Study protocol for the FITR Heart Study: Feasibility, safety, adherence, and efficacy of high intensity interval training in a hospital-initiated rehabilitation program for coronary heart disease

HRV-guided training vs traditional HIIT training in cardiac rehabilitation: a randomized controlled trial

High-intensity interval training is the gold standard for cardiac rehabilitation although current revascularization therapy focuses on the recovery of autonomic nervous system balance through heart rate variability (HRV). The main objective was to analyze the effect of HRV-guided training versus high-intensity interval training on cardiorespiratory fitness, heart rate variability, quality of life, and training volume at high intensity, as well as exercise adherence, safety, and feasibility in ischemic patients. This is an 8-week cluster randomized controlled trial with an HRV-based training group (HRV-G) and a traditional HIIT group (HIIT-G). Maximal oxygen consumption, heart rate, and blood pressure were measured during the Bruce protocol treadmill test. HRV was measured with the HRV4Training application, and quality of life with the MacNew QLMI. The repeated measures ANCOVA was used with the age and the baseline scores as covariables. Forty-six patients (mean age 55 ± 11.03 years) were randomized and assigned either to HRV-G (n = 23) or HIIT-G (n = 23). Both groups improved maximal oxygen consumption and METS (P > .05). However, the resting systolic blood pressure was lower in HRV-G (4.3 ± 1.2 mmHg, P = .05). In HRV-G, the resting diastolic, maximal diastolic, and systolic blood pressure decreased (5.4 ± 5.96 mmHg, P = .007; 11.4 ± 12.46 mmHg, P = .005; and 5 ± 5.98 mmHg, P = .013, respectively) whereas the recovery heart rate increased significantly (-21.5 ± 23.16 beats/min, P = .003). The LnrMSSDcv ([LnrMSSDSD/LnrMSSDMEAN] × 100) was lower in HRV-G (1.23 ± 0.91 mmHg, P = .03) while the training volume at high intensity was higher in HIIT-G (31.4 ± 29.2 min, P = .024). HRV-guided training presents a better cardioprotective effect than HIIT-G at a lower high-intensity training volume.

A Bibliometric Analysis of High-Intensity Interval Training in Cardiac Rehabilitation

As global quality of life has improved, the risk factors for cardiovascular diseases have gradually increased in prevalence. People have consequently sought to improve their health through physical exercise. High-intensity interval training (HIIT) is a cardiac rehabilitation (CR) tool that has been of great interest for several years. However, its feasibility and safety remain controversial. This study aimed to explore hot research topics and new directions regarding the role of HIIT in CR and to describe the dynamic development of the field. We used the Web of Science Core Collection database to develop visualizations using CiteSpace software (v.6.1.R2). The number of articles published, institutional collaboration networks, author partnerships, and keyword co-occurrence and clustering were used to analyze the impact of HIIT on CR. Our results showed that Norway, Canada, and the United States were the most prominent contributors to this field. Articles by Nigam, A and Juneau, M had the highest number of citations. The Norwegian University of Science and Technology had performed the most in-depth research in this area. The European Journal of Preventive Cardiology had published the most articles. The United States had the highest number of publishing journals. Relevant issues focused on coronary artery disease, exercise capacity, heart failure, cardiorespiratory fitness, and physical activity. HIIT in heart transplantation may be at the forefront of research in this field and future studies should focus on this topic. HIIT-based CR can therefore improve the exercise capacity and quality of life of cardiovascular patients and improve patient compliance in a safe manner.

HRV-Guided Training for Elders after Stroke: A Protocol for a Cluster-Randomized Controlled Trial

