Abstract
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 review training interventions to determine which training modality, HIIT, SIT or low- to moderate-intensity continuous training (LIT/ MICT), leads to greater physiological adaptations and performance improvements in trained cyclists; and (2) determine the moderating effects of intervention length on the effectiveness of the 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 [V̇O 2max /V̇O 2peak ] ≥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 outcome level, point estimates favouring HIIT/SIT were negligible in the Wingate model (0.01 [95%CI = -3.56 to 3.57]) and trivial for relative V̇O 2max /V̇O 2peak (0.10 [95%CI = -0.34
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 outcome level, point estimates favouring HIIT/SIT were negligible in the Wingate model (0.01 [95%CI = -3.56 to 3.57]) and trivial for relative V̇O 2max /V̇O 2peak (0.10 [95%CI = -0.34 to 0.54]). There were small improvements in absolute V̇O 2max /V̇O 2peak (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 maximum aerobic power/ peak power output (0.43 [95%CI = -0.09 to 0.95]) and physiological thresholds (0.46 [95%CI = -0.24 to 1.17]) in HIIT/SIT compared to LIT/MICT. Finally, the time-trial/time-to-exhaustion model (0.96 [95%CI = -0.81 to 2.73]) evidenced large improvements in Copyright: © 2023 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
International Journal of Strength and Conditioning. 2023 performance variables following HIIT/SIT compared to controls. However, interval estimates were very imprecise 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 (β Duration : 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. Keywords: cycling, exercise prescription, maximal oxygen consumption, high-intensity, intervention, programme optimisation INTRODUCTION Over recent decades, optimisation of endurance training has attracted considerable attention in the literature, in an attempt to provide a more scientific basis to endurance performance through ‘evidence- informed’ coaching practice. In this sense, training strategies which seek to optimise physiological adaptations have been widely investigated, with a particular emphasis on training intensity distribution [e.g., 1–3], exercise modalities [e.g., 4–9] and the manipulation of training variables [e.g., 10–12]. Ensuring an integrated approach to periodisation which covers all aspects of performance is considered important for continuously eliciting adaptations, managing fatigue/recovery, and avoiding stagnation during an athlete’s competitive season [13–16]. Exercise intensity is an important training variable that influences physiological adaptations and performance [17]. Indeed, in athletes with already high volumes of training, it would appear that appropriate manipulation of training intensity influences the extent to which further performance gains are made [18]. As such, an appropriate blend of high-volume and high-intensity training is required to induce the physiological and metabolic adaptations that ultimately drive performance enhancements [19]. Nonetheless, there remains equivocal evidence regarding the comparative effects of high-intensity training sessions with other approaches and the most appropriate ways to
training intensity influences the extent to which further performance gains are made [18]. As such, an appropriate blend of high-volume and high-intensity training is required to induce the physiological and metabolic adaptations that ultimately drive performance enhancements [19]. Nonetheless, there remains equivocal evidence regarding the comparative effects of high-intensity training sessions with other approaches and the most appropriate ways to prescribe high-intensity training sessions to endurance athletes. High-intensity interval training (HIIT) is recognised as a viable training modality for eliciting physiological adaptations. By its traditional definition, HIIT consists of submaximal or near maximal efforts (often at 85–95% maximum heart rate and ≥80% maximal power output from a graded exercise test [W max / PPO]), performed above the lactate turnpoint (LTP) or critical power (CP) or second ventilatory threshold (VT2), interspersed by periods of rest or low-intensity exercise [17, 20]. HIIT protocols usually incorporate work intervals lasting 2–8 min, with longer intervals (up to ~16 min) being described as “aerobic” interval training (AIT) [21]. Recovery intervals in HIIT are usually prescribed using a fixed work:recovery ratio (e.g., 2:1, 1:1, 1:4) or self-selected recovery durations [22–24]. Different variations of HIIT which are shorter in duration (usually 20–30 s) have also emerged, referred to as sprint interval training (SIT) [4]. SIT is performed in the extreme exercise intensity domain at power outputs or velocities above those associated with maximal/peak oxygen consumption (V̇O 2max /V̇O 2peak ), often with fixed recovery periods of 1.5–4 min [25–28]. Implementing HIIT/SIT has been shown to induce cardiovascular [e.g., 29–32], metabolic [e.g., 33–35], neuromuscular [36, 37], molecular [25, 38, 39] and performance [e.g., 40– 42] adaptations, which are at least comparable to the physiological adaptations observed in traditional (moderate intensity) endurance training despite a substantially lower training volume and/or session duration [31, 43–47]. Prescribing HIIT/SIT can be challenging due to the large number of training variables which may influence the exercise stimulus, including the duration and intensity of individual work intervals and recovery (relief) intervals, the total number of individual work intervals (i.e., repetitions) and the number of series/sets (i.e., groups of work intervals separated by longer recoveries),
