Application of the speed-duration relationship to normalize the intensity of high-intensity interval training

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.

Conventional methods to prescribe exercise intensity are ineffective for exhaustive interval training

PURPOSE

To compare methods of relative intensity prescription for their ability to normalise performance (i.e., time to exhaustion), physiological, and perceptual responses to high-intensity interval training (HIIT) between individuals.

METHODS

Sixteen male and two female cyclists (age: 38 ± 11 years, height: 177 ± 7 cm, body mass: 71.6 ± 7.9 kg, maximal oxygen uptake ([Formula: see text]O2max): 54.3 ± 8.9 ml·kg-1 min-1) initially undertook an incremental test to exhaustion, a 3 min all-out test, and a 20 min time-trial to determine prescription benchmarks. Then, four HIIT sessions (4 min on, 2 min off) were each performed to exhaustion at: the work rate associated with the gas exchange threshold ([Formula: see text]GET) plus 70% of the difference between [Formula: see text]GET and the work rate associated with [Formula: see text]O2max; 85% of the maximal work rate of the incremental test (85%[Formula: see text]max); 120% of the mean work rate of the 20 min time-trial (120%TT); and the work rate predicted to expend, in 4 min, 80% of the work capacity above critical power. Acute HIIT responses were modelled with participant as a random effect to provide estimates of inter-individual variability.

RESULTS

For all dependent variables, the magnitude of inter-individual variability was high, and confidence intervals overlapped substantially, indicating that the relative intensity normalisation methods were similarly poor. Inter-individual coefficients of variation for time to exhaustion varied from 44.2% (85%[Formula: see text]max) to 59.1% (120%TT), making it difficult to predict acute HIIT responses for an individual.

CONCLUSION

The present study suggests that the methods of intensity prescription investigated do not normalise acute responses to HIIT between individuals.

Cardiorespiratory Responses to Constant and Varied-Load Interval Training Sessions

PURPOSE

To compare the cardiorespiratory responses of a traditional session of high-intensity interval training session with that of a session of similar duration and average load, but with decreasing workload within each bout in cyclists and runners.

METHODS

A total of 15 cyclists (maximal oxygen uptake [V˙O2max] 62 [6] mL·kg-1·min-1) and 15 runners (V˙O2max 58 [4] mL·kg-1·min-1) performed both sessions at the maximal common tolerable load on different days. The sessions consisted of four 4-minute intervals interspersed with 3 minutes of active recovery. Power output was held constant for each bout within the traditional day, whereas power started 40 W (2 km·h-1) higher and finished 40 W (2 km·h-1) lower than average within each bout of the decremental session.

RESULTS

Average oxygen uptake during the high-intensity intervals was higher in the decremental session in cycling (89 [4]% vs 86 [5]% of V˙O2max, P = .002) but not in running (91 [4]% vs 90 [4]% of V˙O2max, P = .38), as was the time spent >90% of V˙O2max and the time spent >90% of peak heart rate. Average heart rate (P < .001), pulmonary ventilation (P < .001), and blood lactate concentration (P < .001) were higher during the decremental sessions in both cycling and running.

CONCLUSIONS

Higher levels of physiological perturbations were achieved during decremental sessions in both cycling and running. These differences were, however, more prominent in cycling, thus making cycling a more attractive modality for testing the effects of a training intervention.

Blackcurrant extract does not affect the speed-duration relationship during high-intensity running

