High-Intensity Interval Training in Cycling: Acute response and Chronic adaptations

Introduction
High-Intensity Interval training (HIIT) was highlighted as a safe intervention that allows for multiple health benefits across the lifespan [1]. Previous research indicated variables for characterizing HIIT protocols and categorized four types of HIIT: long intervals (LI, > 60 s), short intervals (SI, ≤ 60 s), repeated sprint training (RST) and sprint interval training (SIT) [2]. It is commonly assumed that the acute physiological response corresponds to the adaptational effect. While recent meta-analyses compared the chronic adaptations to LI and SIT [3], there are several promising HIIT protocols that deserve closer attention at both levels. Hence, this systematic review aims to compare the effects of various HIIT protocols on (1) the acute response and (2) chronic adaptions in cycling.

Methods
PubMed and Web of Science were used to find studies in the timespan from 2013 until the date of the search (15th of February 2021). Inclusion criteria were the comparison of different HIIT protocols, cycling as exercise modality and participants with a mean maximal oxygen uptake (V̇O2max) ≥ 45 ml/min/kg. Risk of bias was assessed by using the PEDro scale. Results were summarized for the acute response (time ≥ 90% V̇O2max, lactate concentration, perceived exertion and power) and chronic adaptations (time trial performance, V̇O2max and lactate threshold) while indicating the participants’ performance levels.

Results
A total of 31 studies (23 acute, 14 chronic, 6 both) with a mean PEDro score of 5.4 ± 0.9 and a total sample size of N = 544 were included. The majority of acute studies applied a crossover design, while longitudinal studies were exclusively group-based. The highest times ≥ 90% V̇O2max were reported for intervals with varied work intensity and protocols that combined SI and LI. SI demonstrated a substantially higher acute response compared to LI while differences in chronic adaptations appeared to be rather small and tended to favor SI in highly-trained/professional athletes. When compared to SI and LI, SIT and RST demonstrated considerably lower improvements in aerobic endurance parameters, whereas anaerobic and sprint parameters were effectively improved.

Discussion and Conclusion
The results indicate that HIIT protocols that are associated with a higher acute response may not necessarily lead to superior chronic adaptations. Whereas SI and LI seem to be equally effective for improving endurance performance, SIT and RST are more adequate for improving sprint performance. In accordance with previous research [3], the reported performance improvements decreased with increasing training status. Limitations arise from the design of longitudinal studies and the applied matching procedures. Future studies are encouraged to perform crossover interventions and include measures of glycolytic capabilities to holistically examine the adaptational effect of various forms of HIIT.

References
1) Martland et al. (2020) J Sports Sci
2) Buchheit & Laursen (2013) Sports Med
3) Rosenblat et al. (2020-22) Sports Med