Optimizing Interval Training Through Power-Output Variation Within the Work Intervals

A narrative review exploring advances in interval training for endurance athletes

Interval training is considered an essential training component in endurance athletes. Recently, there has been a focus on optimization of interval training characteristics to sustain a high fraction of maximal oxygen consumption (≥90% VO2max) to improve physiological adaptations and performance. Herein, we present a synopsis of the latest research exploring both acute and chronic studies in endurance athletes. Further, a decision flowchart was created for athletes and coaches to select the most appropriate interval training regime for specific individualized goals.

A training goal-oriented categorization model of high-intensity interval training

There are various categorization models of high-intensity interval training (HIIT) in the literature that need to be more consistent in definition, terminology, and concept completeness. In this review, we present a training goal-oriented categorization model of HIIT, aiming to find the best possible consensus among the various defined types of HIIT. This categorization concludes with six different types of HIIT derived from the literature, based on the interaction of interval duration, interval intensity and interval:recovery ratio. We discuss the science behind the defined types of HIIT and shed light on the possible effects of the various types of HIIT on aerobic, anaerobic, and neuromuscular systems and possible transfer effects into competition performance. We highlight various research gaps, discrepancies in findings and not yet proved know-how based on a lack of randomized controlled training studies, especially in well-trained to elite athlete cohorts. Our HIIT "toolbox" approach is designed to guide goal-oriented training. It is intended to lay the groundwork for future systematic reviews and serves as foundation for meta-analyses.

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 (% W ˙ max) vs. maximal sustainable work rate in a self-paced interval training session (% W ˙ max-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 % W ˙ max group, but 100% for the % W ˙ max-SP the 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 % W ˙ max-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·week-1) and self-paced interval training performance (standard deviation: 1.451 W·week-1). However, inter-individual 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, % W ˙ max-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 inter-individual variability in adaptive responses cannot always be identified when intra-individual variability is accounted for.

A Perspective on High-Intensity Interval Training for Performance and Health

Interval training is a simple concept that refers to repeated bouts of relatively hard work interspersed with recovery periods of easier work or rest. The method has been used by high-level athletes for over a century to improve performance in endurance-type sports and events such as middle- and long-distance running. The concept of interval training to improve health, including in a rehabilitative context or when practiced by individuals who are relatively inactive or deconditioned, has also been advanced for decades. An important issue that affects the interpretation and application of interval training is the lack of standardized terminology. This particularly relates to the classification of intensity. There is no common definition of the term "high-intensity interval training" (HIIT) despite its widespread use. We contend that in a performance context, HIIT can be characterized as intermittent exercise bouts performed above the heavy-intensity domain. This categorization of HIIT is primarily encompassed by the severe-intensity domain. It is demarcated by indicators that principally include the critical power or critical speed, or other indices, including the second lactate threshold, maximal lactate steady state, or lactate turnpoint. In a health context, we contend that HIIT can be characterized as intermittent exercise bouts performed above moderate intensity. This categorization of HIIT is primarily encompassed by the classification of vigorous intensity. It is demarcated by various indicators related to perceived exertion, oxygen uptake, or heart rate as defined in authoritative public health and exercise prescription guidelines. A particularly intense variant of HIIT commonly termed "sprint interval training" can be distinguished as repeated bouts performed with near-maximal to "all out" effort. This characterization coincides with the highest intensity classification identified in training zone models or exercise prescription guidelines, including the extreme-intensity domain, anaerobic speed reserve, or near-maximal to maximal intensity classification. HIIT is considered an essential training component for the enhancement of athletic performance, but the optimal intensity distribution and specific HIIT prescription for endurance athletes is unclear. HIIT is also a viable method to improve cardiorespiratory fitness and other health-related indices in people who are insufficiently active, including those with cardiometabolic diseases. Research is needed to clarify responses to different HIIT strategies using robust study designs that employ best practices. We offer a perspective on the topic of HIIT for performance and health, including a conceptual framework that builds on the work of others and outlines how the method can be defined and operationalized within each context.

Adding Intermittent Vibration to Varied-intensity Work Intervals: No Extra Benefit

Varied-intensity work intervals have been shown to induce higher fractions of maximal oxygen uptake during high-intensity interval training compared with constant-intensity work intervals. We assessed whether varied-intensity work intervals combined with intermittent vibration could further increase cyclists' fraction of maximal oxygen uptake to potentially optimise adaptive stimulus. Thirteen cyclists (V̇O_2max: 69.7±7.1 ml·kg^-1·min^-1) underwent a performance assessment and two high-intensity interval training sessions. Both comprised six 5-minute varied-intensity work intervals within which the work rate was alternated between 100% (3×30-second blocks, with or without vibration) and 77% of maximal aerobic power (always without vibration). Adding vibration to varied-intensity work intervals did not elicit a longer time above ninety percent of maximal oxygen uptake (415±221 versus 399±209 seconds, P=0.69). Heart rate- and perceptual-based training-load metrics were also not affected (all P≥0.59). When considering individual work intervals, no between-condition differences were found (fraction of maximal oxygen uptake, P=0.34; total oxygen uptake, P=0.053; mean minute ventilation, P=0.079; mean heart rate, P=0.88; blood lactate concentration, P=0.53; ratings of perceived exertion, P=0.29). Adding intermittent vibration to varied-intensity work intervals does not increase the fraction of maximal oxygen uptake elicited. Whether intermittent exposure to vibration can enhance cyclists' adaptive stimulus triggered by high-intensity interval training remains to be determined.