There are many consequences associated with having a stroke, all of which are important factors affecting long-term rehabilitation outcomes; these become important health issues for those of advanced age and require dedicated health strategies. High-intensity interval training (HIIT) is an effective training protocol used in cardiac rehabilitation programs; however, owing to the inter-individual variability in physiological responses to training associated with cardiovascular diseases, the exercise regimen given to each patient should be closely controlled and individualized to ensure the safety and efficiency of the exercise program. Heart rate variability (HRV) is currently being used for this purpose, as it is closely linked to parasympathetic nervous system activation, with high HRV scores associated with good cardiovascular adaptation. The objective of this protocol is to determine the effect of HIIT compared to HRV-guided training in terms of cardiorespiratory fitness, heart rate variability, functional parameters, body composition, quality of life, inflammatory markers, and cognitive function in patients who have had a stroke, as well as to assess the feasibility of patients undertaking an 8-week cardiac rehabilitation program, evaluating its safety and their adherence. The proposed protocol involved cluster-randomized controlled design in which the post-stroke patients are assigned either to an HRV-based training group (HRV-G) or a HIIT-based training group (HIIT-G). HIIT-G will train according to a predefined training program, whereas HRV-G will train depending on the patients' daily HRV. The outcomes considered are peak oxygen uptake (VO_2peak), endothelial and work parameters, heart rate variability, functional parameters, relative weight and body fat distribution, quality of life, inflammatory markers and cognitive function, as well as exercise adherence, feasibility, and safety. It is expected that this HRV-guided training protocol will improve functional performance in patients following a stroke and be safer, more feasible, and generate improved adherence relative to HIIT, providing an improved strategy for to optimize cardiac rehabilitation interventions.

Comparison of high intensity interval training with standard cardiac rehabilitation on vascular function

OBJECTIVES

To compare the short- and long-term effects of high-intensity interval training (HIIT) with usual care moderate intensity continuous training (MICT) on systemic vascular function and stiffness in patients with coronary artery disease undergoing a cardiac rehabilitation program.

DESIGN

Randomized controlled trial.

METHODS

Fifty-four patients (age = 63 ± 8 years, 93% male) were randomized to complete 3 sessions/week (2 supervised, 1 home-based) of either (1) 4 × 4-min HIIT or (2) 40-min MICT, for 4 weeks. Patients then continued 3 unsupervised home-based sessions/week of their allocated training for 11 months. Brachial artery flow-mediated dilation, pulse wave velocity, and blood pressure were measured at baseline, 4 weeks, 3 months, 6 months, and 12 months. Data were analyzed using linear mixed modeling and are presented as mean change from baseline (95% CI).

RESULTS

HIIT showed a greater improvement in flow-mediated dilation compared to MICT after 4 weeks [1.5% (0.9, 2.1) vs 0.1% (-0.5, 0.8); p = 0.004) but not 12 months [1.2% (-0.2, 2.5) vs 0.4% (-0.8, 1.7); p = 0.153). There were no short- or long-term group differences for changes in pulse wave velocity, peripheral or central blood pressure between HIIT and MICT after 4 weeks, or over 12 months.

CONCLUSIONS

A 4-week HIIT program was superior to MICT for improving vascular function, but not arterial stiffness or blood pressure. Over 12 months, changes in vascular function, blood pressure, and arterial stiffness were similar for HIIT and MICT.

Accuracy of dual-energy x-ray absorptiometry for assessing longitudinal change in visceral adipose tissue in patients with coronary artery disease

BACKGROUND/OBJECTIVE

Visceral adipose tissue (VAT) is a key target of interventions for obesity-related diseases. Dual-energy x-ray absorptiometry (DXA) can estimate VAT, however its accuracy to measure longitudinal change in VAT compared to gold-standard techniques such as magnetic resonance imaging (MRI), has not been studied in adults. This study aimed to determine the accuracy of DXA compared with gold-standard MRI for cross-sectional VAT assessment, and for detecting longitudinal change in VAT.

METHODS

Adults with coronary artery disease (64 ± 8 years; BMI 27.8 ± 3.5 kg/m²; 88% male) were assessed for VAT by DXA and MRI at baseline (n = 34) and during implementation of an exercise intervention study at 3- and 12-months (n = 29). To match the 5.2 cm DXA measurement site for Hologic software (InnerCore^TM), VAT cross-sectional area (CSA) was measured by MRI using a single slice at L4/L5 junction, and VAT volume measured by 10 × 5 mm slices over the L4/L5 junction. MRI slices were quantified for VAT using semi-automated specialised software. Relationships between DXA and MRI for cross-sectional VAT and longitudinal change in VAT were determined by linear regression. Accuracy between the methods was assessed by Bland-Altman analysis, with data presented as mean difference (95% confidence interval), lower and upper limits of agreement (LoA).

