Changes in Strength and Muscle Thickness in Physically Active Males and Females in Response to Resistance Exercise Training: Comparison of Absolute and Relative Change Scores

Abstract

Smith, S, Behan, FP, Nolan, D, and Egan, B. Changes in strength and muscle thickness in physically active males and females in response to resistance exercise training: Comparison of absolute and relative change scores. J Strength Cond Res XX(X): 000–000, 2026—Investigations of between-sex differences in adaptive responses to resistance exercise training (RET) have produced equivocal results, some of which may be influenced by the statistical analyses used. This study explored whether different approaches to the analysis of change scores, and accounting for baseline differences, resulted in divergent conclusions about between-sex differences in response to RET. In a parallel group pre-post design, male subjects ( n = 22; age, 29.0 ± 6.3 years; body mass index, 25.6 ± 4.0 kg·m ⁻² ; body fat, 14.0 ± 5.3%) and female subjects ( n = 19; 30.6 ± 6.0 years; 24.9 ± 3.1 kg·m ⁻² ; 26.3 ± 5.8%) with >1 year of RET experience completed 12 weeks of twice weekly supervised, periodized, and progressive RET. Strength (kg) was assessed by one-repetition maximum, and muscle thickness (cm) was assessed by two-dimensional ultrasound measurement. Change scores were analyzed as relative change, as absolute change, and as change scores adjusted for baseline differences using analysis of covariance. Muscle thickness of both the biceps brachii (∼14%) and vastus lateralis (∼10%) muscles increased with RET, but no statistically significant differences between groups were observed. The increase in bench press strength was larger in female subjects (32.8 [26.4, 39.2] % vs. 19.9 [15.1, 24.7] %; p = 0.002 d = 1.07) when reported in relative, but not absolute, terms. The increase in hex bar deadlift strength was larger in male subjects (29.4 [21.8, 37.0] kg vs. 14.4 [10.8, 17.9] kg; p < 0.001 d = 1.15) when reported in absolute, but not relative, terms. Physiological differences between sexes that are linked to mechanisms of adaptation are likely to contribute, but which analytical approaches have been applied should be carefully considered when interpreting such observations.