Refuting the myth of non-response to exercise training: 'non-responders' do respond to higher dose of training

One in five adults following physical activity guidelines are reported to not demonstrate any improvement in cardiorespiratory fitness (CRF). Herein, we sought to establish whether CRF non-response to exercise training is dose-dependent, using a between-and within-subject study design. Seventy-eight healthy adults were divided into five groups (1-5) respectively comprising one, two, three, four and five 60 min exercise sessions per week but otherwise following an identical 6-week endurance training (ET) programme. Non-response was defined as any change in CRF, determined by maximal incremental exercise power output (W-max), within the typical error of measurement (+/- 3.96%). Participants classified as non-responders after the ET intervention completed a successive 6-week ET period including two additional exercise sessions per week. Maximal oxygen consumption ((V) over dotO(2max)), haematology and muscle biopsies were assessed prior to and after each ET period. After the first ET period, Wmax increased (P < 0.05) in groups 2, 3, 4 and 5, but not 1. In groups 1, 2, 3, 4 and 5, 69%, 40%, 29%, 0% and 0% of individuals, respectively, were non-responders. After the second ET period, non-response was eliminated in all individuals. The change in (V) over dotO(2max) with exercise training independently determined Wmax response (partial correlation coefficient, r(partial) >= 0.74, P< 0.001). In turn, total haemoglobin mass was the strongest independent determinant of (V) over dotO(2max) (r(partial) = 0.49, P < 0.001). In conclusion, individual CRF non-response to exercise training is abolished by increasing the dose of exercise and primarily a function of haematological adaptations in oxygen-carrying capacity.