Modelling inter-individual variability in acute and adaptive responses to interval training: insights into exercise intensity normalisation

Purpose: To investigate the influence of exercise intensity normalisation on intra- and inter-individual acute and adaptive responses to an interval training programme. Methods: Nineteen cyclists were split in two groups differing (only) in how exercise intensity was normalised: 80% of the maximal work rate achieved in an incremental test (%Ẇmax) vs. maximalsustainable work rate in a self-paced interval training session (%Ẇmax-SP). Testing duplicates were conducted before and after an initial control phase, during the training intervention, and at the end, enabling the estimation of inter-individual variability in adaptive responses devoid of intra-individual variability. Results: Due to premature exhaustion, the median training completion rate was 88.8% for the %Ẇmax group, but 100% for the %Ẇmax-SP group. Ratings of perceived exertion and heart rates were not sensitive to how intensity was normalised, manifesting similar inter-individual variability, although intra-individual variability was minimised for the %Ẇmax-SP group. Amongst six adaptive response variables, there was evidence of individual response for only maximal oxygen uptake (standard deviation: 0.027 L· min-1 · week1 ) and self-paced interval training performance (standard deviation: 1.451 W· week-1 ). However, interindividual variability magnitudes were similar between groups. Average adaptive responses were also similar between groups across all variables. Conclusions: To normalise completion rates of interval training, %Ẇmax-SP should be used to prescribe relative intensity. However, the variability in adaptive responses to training may not reflect how exercise intensity is normalised, underlining the complexity of the exercise dose-adaptation relationship. True interindividual variability in adaptive responses cannot always be identified when intra-individual variability is accounted for.