RESULTS

Strong correlations were found between DXA-VAT and MRI-VAT at baseline (r = 0.90; p < 0.001), and longitudinal change in DXA-VAT and MRI-VAT over 3- and 12-months (r = 0.67; p < 0.001). In contrast, Bland-Altman analysis revealed significant overestimation by DXA-VAT volume at baseline by 13% [-104 cm³ (-157, -52 cm³), p < 0.001; LoA (-398, 189 cm³)], and underestimation of change in DXA-VAT volume over 3-months by 33% [-41 cm³ (-77, -4 cm³), p = 0.030; LoA (-228, 147 cm³)] and 12-months by 47% [-65 cm³ (-114, -17 cm³), p = 0.010; LoA (-316, 185 cm³)]. Results were similar for VAT CSA.

CONCLUSIONS

Compared with MRI, DXA substantially underestimated longitudinal changes in VAT. Therefore, DXA is not currently a valid alternative to MRI for quantifying VAT changes and may under-represent the effectiveness of interventions for obesity management.

High-Intensity Interval Training in Patients with Pulmonary Embolism: A Randomized Controlled Trial

PURPOSE

High-intensity interval training (HIIT) appears to be safe and effective in cardiovascular diseases. However, there is a paucity of data on the effect of HIIT for patients with acute pulmonary embolism (PE). The present randomized controlled trial (RCT) therefore examined the efficiency and safety of HIIT in patients with acute PE.

METHODS

In single-center parallel open-label RCT, 24 patients (5 women) discharged recently with a diagnosis of intermediate-high risk acute PE were randomized (1:1) to supervised HIIT (n = 12) or control (n = 12) group. The primary outcomes were exercise capacity evaluated in terms of the estimated maximal oxygen uptake (eVO2max), lung function (forced expiratory volume in 1 second, FEV1), right ventricular (RV) function, (RV/left ventricle diameter (LV) ratio) and health related quality of life (HRQoL). Safety was the secondary outcome.

RESULTS

8-weeks of HIIT improved eVO2max (+65%, p < 0.001), FEV1 (%) (+17%, p = 0.031), and RV/LV ratio diameter (-27%, p = 0.005), as well as HRQoL. All patients in the HIIT group tolerated exercise training without serious adverse events. The control group did not improve (p > 0.05) eVO2max, RV/LV ratio diameter, or HRQoL; however, FEV1 (%) was slightly reduced (-6%, p = 0.030).

CONCLUSION

The present RCT of a tailored center based HIIT intervention provides preliminary evidence that this intervention could improve exercise capacity, lung function, RV function, and HRQoL without serious adverse events, which could provide marked clinical benefits following PE. Further larger multicenter randomized controlled studies are needed to confirm these promising findings.

Adherence to High-Intensity Interval Training in Cardiac Rehabilitation: A REVIEW AND RECOMMENDATIONS

PURPOSE

High-intensity interval training (HIIT) is gaining popularity as a training approach for patients attending cardiac rehabilitation (CR). While the literature has focused on the efficacy of HIIT for improving cardiorespiratory fitness (CRF), particularly when compared with moderate intensity exercise, less emphasis has been placed on adherence to HIIT. The aim of this review was to summarize the current literature regarding adherence to HIIT in CR patients with coronary artery disease.

REVIEW METHODS

A review identified 36 studies investigating HIIT in CR patients with coronary artery disease. Methods and data were extracted for exercise or training adherence (to attendance, intensity, and duration), feasibility of protocols, and CRF. The review summarizes reporting of adherence; adherence to HIIT and comparator/s; the influence of adherence on changes in CRF; and feasibility of HIIT.

SUMMARY

Adherence to the attendance of HIIT sessions was high and comparable with moderate-intensity exercise. However, adherence to the intensity and duration of HIIT was variable and underreported, which has implications for determining the treatment effect of the exercise interventions being compared. Furthermore, additional research is needed to investigate the utility of home-based HIIT and long-term adherence to HIIT following supervised programs. This review provides recommendations for researchers in the measurement and reporting of adherence to HIIT and other exercise interventions to facilitate a sufficient and consistent approach for future studies. This article also highlights strategies for clinicians to improve adherence, feasibility, and enjoyment of HIIT for their patients.

Short-term and Long-term Feasibility, Safety, and Efficacy of High-Intensity Interval Training in Cardiac Rehabilitation: The FITR Heart Study Randomized Clinical Trial

Importance

High-intensity interval training (HIIT) is recognized as a potent stimulus for improving cardiorespiratory fitness (volume of oxygen consumption [VO2] peak) in patients with coronary artery disease (CAD). However, the feasibility, safety, and long-term effects of HIIT in this population are unclear.

