. Load and mean concentric velocity (MCV) were obtained individually during an incremental test (30-90% one-repetition maximum-1RM). Results We obtained strong linear relationships between the individual MCV and relative load ( R-2 > 0.95). Overall, relatively strong persons reached faster MCVs throughout the full range of relative loads (effect size [ES]: 0.48-0.93), except at 1RM. We also obtained steeper slopes and higher velocity intercepts (y intercepts) in relatively strong participants (ES: 0.05 and 0.83, respectively; p < 0.01). Finally, we found that males attained steeper load-velocity relationships (ES favoring relatively strong and weak males: 2.8 and 1.4, respectively) and higher velocity intercepts (ES favoring relatively strong and weak males: 2.0 and 1.0, respectively) than females (p < 0.0001). Conclusion MCV can be used to estimate relative load on the Smith machine concentric-back squat in males and females with different levels of muscle strength; however, the load-velocity relationship is affected by muscle strength and sex. Therefore, the extraction of individual load-velocity relationships is advisable instead of computing group estimations.