Fitness and strength responses to distinct exercise modes in twins: Studies of Twin Responses to Understand Exercise as a THerapy (STRUETH) study.

Exercise response is idiosyncratic, although the degree of responsiveness, concordance in response between modalities and genetic contribution to responsiveness are not well understood. We investigated this using a novel randomized cross-over design of dichotomous exercise interventions in mono-(MZ) and di-zygotic (DZ) twin pairs. We studied strength (1RM) and fitness ( V ̇ O 2 max ) responses in 84 same-sex untrained twins (30 MZ, 12 DZ pairs; 24.9 ± 5.4 years). Twins, as pairs, underwent 3 months of resistance (RES) and endurance (END) training, separated by a 3 month washout period. Training responses and genetic/environmental contributions to responses were assessed. Leg strength 1RM increased following RES but not END (△47 ± 29 vs. 3 ± 26 kg; P < 0.001), whereas V ̇ O 2 max increased following END but not RES (△0.25 ± 0.26 vs. 0.04 ± 0.25 L min-1 ; P < 0.001). A higher percentage of individuals responded to RES for strength and to END for V ̇ O 2 max (P < 0.0001). Within-individual responses to each mode were not correlated (P > 0.05). Cross-sectional intraclass correlations were higher for MZ than DZ pairs for all variables, largely as a result of shared environment. Following training, MZ, but not DZ pairs, were significantly correlated for strength change to RES (rMZ = 0.62, P = 0.002) and END (rMZ = 0.36, P = 0.04), and for V ̇ O 2 max change to END (L min-1 , rMZ = 0.45, P = 0.02) with a mixture of unshared/shared environmental contributions. Our findings indicate that individual responsiveness differs between modalities and low-responders to one mode may be 'rescued' by switching to an alternate mode. Additionally, genes may not play as large a role as previously estimated from cross-sectional data for training adaptation, and/or cross-sectional data do not reflect longitudinal training effects. The present study has implications for optimizing the individualization of exercise prescription.