Physiological and Performance Adaptations to Interval Training in Endurance-Trained Cyclists: An Exploratory Systematic Review and Meta-Analysis

Background: In endurance cycling, both high-intensity interval training (HIIT) and sprint interval training (SIT) have become popular training modalities due to their ability to elicit improvements in performance. Studies have attempted to ascertain which form of interval training might be more beneficial for maximising cycling performance as well as a range of physiological parameters, but an amalgamation of results which explores the influence of different interval training programming variables in trained cyclists has not yet been conducted. Objective: The aims of this study were to: (1) systematically investigate training interventions to determine which training modality, HIIT, SIT or low- to moderate-intensity continuous training (LIT/MICT), leads to greater physiological and performance adaptations in trained cyclists; and (2) determine the moderating effects of interval work-bout duration and intervention length on the overall HIIT/SIT programme. Data Sources: Electronic database searches were conducted using SPORTDiscus and PubMed. Study Selection: Inclusion criteria were: (1) at least recreationally-trained cyclists aged 18–49 years (maximum/peak oxygen uptake [V̇O2max/V̇O2peak] ≥45 mL·kg-1·min-1); (2) training interventions that included a HIIT or SIT group and a control group (or two interval training groups for direct comparisons); (3) minimum intervention length of 2 weeks; (4) interventions that consisted of 2–3 weekly interval training sessions­. Results: Interval training leads to small improvements in all outcome measures combined (overall main effects model, SMD: 0.33 [95%CI = 0.06 to 0.60]) when compared to LIT/MICT in trained cyclists. At the individual level, point estimates favouring HIIT/SIT were negligible (Wingate model: 0.01 [95%CI = -3.56 to 3.57]), trivial (relative V̇O2max/V̇O2peak: 0.10 [95%CI = -0.34 to 0.54]), small (absolute V̇O2max/V̇O2peak: 0.28 [95%CI = 0.15 to 0.40], absolute maximum aerobic power/peak power output: 0.38 [95%CI = 0.15 to 0.61], relative absolute maximum aerobic power/peak power output: 0.43 [95%CI = -0.09 to 0.95], physiological thresholds: 0.46 [95%CI = -0.24 to 1.17]), and large (time-trial/time-to-exhaustion: 0.96 [95%CI = -0.81 to 2.73]) improvements in physiological/performance variables compared to controls, with very imprecise interval estimates for most outcomes. In addition, intervention length did not contribute significantly to the improvements in outcome measures in this population, as the effect estimate was only trivial (βDuration: 0.04 [ 95%CI = -0.07 to 0.15]). Finally, the network meta-analysis did not reveal a clear superior effect of any HIIT/SIT types when directly comparing interval training differing in interval work-bout duration. Conclusion: The results of the meta-analysis indicate that both HIIT and SIT are effective training modalities to elicit physiological adaptations and performance improvements in trained cyclists. Our analyses highlight that the optimisation of interval training prescription in trained cyclists cannot be solely explained by interval type or interval work-bout duration and an individualised approach that takes into account the training/competitive needs of the athlete is warranted.