INTRODUCTION
Epidemiology and Pathophysiology
Obesity-related diseases, such as prediabetes, type 2 diabetes, dyslipidemia, and metabolic syndrome, are considered major public health issues across the globe, resulting in increased individual risk of cardiovascular and pulmonary diseases as well as certain cancers (1). The prevalence of metabolic disease continues to increase such that a significant percentage of the global adult population are affected today (1). It is notable that the annual global medical cost of treating obesity-related diseases has been estimated at $2 trillion USD, representing a substantial burden for health care systems worldwide (2). Overweight and obesity, combined with physical inactivity, result in significant risk factors for impaired metabolic health and subsequent comorbidities (3) (Table, Supplemental Digital Content 1, https://links.lww.com/FIT/A175).
HIIT IN METABOLIC DISEASE
High-intensity interval training (HIIT) has been documented as an effective (4,5) and popular (6) exercise mode for athletic and general populations, as well as for individuals with obesity-related metabolic diseases (7,8). Recently, the American College of Sports Medicine (ACSM) underlined the role of HIIT in cardiometabolic disease prevention, indicating that HIIT and moderate-intensity continuous training (MICT) elicit similar improvements in body composition, insulin sensitivity, and blood pressure in overweight and obese adults (9).
Physiological adaptations
Cardiorespiratory Fitness
There is accumulating evidence linking cardiorespiratory fitness (CRF) with all-cause mortality, thus making CRF an important clinical vital sign (10). Although all exercise modes are considered effective in increasing aerobic capacity or CRF (11), evidence suggests that vigorous exercise is more beneficial for improving CRF than moderate-intensity exercise (12). In fact, HIIT appears to be superior to MICT in improving maximal aerobic capacity in populations with unhealthy body mass (13,14). In addition, CRF improvements appear to be greater when the duration of work intervals exceed 2 minutes (14). Regarding HIIT, both short- (≤12 weeks) and long-term (>12 weeks) interventions may significantly improve CRF in overweight/obese populations (15). Further, hybrid-type exercise protocols integrating HIIT and functional resistance training into the same session have been reported to induce meaningful increases in CRF after 9-week (16) and 40-week (17) supervised interventions in previously inactive overweight or obese women. In addition, both exercise modes showed similar improvements in CRF for overweight adults after a 12-month unsupervised intervention (18). Collectively, such positive responses of CRF observed in response to HIIT are associated with physiological adaptations that are linked to low risk of all-cause mortality and adverse cardiovascular events (19).
Further research is warranted to investigate the sustainability of HIIT and whether such a vigorous exercise mode can promote long-term exercise adherence in individuals with metabolic diseases.
Body Composition
Current evidence suggests that HIIT elicits modest improvements in various anthropometric and body composition indices (15); however, these changes are similar to those reported for MICT in overweight and obese adults (20,21) as well as adults with prediabetes and type 2 diabetes (22). HIIT also has been reported as a time-efficient exercise strategy, requiring approximately 40% less training time compared with MICT for overweight/obese populations (20) to yield similar outcomes. Although exercise alone is not optimal for weight loss (23), HIIT itself can produce positive changes in anthropometry and body composition (13) regardless of clinical outcomes (21). In comparing aerobic-based HIIT formats, running appears to provide greater reductions in abdominal and visceral fat mass compared with cycling in overweight and obese adults (24). Recently, hybrid HIIT-based training programs, mixing aerobic and resistance, have been shown to be safe and effective for improving various anthropometric and body composition variables in inactive, overweight adults (16–18).
HIIT seems to be a time-efficient, powerful tool for improving CRF, body composition, glucose homeostasis, and lipid metabolism through either short- or long-term interventions in adults with impaired metabolic health.
Metabolic Health
HIIT appears to be effective for improving various cardiometabolic risk factors such as resting blood pressure, blood lipids, and glycemic regulation (15), with low-volume (51 minutes/week), high-volume (114 minutes/week), and MICT (150 minutes/week) programs all showing similar effectives in ameliorating the severity of metabolic syndrome (25). HIIT also elicits more beneficial changes in insulin resistance compared with aerobic, resistance, or combined training in adults with poor glycemic control (14,22). Recently, short-term (6–8 weeks) interventions using hybrid HIIT-style protocols revealed significant improvements in glucose control in adults with type 2 diabetes (26,27), but not in those with overweight and obesity (28). However, longer interventions (20 weeks) appeared to produce meaningful changes in various cardiometabolic health indices in overweight or obese women (29).