Anthocyanin-rich blackcurrant extract (BC) has been shown to ergogenically aid high-intensity exercise. Capacity for such exercise is evaluated by the hyperbolic speed-tolerable duration (S-D_tol) relationship. Therefore, in double-blinded and cross-over randomised controlled trials, 15 males underwent treadmill running incremental exercise testing and were assessed for S-D_tol, quantified by critical speed (CS) and D' (distance), and assessments of time to exhaustion performance to empirically test the limits of the S-D_tol relationship, after daily supplementation of 300 mg/d BC (105 mg/d anthocyanin) or placebo. Supplementation with BC did not change CS (placebo 12.1 ± 1.0 km/h vs BC 11.9 ± 1.0 km/h, p > .05) or D' (placebo 918.6 ± 223.2 m vs BC 965.2 ± 231.2 m, p > .05), although further analysis indicated D' increased in 60% of subject (p = .08), indicating a trend toward cohorts potentially benefiting from BC supplementation. BC supplementation did not change time to exhaustion at or above CS, maximal oxygen uptake (VO_2max), lactate threshold (LT), submaximal running economy (C_R), or substrate utilisation during exercise (all p > .05). In conclusion, we could not detect any beneficial effect of BC supplementation during high-intensity running exercise, including the determining factors S-D_tol relationship, VO_2max, LT or C_R. Hence, no ergogenic effect was observed.

Intensity-dependence of exercise and active recovery in high-intensity interval training

BACKGROUND

High-intensity interval training (HIIT) with interspersing active recovery is an effective mode of exercise training in cohorts ranging from athletes to patients. Here, we assessed the intensity-dependence of the intervals and active recovery bouts for permitting a sustainable HIIT protocol.

METHODS

Fourteen males completed 4x4-minute HIIT protocols where intensities of intervals ranged 80-100% of maximal oxygen uptake (VO2max) and active recovery ranged 60-100% of lactate (La-) threshold (LT). Blood La- measurements indicated fatigue, while tolerable duration of intervals indicated sustainability.

RESULTS

HIIT at 100% of VO2max allowed 44±10% [30-70%] completion, i.e. fatigue occurred after 7minutes:6seconds of the intended 16 minutes of high intensity, whereas HIIT at 95-80% of VO2max was 100% sustainable (P<0.01). Measured intensity did not differ from intended intensity across the protocols (P>0.05). Blood La- concentration [La-] increased to 9.3±1.4mM during HIIT at 100% of VO2max, whereas at 80-95% of VO2max stabilized at 2-6mM in an intensity-dependent manner (P<0.01 vs. 100% of VO2max and P<0.05 vs. baseline). Active recovery at 60-70% of LT during HIIT associated with steady-state blood [La-] peaking at 6-7mM, whereas at 80-100% of LT, blood [La-] accumulated to 10-13mM (P<0.05). After HIIT, active recovery at 80-90% of LT cleared blood [La-] 90% faster than at 60-70% of LT (P<0.05).

CONCLUSIONS

To permit highest exercise stress during 4x4-minute HIIT, exercise intensity should be set to 95% of VO2max, whereas active recovery should be set to 60-70% of LT during HIIT and 80-90% of LT after HIIT to most efficiently prevent excess La- and aid recovery.

Different durations within the method of best practice affect the parameters of the speed-duration relationship

The aim of the study was to determine whether estimates of the speed-duration relationship are affected using different time-trial (TT) field-based testing protocols, where exhaustive times were located within the generally recommended durations of 2-15 min. Ten triathletes (mean ± SD age: 31.0 ± 5.7 years; height: 1.81 ± 0.05 m; body mass: 76.5 ± 6.8 kg) performed two randomly assigned field tests to determine critical speed (CS) and the total distance covered above CS (D́). CS and D́ were obtained using two different protocols comprising three TT that were interspersed by 60 min passive rest. The TTs were 12, 7, and 3 min in Protocol I and 10, 5, and 2 min in Protocol II. A linear relationship of speed vs. the inverse of time (s = D́ × 1/t + CS) was used to determine parameter estimates. Significant differences were found for CS (p = 0.026), but not for D́ (p = 0.123). The effect size for CS (d = 0.305) was considered small, while that for D́ was considered moderate (d = 0.742). CS was significantly correlated between protocols (r = 0.934; p < 0.001), however, no correlation was found for D́ (r = 0.053; p = 0.884). The 95% limits of agreement were ±0.28m s^-1 and ±73.9 m for CS and D́, respectively. These findings demonstrate that the choice of exhaustive times within commonly accepted durations results in different estimates of CS and D́, and thus protocols cannot be used interchangeably. The use of a consistent protocol is therefore recommended, when investigating or monitoring the speed-duration relationship estimates in well-trained athletes.