and/or session duration [31, 43–47]. Prescribing HIIT/SIT can be challenging due to the large number of training variables which may influence the exercise stimulus, including the duration and intensity of individual work intervals and recovery (relief) intervals, the total number of individual work intervals (i.e., repetitions) and the number of series/sets (i.e., groups of work intervals separated by longer recoveries), and the duration and intensity of the between-series recovery periods [4]. The differences in the application of interval training between HIIT and SIT lie primarily in the duration and intensity of the exercise bouts, reflecting distinct acute metabolic processes that, consequently, may lead to different chronic adaptations to training [48]. The moderating effects of recovery durations should also be weighed, and likely contribute to the overall Physiological Adaptations and Performance Improvements to Interval Training in Endurance-Trained Cyclists: An Exploratory Systematic Review and Meta-Analysis 2 Copyright: © 2023 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
International Journal of Strength and Conditioning. 2023 Norte, B., Steele, J., & Wright, J. physiological stimulus of a training session in distinct ways depending on the interval training modality [21, 49]. Moreover, other programming variables (e.g., session frequency, weekly volume, training intensity distribution, the inclusion of resistance training or other forms of exercise, and period of the season) [50–54] and population characteristics (e.g., training history, sex, age, baseline physiological measures, phenotype) [55, 56] also influence the magnitude of training responses/ adaptations and, in turn, the potential of a given training intervention to elicit performance improvements. Despite the lack of standardisation enabling our understanding of different periodisation models and exercise protocols using HIIT and SIT [57], the evidence has consistently shown that both interval training modalities produce beneficial physiological adaptations that enhance endurance performance. In cycling, high-intensity training programmes lead to performance gains in participants ranging from recreationally-trained [58] to elite-level cyclists [59]. Improvements in V̇O 2max [26, 60, 61], CP [62], power output at different blood lactate markers [58, 60, 61] and ventilatory thresholds (VT 1 /VT 2 ) [63, 64] have been reported following HIIT/SIT training regimens lasting up to 10–12 weeks, with 2 weeks being the minimum intervention length required to elicit adaptations even in highly trained cyclists [63]. Other performance measures such as time- trials (TT) [59, 60, 65, 66] and time-to-exhaustion (TTE) [58] are also improved, which could be partly explained by an increased ability to tolerate higher blood lactate concentrations [64] after a period of HIIT/SIT. Importantly, physiological adaptations are dictated by the aforementioned programming variables and population characteristics. Given the complexity of endurance training, the mechanisms driving improvements in performance are likely multifactorial and warrant further investigation to optimise HIIT/SIT prescription. Previous reviews have shown that interval training (HIIT/SIT) may lead to greater improvements in V̇O 2max [67, 68] and fat oxidation in overweight/ obese individuals [69] than moderate-intensity continuous training (MICT), whilst others [70–74] revealed no clear superior benefits in a range of physiological and body composition measures. The effectiveness of HIIT/SIT interventions has been systematically investigated in overweight/obese adults [74], trained athletes
shown that interval training (HIIT/SIT) may lead to greater improvements in V̇O 2max [67, 68] and fat oxidation in overweight/ obese individuals [69] than moderate-intensity continuous training (MICT), whilst others [70–74] revealed no clear superior benefits in a range of physiological and body composition measures. The effectiveness of HIIT/SIT interventions has been systematically investigated in overweight/obese adults [74], trained athletes in a range of sports [75], healthy/sedentary adults [67, 70, 73], mixed populations [68, 71, 72], and young athletes [76], but not solely in trained cyclists. The aforementioned systematic reviews compare interval training with MICT in health and disease, which albeit important for public health guidance and disease prevention/ amelioration, provides very little information with regard to endurance training optimisation in athletes with already high-volume training backgrounds. To our knowledge, only two systematic reviews have focused on chronic adaptations to cycling training in trained cyclists [53, 77], with a particular focus on cycling cadence [77] and periodisation models [53] rather than specific exercise prescription. In addition, although it is undeniable that both HIIT and SIT improve physiological adaptations in various populations, the number of reviews directly comparing both interval training modalities is sparse [78–80]. Therefore, the purpose of the present review was to systematically investigate the effects of different HIIT/SIT interventions in comparison to low-intensity training (LIT) or MICT on physiological adaptations and performance in trained cyclists. To address the lack of reviews discriminating between HIIT and SIT, the secondary aims of this investigation were: (1) to examine the potential effects of HIIT differing in interval work-bout duration on performance outcomes; (2) to determine whether traditional HIIT modality is superior in inducing performance adaptations in comparison with SIT (or vice-versa); and (3) to investigate the moderating effects of intervention length in relation to overarching training adaptations. METHODS The review was conducted in accordance with the guidelines recommended in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [81]. This review was not pre-registered as it was conducted as part of an undergraduate dissertation and thus is considered exploratory. Literature Search Strategy Electronic database searches were performed using SPORTDiscus