The aforementioned findings suggest that HIIT may reduce fasting blood glucose levels even after a short-term, low-volume, exercise protocol (30), which is a critical outcome for individuals with poor glucose regulation. HIIT and MICT exhibit similar efficacy in normalizing blood lipids in overweight or obese adults (14) and with poor glucose control (22). The implementation of HIIT for individuals with impaired blood lipid metabolism has been reported as a beneficial part of the exercise prescription (8,9) pointing to greater cardiovascular and hemodynamic adaptations through autonomic nervous system adjustments in obesity and diabetes (31).
HIIT Programming
Safety Considerations
According to ACSM’s exercise preparticipation health screening guidelines, medical clearance is recommended for previously active or inactive individuals diagnosed with any metabolic disease before engaging in any structured exercise program at any intensity (32). Furthermore, ΗΙΙΤ does not have to be an all-out exercise experience, as the level of intensity can be carefully increased over time, as evidence supports moderate-intensity interval training as an effective exercise strategy for improving glycemic control, body composition, and physical fitness in adults with type 2 diabetes (33). Heart rate monitoring and rating of perceived exertion can be used for assessing the exercise intensity throughout the session (34). In summary, guidelines for the delivery of HIIT for clinical populations, including those with metabolic diseases, have been recently published, highlighting the importance of monitoring training progression and recording as well as reporting exercise training intensities (35).
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Prescription
Given the international guidelines for physical activity and exercise (34), and those for sedentary behavior (36), obesity (37), and type 2 diabetes (38), such populations should establish a foundational level of fitness before entering a HIIT protocol. Individuals are best advised to participate in a preparatory training plan before applying HIIT, to reduce the risk of musculoskeletal injury, while ensuring a positive exercise experience from the outset. A periodized preparatory training plan for medically cleared individuals with metabolic diseases is shown in Table 1.
TABLE 1 -
Supervised, Periodized Preparatory Training Plan
|
|
|
|
Intensity |
Total Time
a
|
| Phase |
Duration |
Mode |
Frequency |
Classification |
%HRR |
%MHR |
RPE
b
|
Per Session |
Per Week |
| 1 |
4–6 weeks |
MICT |
3–5 days/week |
Moderate |
40–59 |
55–69 |
11–13 |
20–60 minutes |
150–300 minutes |
| 2 |
4–6 weeks |
MIIT |
2–3 days/week |
Moderate–Hard |
60–79 |
70–84 |
13–15 |
30–40 minutes |
75–150 minutes |
| 3 |
4–6 weeks |
HIIT |
1–2 days/week |
Hard |
≥80 |
≥85 |
15–16 |
15–25 minutes |
15–50 minutes |
| 4 |
4–6 weeks |
HIIT |
1–2 days/week |
Very hard |
≥85 |
≥90 |
16–17 |
10–20 minutes |
10–40 minutes |
MICT, moderate-intensity continuous training; MIIT, moderate-intensity interval training; HIIT, high-intensity interval training; HRR, heart rate reserve; MHR, maximal heart rate; RPE, rating of perceived exertion.
aIncluding warm-up and cooldown (gradually increase by 10%–20% per week).
b6–20 scale.
Overall, intermittent-based exercise prescription for populations with metabolic diseases mainly involves two models: 1) traditional, aerobic-based training as a single-component session (25,39–41) and 2) hybrid, resistance-based training as a multicomponent session (16,17,26–28). Table 2 shows the definitions of these two different models. In general, there are many different HIIT protocols, and thus careful planning and a customized approach are needed when designing individual HIIT workouts (8). The duration of the work and the duration of the recovery intervals are considered important training parameters, and therefore practitioners should focus on intensity, not volume, when implementing progressive HIIT protocols. Ideally, work-to-rest ratios should range from 1:4 to 1:1 (17). Lastly, considering that safety should always be a priority, only low- to moderate-impact drills, i.e., multiplanar, resistance-based exercises, should be blended with high intensity when designing hybrid-type programs for previously inactive individuals with impaired metabolic health or poor functional capacity (42). Table 3 provides two sample workout routines that can be implemented in most settings.