Increased oxygen uptake in well-trained runners during uphill high intensity running intervals: A randomized crossover testing

The time spent above 90% of maximal oxygen uptake ( V ˙ O₂max) during high-intensity interval training (HIIT) sessions is intended to be maximized to improve V ˙ O₂max. Since uphill running serves as a promising means to increase metabolic cost, we compared even and moderately inclined running in terms of time ≥90% V ˙ O₂max and its corresponding physiological surrogates. Seventeen well-trained runners (8 females & 9 males; 25.8 ± 6.8yrs; 1.75 ± 0.08m; 63.2 ± 8.4kg; V ˙ O₂max: 63.3 ± 4.2 ml/min/kg) randomly completed both a horizontal (1% incline) and uphill (8% incline) HIIT protocol (4-times 5min, with 90s rest). Mean oxygen uptake ( V ˙ O₂mean), peak oxygen uptake ( V ˙ O₂peak), lactate, heart rate (HR), and perceived exertion (RPE) were measured. Uphill HIIT revealed higher (p ≤ 0.012; partial eta-squared (pes) ≥ 0.351) V ˙ O₂mean (uphill: 3.3 ± 0.6 vs. horizontal: 3.2 ± 0.5 L/min; standardized mean difference (SMD) = 0.15), V ˙ O₂peak (uphill: 4.0 ± 0.7 vs. horizontal: 3.8 ± 0.7 L/min; SMD = 0.19), and accumulated time ≥90% V ˙ O₂max (uphill: 9.1 ± 4.6 vs. horizontal: 6.4 ± 4.0 min; SMD = 0.62) compared to even HIIT. Lactate, HR, and RPE responses did not show mode*time rANOVA interaction effects (p ≥ 0.097; pes ≤0.14). Compared to horizontal HIIT, moderate uphill HIIT revealed higher fractions of V ˙ O₂max at comparable perceived efforts, heartrate and lactate response. Therefore, moderate uphill HiiT notably increased time spent above 90% V ˙ O₂max.

Adding Vibration During Varied-Intensity Work Intervals Increases Time Spent Near Maximal Oxygen Uptake in Well-Trained Cyclists

PURPOSE

Previous research suggests that the percentage of maximal oxygen uptake attained and the time it is sustained close to maximal oxygen uptake (eg, >90%) can serve as a good criterion to judge the effectiveness of a training stimulus. The aim of this study was to investigate the acute effects of adding vibration during varied high-intensity interval training (HIIT) sessions on physiological and neuromuscular responses.

METHODS

Twelve well-trained cyclists completed a counterbalanced crossover protocol, wherein 2 identical varied HIIT cycling sessions were performed with and without intermittent vibration to the lower-intensity workloads of the work intervals (6 × 5-min work intervals and 2.5-min active recovery). Each 5-minute work interval consisted of 3 blocks of 40 seconds performed at 100% of maximal aerobic power interspersed with 60-second workload performed at a lower power output, equal to the lactate threshold plus 20% of the difference between lactate threshold and maximal aerobic power. Oxygen uptake and electromyographic activity of lower and upper limbs were recorded during all 5-minute work intervals.

RESULTS

Adding vibration induced a longer time ≥90% maximal oxygen uptake (11.14 [7.63] vs 8.82 [6.90] min, d = 0.64, P = .048) and an increase in electromyographic activity of lower and upper limbs during the lower-intensity workloads by 20% (16%) and 34% (43%) (d = 1.09 and 0.83; P = .03 and .015), respectively.

CONCLUSION

Adding vibration during a varied HIIT session increases the physiological demand of the cardiovascular and neuromuscular systems, indicating that this approach can be used to optimize the training stimulus of well-trained cyclists.