in relation to overarching training adaptations. METHODS The review was conducted in accordance with the guidelines recommended in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [81]. This review was not pre-registered as it was conducted as part of an undergraduate dissertation and thus is considered exploratory. Literature Search Strategy Electronic database searches were performed using SPORTDiscus and PubMed. All available records published from inception to 3 July 2023 were considered for initial analysis. Articles were retrieved from each database using the following search criteria in the search query box: (High-intensity interval training OR HIIT OR HIT OR High-intensity training OR Sprint interval training OR Repeated sprint training) AND (cycling performance). Additional articles were identified through reference lists of potentially eligible papers. 3 Copyright: © 2023 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
4 Copyright: © 2023 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Strength and Conditioning. 2023 Physiological Adaptations and Performance Improvements to Interval Training in Endurance-Trained Cyclists: An Exploratory Systematic Review and Meta-Analysis Search Limits During the initial search, the following search limits were selected to optimise the search strategy: (1) Abstract available, (2) Journal articles, (3) Humans, and (4) English language. Eligibility Criteria Inclusion and exclusion criteria were used according to the PICO criteria (i.e., participants, intervention, comparators, outcome) to guide the study selection process. Type of Study The systematic review included randomised and matched controlled trials. Type of Participants Healthy recreationally–highly trained cyclists aged 18–49 years with a minimum relative V̇O 2max /V̇O 2peak of 45 mL·kg -1 ·min -1 were considered. The inclusion of males and females aged 49 years or under was based on a previous study which demonstrated that physiological adaptations to endurance training are not impacted by sex in this age group [82]. Training categorisation of cyclists followed the guidelines of Quesada et al. [83] regarding training volume and frequency, and that of De Pauw et al. [84] based on the need for physiological information as a means of classifying subject groups. It is important to note that not all included studies reported metabolic ‘threshold’ data (Table 1). Nevertheless, we believe cyclists in all included studies were at least recreationally-trained as per De Pauw et al., [84] and indeed physiological data suggest a sufficient training level was met. Finally, studies performed on participants with underlying health conditions, and acute or chronic diseases (e.g., diabetes, heart problems) were excluded from this review. Type of Interventions Training interventions were required to last a minimum duration of 2 weeks (with at least two interval training sessions per week), which has been shown to be sufficient to induce positive physiological adaptations in highly trained cyclists [63]. Participants had to be allocated to an interval training group (HIIT/SIT) or a matched comparator group
from this review. Type of Interventions Training interventions were required to last a minimum duration of 2 weeks (with at least two interval training sessions per week), which has been shown to be sufficient to induce positive physiological adaptations in highly trained cyclists [63]. Participants had to be allocated to an interval training group (HIIT/SIT) or a matched comparator group that performed either LIT or MICT (or both) referred to as the control group (CON). Studies that did not have a control group but included multiple interval training groups were considered for analyses involving direct comparisons of both interval training modalities, and between HIIT differing in work-bout duration. In contrast, studies comparing an interval training group solely with a no-exercise CON were excluded from the review. Articles which incorporated both HIIT and SIT (i.e., ‘combined’ HIIT/SIT) in the same training intervention were considered for analysis as long as the abovementioned criteria were met (i.e., the study allowed for comparisons against other interval training groups and/or CON performing LIT/MICT). Studies reporting the effects of HIIT interventions consisting of intense overloading strategies (e.g., block periodisation) were excluded. Performing two to three weekly HIIT sessions is sufficient to signal physiological adaptations and further increases may induce symptoms of overreaching/overtraining [1]. In this sense, training interventions consisting of more than 3 weekly interval training sessions were not considered. Studies in which participants were under supplement administration were excluded from this review due to potential performance enhancements [85] and, thus, lead to confusion in ascertaining the true effects of HIIT/SIT. For the same reason, studies that manipulated environmental conditions or combined cycling training with strength training were also excluded, similar to that of other systematic reviews in this area [67, 74]. For the purpose of this review, HIIT was defined as near maximal exercise at 85–95% maximum heart rate and ≥80% W max /PPO, lasting anywhere from ~1–8 min. HIIT incorporating longer submaximal work intervals (up to 16 min) was described as AIT, despite work intensities being undeniably high. SIT was defined as ‘all-out’ or ‘supramaximal’ exercise lasting 20–30 s, interspersed by fixed recovery periods.
