Repeated Resistance Training Reveals the Reproducibility of Muscle Strength and Size Responses Within Individuals

This study investigated whether the responses of upper and lower limb muscle strength and size to resistance training (RT) are reproducible across two RT periods. Untrained males and females (age 32 ± 5 years) were randomly assigned to RT (n = 20) or a control group (n = 27). The RT group completed two identical 10-week RT periods separated by 10 weeks of detraining. The control group underwent a 10-week non-RT period. Before and after each period, the muscle cross-sectional area (CSA) of the vastus lateralis (VL) and biceps brachii (BB) was measured by ultrasound. One-repetition maximum (1RM) was measured in leg press (LP) and biceps curl (BC). Minimal detectable changes were determined. The RT group showed greater gains (p < 0.001) in muscle size and strength than the control group, with variability observed among individual responses. The reproducibility of responses within-participants was demonstrated by correlations (p ≤ 0.001) between the first and second RT periods in VLCSA (r = 0.697), BBCSA (r = 0.761), and LP1RM (r = 0.671), with a trend for BC1RM (r = 0.393 and p = 0.095). Nonresponders were identified, but none were detected in both RT periods for more than one variable. First RT correlated negatively (p < 0.05) with subsequent detraining in BBCSA (r = -0.673) and LP1RM (r = -0.488), with a trend for VLCSA (r = -0.422 and p = 0.072). In conclusion, responses to RT are reproducible when RT is repeated, indicating that the individuality of the training response has a physiological origin. Nonresponsiveness is rare and should not be justified solely by one variable. Responses to detraining suggest that greater RT gains may also diminish faster.