Reliability of the parameters of the power-duration relationship using maximal effort time-trials under laboratory conditions

The purpose of this study was to assess the reliability of critical power (CP) and the total amount of work accomplished above CP (W´) across repeated tests using ecologically valid maximal effort time-trials (TT) under laboratory conditions. After an initial incremental exercise test, ten well-trained male triathletes (age: 28.5 ± 4.7 years; body mass: 73.3 ± 7.9 kg; height: 1.80 ± 0.07 m; maximal aerobic power [MAP]: 329 ± 41 W) performed three testing sessions (Familiarization, Test I and Test II) each comprising three TT (12, 7, and 3 min with a passive recovery of 60 min between trials). CP and W´ were determined using a linear regression of power vs. the inverse of time (1/t) (P = W´ ∙ 1/t + CP). A repeated-measures ANOVA was used to detect differences in CP and W´ and reliability was assessed using the intra-class correlation coefficient (ICC) and the coefficient of variation (CoV). CP and W´ values were not significantly different between repeated tests (P = 0.171 and P = 0.078 for CP and W´, respectively). The ICC between Familiarization and Test I was r = 0.86 (CP) and r = 0.58 (W´) and between Tests I and II it was r = 0.94 (CP) and r = 0.95 (W´). The CoV notably decreased from 4.1% to 2.6% and from 25.3% to 8.2% for CP and W´, respectively. Despite the non-significant differences for both parameter estimates between Familiarization, Test I, and Test II, ICC and CoV values improved notably after the familiarization trial. Our novel findings indicate that for both, CP and W´ a familiarization trial increased reliability. It is therefore advisable to familiarize well-trained athletes when determining the power-duration relationship using TT under laboratory conditions.

Determinants of curvature constant (W') of the power duration relationship under normoxia and hypoxia: the effect of pre-exercise alkalosis

Purpose

This study investigated the effect of induced alkalosis on the curvature constant (W’) of the power-duration relationship under normoxic and hypoxic conditions.

Methods

Eleven trained cyclists (mean ± SD) Age: 32 ± 7.2 years; body mass (bm): 77.0 ± 9.2 kg; VO_2peak: 59.2 ± 6.8 ml·kg⁻¹·min⁻¹ completed seven laboratory visits which involved the determination of individual time to peak alkalosis following sodium bicarbonate (NaHCO₃) ingestion, an environment specific ramp test (e.g. normoxia and hypoxia) and four x 3 min critical power (CP) tests under different experimental conditions. Participants completed four trials: alkalosis normoxia (ALN); placebo normoxia (PLN); alkalosis hypoxia (ALH); and placebo hypoxia (PLH). Pre-exercise administration of 0.3 g.kg⁻¹ BM of NaHCO₃ was used to induce alkalosis. Environmental conditions were set at either normobaric hypoxia (FiO₂: 14.5%) or normoxia (FiO₂: 20.93%).

Results

An increase in W’ was observed with pre-exercise alkalosis under both normoxic (PLN: 15.1 ± 6.2 kJ vs. ALN: 17.4 ± 5.1 kJ; P = 0.006) and hypoxic conditions (ALN: 15.2 ± 4.9 kJ vs. ALN: 17.9 ± 5.2 kJ; P < 0.001). Pre-exercise alkalosis resulted in a larger reduction in bicarbonate ion (HCO₃⁻) concentrations during exercise in both environmental conditions (p < 0.001) and a greater blood lactate accumulation under hypoxia (P = 0.012).

Conclusion

Pre-exercise alkalosis substantially increased W’ and, therefore, may determine tolerance to exercise above CP under normoxic and hypoxic conditions. This may be due to NaHCO₃ increasing HCO₃⁻ buffering capacity to delay exercise-induced acidosis, which may, therefore, enhance anaerobic energy contribution.