High-Intensity Interval Training (HIIT) in Hypoxia Improves Maximal Aerobic Capacity More Than HIIT in Normoxia: A Systematic Review, Meta-Analysis, and Meta-Regression

The present study aimed to determine the effect of high intensity interval training (HIIT) in hypoxia on maximal oxygen uptake (VO_2max) compared with HIIT in normoxia with a Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA)-accordant meta-analysis and meta-regression. Studies which measured VO_2max following a minimum of 2 weeks intervention featuring HIIT in hypoxia versus HIIT in normoxia were included. From 119 originally identified titles, nine studies were included (n = 194 participants). Meta-analysis was conducted on change in (∆) VO_2max using standardised mean difference (SMD) and a random effects model. Meta-regression examined the relationship between the extent of environmental hypoxia (fractional inspired oxygen [FiO₂]) and ∆VO_2max and intervention duration and ∆VO_2max. The overall SMD for ∆VO_2max following HIIT in hypoxia was 1.14 (95% CI = 0.56-1.72; p < 0.001). Meta-regressions identified no significant relationship between FiO₂ (coefficient estimate = 0.074, p = 0.852) or intervention duration (coefficient estimate = 0.071, p = 0.423) and ∆VO_2max. In conclusion, HIIT in hypoxia improved VO_2max compared to HIIT in normoxia. Neither extent of hypoxia, nor training duration modified this effect, however the range in FiO₂ was small, which limits interpretation of this meta-regression. Moreover, training duration is not the only training variable known to influence ∆VO_2max, and does not appropriately capture total training stress or load. This meta-analysis provides pooled evidence that HIIT in hypoxia may be more efficacious at improving VO_2max than HIIT in normoxia. The application of these data suggest adding a hypoxic stimuli to a period of HIIT may be more effective at improving VO_2max than HIIT alone. Therefore, coaches and athletes with access to altitude (either natural or simulated) should consider implementing HIIT in hypoxia, rather than HIIT in normoxia where possible, assuming no negative side effects.

Increasing Oxygen Uptake in Cross-Country Skiers by Speed Variation in Work Intervals

PURPOSE

Accumulated time at a high percentage of peak oxygen consumption (VO2peak) is important for improving performance in endurance athletes. The present study compared the acute physiological and perceived effects of performing high-intensity intervals with roller ski double poling containing work intervals with (1) fast start followed by decreasing speed (DEC), (2) systematic variation in exercise intensity (VAR), and (3) constant speed (CON).

METHODS

Ten well-trained cross-country skiers (double-poling VO2peak 69.6 [3.5] mL·min-1·kg-1) performed speed- and duration-matched DEC, VAR, and CON on 3 separate days in a randomized order (5 × 5-min work intervals and 3-min recovery).

RESULTS

DEC and VAR led to longer time ≥90% VO2peak (P = .016 and P = .033, respectively) and higher mean %VO2peak (P = .036, and P = .009) compared with CON, with no differences between DEC and VAR (P = .930 and P = .759, respectively). VAR, DEC, and CON led to similar time ≥90% of peak heart rate (HRpeak), mean HR, mean breathing frequency, mean ventilation, and mean blood lactate concentration ([La-]). Furthermore, no differences between sessions were observed for perceptual responses, such as mean rate of perceived exertion, session rate of perceived exertion or pain score (all Ps > .147).

CONCLUSIONS

In well-trained XC skiers, DEC and VAR led to longer time ≥90% of VO2peak compared with CON, without excessive perceptual effort, indicating that these intervals can be a good alternative for accumulating more time at a high percentage of VO2peak and at the same time mimicking the pronounced variation in exercise intensities experienced during XC-skiing competitions.

Exercise Intensity and Pacing Pattern During a Cross-Country Olympic Mountain Bike Race

Objective: To examine the power profiles and pacing patterns in relation to critical power (CP) and maximal aerobic power (MAP) output during a cross-country Olympic (XCO) mountain bike race.

Methods: Five male and two female national competitive XCO cyclists completed a UCI Cat. 1 XCO race. The races were 19 km and 23 km and contained five (female) and six (male) laps, respectively. Power output (PO) during the race was measured with the cyclists’ personal power meters. On two laboratory tests using their own bikes and power meters, CP and work capacity above CP (W') were calculated using three time trials of 12, 7, and 3 min, while MAP was established based on a 3-step submaximal test and the maximal oxygen uptake from the 7-min time trial.

Results: Mean PO over the race duration (96 ± 7 min) corresponded to 76 ± 9% of CP and 63 ± 4% of MAP. 40 ± 8% of race time was spent with PO > CP, and the mean duration and magnitude of the bouts >CP was ~8 s and ~120% of CP. From the first to last lap, time >CP and accumulated W' per lap decreased with 9 ± 6% and 45 ± 17%, respectively. For single >CP bouts, mean magnitude and mean W' expended decreased by 25 ± 8% and 38 ± 15% from the first to the last lap, respectively. Number and duration of bouts did not change significantly between laps.

Conclusion: The highly variable pacing pattern in XCO implies the need for rapid changes in metabolic power output, as a result of numerous separate short-lived >CP actions which decrease in magnitude in later laps, but with little lap-to-lap variation in number and duration.