TABLE 2 -
Definition of the Models of Intermittent-Based Exercise Training
| Model |
Format |
Training Parameters |
Modalities |
| Traditional |
Single component (aerobic-based) |
Frequency: 2–3 days per week; work intervals: 2–4 minutes (85%–95% MHR); recovery intervals: 1–3 minutes (60%–70% MHR); series per session: 4–6 times Total time: 20–30 minutes |
One of the following: walking, running, cycling, stair climbing, elliptical, rowing, swimming |
| Hybrid |
Multicomponent (resistance based) |
Frequency: 2–3 days per week; work intervals: 30–60 seconds (>85% MHR); recovery intervals: 30–60 seconds (passive); series per session: 8–12 times (2–3 rounds); recovery time per round: 2–3 minutes (passive); total time: 20–30 minutes |
Full-body movements using either bodyweight or integrated neuromuscular exercises with adjunct equipment (e.g., kettlebells, medicine balls, suspension exercise devices, battle ropes, resistance bands, and balance/stability balls) |
TABLE 3 -
Traditional and Hybrid Supervised, Intermittent-Based Workout Routines
| Phase |
Training Parameters |
Modality |
|
Traditional (single-component bout)
|
| Warm-up |
3 minutes at 60%–70% MHR (RPE 10–11) plus dynamic stretching |
Brisk walking |
| Conditioning |
4–5 × 3–4 minutes work intervals at 85%–95% (RPE 15–16); 3–4 × 2–3 minutes recovery intervals at 60%–70% MHR (RPE 11–13); progression of work-to-rest ratios: 0.75:1 (weeks 1–4), 1:1 (weeks 5–8), and 1:0.75 (weeks 9–12) |
Aerobic-based activities
a
|
| Cool-down |
2 minutes at 50%–60% MHR plus static stretching |
Easy walking |
| Hybrid (multicomponent bout) |
| Warm-up |
5 minutes at 60%–70% MHR (RPE 10–11); movement preparation and dynamic stretching |
Brisk walking; fundamental patterns |
| Conditioning |
6–12 × 30–60 seconds work intervals (>85% MHR, RPE 15–17); 5–11 × 30–60 seconds recovery intervals (passive); volume: 2–3 rounds (2–3 minutes rest per round); progression of work-to-rest ratios: 1:3 (weeks 1–4), 1:2 (weeks 5–8), and 1:1 (weeks 9–12) |
Resistance-based exercises
b
combined with dynamic bodyweight movements
c
|
| Cool-down |
2 minutes at 50%–60% MHR plus static stretching |
Easy walking |
MHR, maximal heart rate; RPE, rating of perceived exertion.
aWalking, jogging, running, stair climbing, elliptical, rowing, or swimming.
bIntegrated neuromuscular movements using fundamental patterns (e.g., bend and lift, pushing, pulling, carry, single-leg, and twist).
cLow knee skips, hops in place, jogging in place, jumping jacks, split jacks, ice skaters, mountain climbers, and burpees.
Supervised progressive HIIT can serve as a piece of the exercise programming puzzle for medically cleared individuals with controlled metabolic diseases in the real world.
Summary
HIIT is a time-efficient strategy to elicit physiological and psychological adaptations linked to improved physical fitness and metabolic health. In addition, HIIT can increase both exercise adherence and exercise enjoyment in populations with metabolic diseases. Current evidence suggests that brief, vigorous, intermittent exercise can be an enjoyable and effective part of any exercise program for certain clinical populations, yet future studies need to focus on the optimal implementation of training parameters when designing effective HIIT protocols for individuals with impaired metabolic health.
BRIDGING THE GAP
HIIT is an effective exercise mode inducing meaningful benefits on physical fitness, metabolic health, and cardiovascular function. This training approach requires adults with metabolic diseases to engage in physically and mentally demanding conditions with respect to the exercise intensity commonly used in other modalities. However, emerging research reveals that incorporating HIIT as part of a comprehensive exercise program for individuals with impaired metabolic health could be a pleasurable and feasible training option aimed at improving health and fitness.
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