Several weeks of systematic resistance exercise, termed resistance training (RT), increases muscle size and strength in both younger and older adults and is recognized as a key measure towards combatting age-related neuromuscular decline. Considerable inter-individual variation exists, however, in the adaptations to RT. Whether this inter-individual variation differs between younger and older adults has not been extensively studied. Further, whether baseline characteristics such as pre-training muscle size and strength can predict individual responses to RT is not definitively known. The purpose of this study was to determine the magnitude of inter-individual variability in responses to prolonged RT, whether it differs between younger and older adults, and whether pre-training characteristics are related to an individual’s response to RT.

Data from three previous studies in untrained younger men and/or older men and women who performed 6-12 months of supervised, progressive, whole-body RT were pooled for this retrospective analysis. Participants (n = 156) were divided into a younger group (YOUNG, n = 65, 31.6 ± 7.0 years) and an older group (OLD, n = 91, 69.2 ± 2.7 years). Measurements of muscle size – vastus lateralis cross-sectional area (VLCSA) via ultrasound – and strength – quadriceps maximal voluntary contraction (QMVC) by isometric dynamometer – were completed pre- and post-intervention.

Relative changes in VLCSA (ΔVLCSA) were greater in YOUNG (12.8 ± 9.3 %, range: -6.0 to +40.7 %) compared to OLD (5.3 ± 13.0 %, range: -19.5 to +49.9 %) (p < 0.001). Ten YOUNG participants (15 %) were classified as muscle size non-responders (post-testing score below the smallest worthwhile change of the measurement) compared to 58 OLD participants (64 %). There was a significant difference in the variability of muscle size changes between YOUNG and OLD (p = 0.014). Relative change in QMVC (ΔQMVC) did not differ between YOUNG (6.4 ± 17.3 %, range: -22.8 to +57.2 %) and OLD (9.2 ± 17.2 %, range: -43.0 to +67.8 %) (p = 0.321). Twenty-seven YOUNG participants (42 %) were classified as muscle strength non- responders compared to 26 OLD participants (29 %). The variability in muscle strength changes did not differ between YOUNG and OLD (p = 0.802). Smaller pre-training VLCSA was related to greater ΔVLCSA in YOUNG (r = -0.308, p = 0.012) but not OLD (r = -0.044, p = 0.679). Lower pre-training QMVC was related to greater ΔQMVC in both YOUNG (r = -0.353, p = 0.004) and OLD (r = -0.283, p = 0.007).

This investigation shows the considerable heterogeneity that exists in the muscle size and strength adaptations to RT. Older adults appear to exhibit diminished and more variable muscle size but not strength responses to RT compared to younger adults. This indicates that RT prescriptions aimed at maximizing muscle growth may need to be differentiated for older populations. Additionally, pre-training values are only weakly correlated to the RT-induced changes in muscle size and strength indicating that many other factors contribute to the inter- individual variability in muscle size and strength responses to RT.