this review, HIIT was defined as near maximal exercise at 85–95% maximum heart rate and ≥80% W max /PPO, lasting anywhere from ~1–8 min. HIIT incorporating longer submaximal work intervals (up to 16 min) was described as AIT, despite work intensities being undeniably high. SIT was defined as ‘all-out’ or ‘supramaximal’ exercise lasting 20–30 s, interspersed by fixed recovery periods. Outcome Measures Studies comparing measures of cycling performance between two or more interval training groups (or with CON) as the primary or secondary aim of the study were included. In order to be included in the systematic review, each study had to include at least one of the following physiological and performance variables typically measured in endurance training studies: (1) V̇O 2max /V̇O 2peak ; (2) Lactate Threshold (LT)/LTP; (3) VT 1 /VT 2 ; (4) OBLA; (5) MLSS; (6) TT performance; (7) Power output associated with V̇O 2max /V̇O 2peak (in individual studies, referred to as “Maximum Aerobic Power” [MAP] or W max /PPO); (10) TTE; (11) CP; (12) Work capacity above CP (W′) (13) Anaerobic capacity (e.g. Wingate test variables); (14) Gross Efficiency (GE); (15) Cycling economy.
International Journal of Strength and Conditioning. 2023 Norte, B., Steele, J., & Wright, J. 5 Copyright: © 2023 by the authors. Licensee IUSCA, London, UK. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Study Selection All potential articles identified at the search phase were exported to an external citation management software (EndNote20, Clarivate, Philadelphia, PA), where all the duplicates were removed. The lead author (BN) independently screened titles, followed by abstracts, and full-text articles. In instances where the inclusion or exclusion of studies could not be ascertained through titles/abstracts only, these studies progressed to the next stage of the screening process and full-text articles were assessed. The study selection process was reviewed by one author (JS) and possible disagreements were resolved by consulting a third author (JW). Studies that did not meet the inclusion and exclusion criteria listed above were excluded from this review. Data Extraction The following characteristics were extracted from each included study by one author (BN) and checked by two other authors (JW and JS): article title and author(s), participant information (age, sex, stature, body mass, training level/ status, baseline physiological measures), method (intervention length, research design), description of the intervention protocol (HIIT modality, interval intensity, duration and frequency) and study outcomes (relevant findings based on the parameters measured). Data on physiological and performance parameters were extracted in the form of pre- and post-training intervention Means and Standard Deviations (SD) (Mean ± SD) or 95% Confidence Intervals (CI) (Mean ± 95% CI), p values and relationships between performance variables (if appropriate). We asked the corresponding authors of one article [86] to provide additional data but the authors no longer had access to it. Despite this, we managed to retrieve relevant baseline and post-intervention data (V̇O 2max /V̇O 2peak , MAP/PPO, Wingate, TT/TTE performance outcomes) from this study which had been reported in another meta- analysis [78] for all but one group that performed 8-min intervals and did not feature in the analysis of this investigation. One study [65] did not present
Description
Explores the impact of HIIT and SIT on cycling performance.