Objective

To compare HIIT with moderate-intensity continuous training (MICT) for feasibility, safety, adherence, and efficacy of improving VO2 peak in patients with CAD.

Design, Setting, and Participants

In this single-center randomized clinical trial, participants underwent 4 weeks of supervised training in a private hospital cardiac rehabilitation program, with subsequent home-based training and follow-up over 12 months. A total of 96 participants with angiographically proven CAD aged 18 to 80 years were enrolled, and 93 participants were medically cleared for participation following a cardiopulmonary exercise test. Data were collected from May 2016 to December 2018, and data were analyzed from December 2018 to August 2019.

Interventions

A 4 × 4-minute HIIT program or a 40-minute MICT program (usual care). Patients completed 3 sessions per week (2 supervised and 1 home-based session) for 4 weeks and 3 home-based sessions per week thereafter for 48 weeks.

Main Outcomes and Measures

The primary outcome was change in VO2 peak during the cardiopulmonary exercise test from baseline to 4 weeks. Further testing occurred at 3, 6, and 12 months. Secondary outcomes were feasibility, safety, adherence, cardiovascular risk factors, and quality of life.

Results

Of 93 randomized participants, 78 (84%) were male, the mean (SD) age was 65 (8) years, and 46 were randomized to HIIT and 47 to MICT. A total of 86 participants completed testing at 4 weeks for the primary outcome, including 43 in the HIIT group and 43 in the MICT group; 69 completed testing at 12 months for VO2 peak, including 32 in the HIIT group and 37 in the MICT group. After 4 weeks, HIIT improved VO2 peak by 10% compared with 4% in the MICT group (mean [SD] oxygen uptake: HIIT, 2.9 [3.4] mL/kg/min; MICT, 1.2 [3.4] mL/kg/min; P = .02). After 12 months, there were similar improvements from baseline between groups, with a 10% improvement in the HIIT group and a 7% improvement in the MICT group (mean [SD] oxygen uptake: HIIT, 2.9 [4.5] mL/kg/min; MICT, 1.8 [4.3] mL/kg/min; P = .30). Both groups had high feasibility scores and low rates of withdrawal due to serious adverse events (3 participants in the HIIT group and 1 participant in the MICT group). One event occurred following exercise (hypotension) in the HIIT group. Over 12 months, both home-based HIIT and MICT had low rates of adherence (HIIT, 18 of 34 [53%]; MICT, 15 of 37 [41%]; P = .35) compared with the supervised stage (HIIT, 39 of 44 [91%]; MICT, 39 of 43 [91%]; P > .99).

Conclusions and Relevance

In this randomized clinical trial, a 4-week HIIT program improved VO2 peak compared with MICT in patients with CAD attending cardiac rehabilitation. However, improvements in VO2 peak at 12 months were similar for both groups. HIIT was feasible and safe, with similar adherence to MICT over 12-month follow-up. These findings support inclusion of HIIT in cardiac rehabilitation programs as an adjunct or alternative modality to moderate-intensity exercise.

Trial Registration

Australian New Zealand Clinical Trials Registry Identifier: ACTRN12615001292561.

High intensity interval training does not result in short- or long-term dietary compensation in cardiac rehabilitation: Results from the FITR heart study

The aim of this study was to investigate short- and long-term compensatory effects on dietary intake following high intensity interval training (HIIT) compared with usual care moderate intensity continuous training (MICT) during and following a cardiac rehabilitation program. This study investigates secondary outcomes of a clinical trial. Ninety-three participants with coronary artery disease enrolled in a 4-week cardiac rehabilitation program, were randomised to 1) 4x4-minute HIIT; or 2) 40-min of MICT (usual care). Patients were instructed to complete 3 weekly sessions (2 supervised, 1 home-based) for 4-weeks, and 3 weekly home-based sessions thereafter for another 48-weeks. Dietary intake was measured by telephone-based 24-h recall over 2 day at baseline, 4-weeks, 3-months, 6-months, and 12-months. Three-Factor Eating Questionnaire was used to measure dietary behaviour and Leeds Food Preference Questionnaire used to measure food preferences. Appetite was assessed by a visual analogue scale and appetite-regulating hormones. There was no change over the study period or differences between groups for daily energy intake at 4-weeks or 12-months. There were also no group differences for any other measures of dietary intake, fasting hunger or appetite-related hormones, dietary behaviour, or food preferences. These findings suggest that compared to moderate intensity exercise, HIIT does not result in compensatory increases of energy intake or indicators of poor diet quality. This finding appears to be the same for patients with normal weight and obesity. HIIT can therefore be included in cardiac rehabilitation programs as an adjunct or alterative to MICT, without concern for any undesirable dietary compensation.

