Introduction: Endurance (EET) and resistance exercise training (RET) each initiate specific physiological adaptions; EET enhances aerobic capacity(1) and RET muscle mass and strength(2). Yet both promote similar health benefits e.g. in insulin sensitivity and blood pressure. Despite it being known that adaptive responses to both EET and RET vary markedly among individuals(3,4), how, a) the patterns of intra-individual adaptations to distinct exercise modes vary, and; b) the core physiological adaptations to EET/RET relate to health benefits, is unknown.
 
Method: We conducted a 14-week randomised cross-over trial (4-weeks RET/EET; 6-week wash-out; then 4-weeks EET/RET) in (n=16) young male subjects (23.4±4.6 years). Aerobic capacity and muscle mass/strength were measured before and after each training cycle. Paired t-tests determined primary adaptive responses, as above. Two-way ANOVA was used to assess clinical health-related changes. Linear correlation and linear discriminant analysis (LDA) were used to associate changes in VO2max and appendicular lean body mass (ALBM) and to identify biomarkers predicting training responses, respectively. Individuals were categorized as responders or non-responders according to calculated cut-off values (typical error). P<0.05 was considered significant. Data are mean±SD.
 
Result: As expected, the 4-weeks EET enhanced aerobic capacity (VO2max: 43.2±9.7 vs. 46.2±9.2 ml/kg/min, P<0.001), while 4-weeks RET increased lean body mass (ALBM: 26.1±4.4 vs. 26.8±4.2 kg, P<0.01). The health benefits of both types of exercise were seen on body composition variables; e.g. both EET and RET reduced total body (EET: 22.5±6.2 vs. 21.8±6.2%, P<0.05; RET: 22.7±6.0 vs. 21.8±5.9%, P<0.01) and abdominal fat % (EET: 23.0±10.2 vs. 21.6±10%; RET: 23.7±9.5 vs. 22.3±10%, both P<0.05). Notably, there was a high probability (~85%) for a RET “non-responder” to be a responder to EET. In contrast, responders to RET had low probability (~22%) of being a responder to EET. Mixed model logistic regression confirmed these relationships with <1 odds ratio for individuals to respond to both training modes (0.046, P<0.05). In terms of health-related outcomes, ~63% of the non-responders to EET (of 8) demonstrated improvement in at least one of the three health parameters (diastolic blood pressure (DBP), HOMA-IR, fat %). Conversely, ~86% of the non-responders to RET (of 7) demonstrated no improvement in any. Finally, LDA analysis suggested reduction in abdominal fat was the most defining feature in individuals’ response to EET, while changes in insulin sensitivity and body fat were the best predictors of response to RET.         
 
Conclusion: Four weeks of EET/RET elicited predicted mean exercise-mode adaptions i.e., aerobic capacity gains with EET and muscle mass gains with RET, while promoting improvements in health-related parameters such as in DBP and body composition even in healthy individuals. Strikingly, individuals showed favour for adaptation to one exercise mode over another i.e., it is unlikely a given individual be a responder to both modes. Intriguingly, health benefits were observed with EET regardless of improvement in aerobic capacity, while gaining muscle mass seems essential to promote health benefits by RET. Finally, changes in biomarkers such as abdominal fat and insulin sensitivity can help predict an individual’s responses to EET/RET, respectively.