Effect of high intensity interval training on body composition in women before and after menopause: a meta-analysis

NEW FINDINGS

What is the topic of this review? A meta-analysis of the efficacy of high intensity interval training (HIIT) in reducing weight, total fat mass (FM) and (intra)-abdominal FM in normal-weight and overweight/obese women before and after menopause. What advances does it highlight? HIIT programmes in women significantly decrease body weight and total and abdominal FM. Their effects are more evident in pre- than in postmenopausal women. Cycling HIIT seems more effective than running, especially in postmenopausal women, and training interventions longer than 8 weeks comprising three sessions a week should be promoted.

ABSTRACT

High-intensity interval training (HIIT) is a stimulating modality for reducing body weight and adipose tissue. The purpose of this meta-analysis was to assess the efficacy of HIIT in reducing weight, total fat mass (FM) and (intra)-abdominal FM in normal-weight and overweight/obese women before and after menopause. A structured electronic search was performed to find all publications relevant to our review. Stratified analyses were made of hormonal status (pre- vs. postmenopausal state), weight, HIIT modalities (cycling vs. running), programme duration (< or ≥8 weeks) and the methods used to measure body composition (dual-energy X-ray absorptiometry vs. computed tomography, Magnetic Resonance Imaging and others). A total of 38 studies involving 959 subjects were included. Our meta-analysis showed that overall HIIT programmes significantly decrease weight, total and abdominal FM in women. Both normal weight and overweight/obese women lost total FM after HIIT protocols whereas HIIT was only effective in decreasing abdominal FM in women with excess adiposity. When pre- and postmenopausal women were considered separately, the effect of HIIT on weight, total and abdominal FM were only significant before menopause. Cycling HIIT seemed more effective than running, especially in postmenopausal women, and training interventions longer than 8 weeks comprising three sessions were more efficient. HIIT is a successful strategy to lose weight and FM in normal weight and overweight/obese women. However, further studies are still needed to draw meaningful conclusions about the real effectiveness of HIIT protocols in postmenopausal women.

Effect of High-Intensity Interval Training on Visceral and Liver Fat in Cardiac Rehabilitation: A Randomized Controlled Trial

OBJECTIVE

This study aimed to investigate the effect of exercise intensity on visceral adipose tissue (VAT) and liver fat reduction in patients with coronary artery disease (CAD) over 3 months and the maintenance of improvements over 12 months.

METHODS

Forty-two participants with CAD were randomized to three sessions/week of either 4 × 4-minute high-intensity interval training (HIIT) or 40 minutes of usual care moderate-intensity continuous training (MICT) for a 4-week supervised cardiac rehabilitation program, followed by three home-based sessions/week for 11 months. Liver fat (as intrahepatic lipid) and VAT were measured via magnetic resonance techniques. Data are mean change (95% CI).

RESULTS

HIIT and MICT significantly reduced VAT over 3 months (-350 [-548 to -153] cm³ vs. -456 [-634 to -278] cm³ ; time × group effect: P = 0.421), with further improvement over 12 months (-545 [-818 to -271] cm³ vs. -521 [-784 to -258] cm³ ; time × group effect: P = 0.577) and no differences between groups. Both groups improved liver fat over 3 months, with HIIT tending to show greater reduction than MICT (-2.8% [-4.0% to -1.6%] vs. -1.4% [-2.4% to -0.4%]; time × group effect: P = 0.077). After 12 months, improvements were maintained to a similar degree. Higher exercise intensity predicted liver fat reduction (β = -0.3 [-0.7 to 0.0]; P = 0.042).

CONCLUSIONS

HIIT and MICT reduced VAT over 3 and 12 months. For liver fat, HIIT tended to provide a slightly greater reduction compared with MICT. These findings support HIIT as a beneficial adjunct or alternative to MICT for reducing visceral and liver fat in patients with CAD.

The feasibility of implementing high-intensity interval training in cardiac rehabilitation settings: a retrospective analysis

Background

Cardiovascular disease is the leading cause of death worldwide. Notwithstanding the well-known benefits of cardiac rehabilitation (CR), adherence to CR remains low, particularly in women. High-intensity interval training (HIIT) has received specific attention as an emerging exercise-training paradigm that addresses frequently cited barriers to CR (i.e. lack of motivation/enjoyment and time, perceiving exercise regime as tiring/boring) and improves cardiovascular risk factors. Previous studies have examined the safety of HIIT in CR; there is little evidence on the feasibility of HIIT in CR. The aims of this study were to evaluate the feasibility of HIIT within a CR setting and examine the sex differences regarding the feasibility of such programming.

Methods

Patients attended an on-site HIIT CR program (10-min warm-up, 25 min of interspersed high-intensity [HI - 4 min at 85–95% HRpeak] and lower intensity [LO - 3 min at 60–70% HRpeak] intervals, 10-min cool-down) twice weekly for 10 weeks. Heart rate (HR) and the Borg rating of perceived exertion (RPE) scale (6–20 points) were recorded at each session. Feasibility was assessed by: [1] attendance and compliance: the number of sessions attended and the compliance to the prescribed HI and LO HR ranges; [2] the patient experience: patients’ perceived effort, program difficulty, if the program was challenging and satisfying; and, [3] safety. Descriptive statistics were used to report the means and their variations. Mann-Whitney U tests and Chi-square analyses were performed to examine sex-differences.

Results

A total of 151 patients (33% women, 57.5 ± 9.1 years) attended the HIIT program and completed 16 ± 5 classes with a low attrition rate (11.3%). Most patients met or exceeded the prescribed target HR for the HI (80%) and LO (84%) intervals, respectively. Patients reported a “somewhat hard” RPE for HI (14 ± 2 points) and “very light” for LO (10 ± 2 points) intervals. All patients were satisfied with the program and found it challenging. Most patients found HIIT to be difficult (7 ± 2 points, scale range 0–10 points), yet safe (97%). Three vasovagal episodes occurred and more women dropped-out of the program than men (p < 0.01).

Conclusions

HIIT is a feasible, safe and well-received exercise paradigm in a CR setting.

A Multi-Center Comparison of O2peak Trainability Between Interval Training and Moderate Intensity Continuous Training

There is heterogeneity in the observed O_2peak response to similar exercise training, and different exercise approaches produce variable degrees of exercise response (trainability). The aim of this study was to combine data from different laboratories to compare O_2peak trainability between various volumes of interval training and Moderate Intensity Continuous Training (MICT). For interval training, volumes were classified by the duration of total interval time. High-volume High Intensity Interval Training (HIIT) included studies that had participants complete more than 15 min of high intensity efforts per session. Low-volume HIIT/Sprint Interval Training (SIT) included studies using less than 15 min of high intensity efforts per session. In total, 677 participants across 18 aerobic exercise training interventions from eight different universities in five countries were included in the analysis. Participants had completed 3 weeks or more of either high-volume HIIT (n = 299), low-volume HIIT/SIT (n = 116), or MICT (n = 262) and were predominately men (n = 495) with a mix of healthy, elderly and clinical populations. Each training intervention improved mean O_2peak at the group level (P < 0.001). After adjusting for covariates, high-volume HIIT had a significantly greater (P < 0.05) absolute O_2peak increase (0.29 L/min) compared to MICT (0.20 L/min) and low-volume HIIT/SIT (0.18 L/min). Adjusted relative O_2peak increase was also significantly greater (P < 0.01) in high-volume HIIT (3.3 ml/kg/min) than MICT (2.4 ml/kg/min) and insignificantly greater (P = 0.09) than low-volume HIIT/SIT (2.5 mL/kg/min). Based on a high threshold for a likely response (technical error of measurement plus the minimal clinically important difference), high-volume HIIT had significantly more (P < 0.01) likely responders (31%) compared to low-volume HIIT/SIT (16%) and MICT (21%). Covariates such as age, sex, the individual study, population group, sessions per week, study duration and the average between pre and post O_2peak explained only 17.3% of the variance in O_2peak trainability. In conclusion, high-volume HIIT had more likely responders to improvements in O_2peak compared to low-volume HIIT/SIT and MICT.