**Background:** Metabolic syndrome (MetS) affects approximately 13–31% of adults globally and is associated with increased risk of cardiovascular disease, cancer, all-cause mortality, and diminished quality of life. Exercise is integral to MetS treatment, but time constraints are among the most frequently reported barriers to regular exercise. Low-volume high-intensity interval training (LOW-HIIT) has emerged as a time-efficient approach, typically requiring ≤10 min of intense interval bouts within a total session of ≤30 min. Exercise intensity for LOW-HIIT is most commonly prescribed based on percentages of maximum heart rate (HRmax), but HRmax determination requires maximal effort during exercise testing, which may not always be feasible or safe for MetS patients. Lactate threshold (LT) determination requires only submaximal effort. This study compared the effects of a 12-week LOW-HIIT program prescribed using HRmax (HIIT-HR) versus LT (HIIT-LT) on cardiometabolic health and quality of life in MetS patients.
**Methods:** Seventy-five obese MetS patients were randomized to HIIT-HR (n=25), HIIT-LT (n=25), or a non-exercising control group (CON, n=25). All patients received standard care nutritional consultation targeting a 500 kcal/day reduction. Both exercise groups performed supervised LOW-HIIT on cycle ergometers twice weekly for 12 weeks (24 sessions total). The protocol consisted of 2 min warm-up, 5 × 1 min vigorous intervals separated by 1 min recovery, and 3 min cool-down (14 min total). HIIT-HR intervals progressed from 80–85% HRmax (weeks 1–4) to 90–95% HRmax (weeks 9–12). HIIT-LT intervals progressed from 95–100% LT (weeks 1–4) to 100–105% LT (weeks 5–12). The LT was defined at 4 mmol/L blood lactate using a step incremental exercise test. Outcomes included VO2max (primary), body composition, waist circumference, blood pressure, HbA1c, fasting insulin, HOMA-index, MetS z-score, and EQ-5D-5L quality of life. Seventeen patients dropped out (HIIT-HR=5, HIIT-LT=5, CON=7), leaving 58 patients for final analysis.
**Key Results:** Compliance was high in both exercise groups (HIIT-HR: 96±6%; HIIT-LT: 94±8%). No adverse events were related to the LOW-HIIT interventions. The HR at LT corresponded to 94±4% of HRmax. Average peak HR during intervals was 93±7% HRmax (HIIT-HR) and 96±3% HRmax (HIIT-LT), confirming target intensities were achieved. Both exercise groups showed significant and similar improvements in relative VO2max (HIIT-HR: +3.6 mL/kg/min, 95% CI 2.5–4.7, p<0.001; HIIT-LT: +3.7 mL/kg/min, 95% CI 2.3–5.0, p<0.001), while CON showed no change. Both exercise groups significantly reduced systolic blood pressure (HIIT-HR: −11 mmHg, p<0.001; HIIT-LT: −13 mmHg, p<0.001), diastolic blood pressure (HIIT-HR: −8 mmHg, p<0.001; HIIT-LT: −10 mmHg, p<0.001), HbA1c (HIIT-HR: −0.2%, p=0.012; HIIT-LT: −0.3%, p<0.001), HOMA-index (HIIT-HR: −1.3 units, p=0.005; HIIT-LT: −1.0 units, p=0.014), and MetS z-score (HIIT-HR: −1.9 units, p<0.001; HIIT-LT: −2.5 units, p<0.001). Quality of life (EQ-VAS) improved by 10 points (p=0.029) in HIIT-HR and 11 points (p=0.002) in HIIT-LT. All groups lost weight (HIIT-HR: −3.9 kg, p<0.001; HIIT-LT: −5.6 kg, p<0.001; CON: −2.6 kg, p=0.003), with no significant between-group difference (p=0.064). However, only the exercise groups significantly reduced waist circumference and fat mass compared to CON. The CON group showed no improvements in any cardiometabolic or fitness outcomes, and absolute VO2max (−214 mL/min, p=0.037) and ventilatory threshold workload (−17 W, p=0.003) significantly decreased in CON.
**Clinical Implications:** This study demonstrates that LOW-HIIT prescribed using either HRmax or LT produces comparable and clinically meaningful improvements in cardiorespiratory fitness, blood pressure, glycemic control, metabolic syndrome severity, and quality of life in obese MetS patients. The improvements in VO2max (~3.7 mL/kg/min) are clinically significant, as each 1 mL/kg/min increase in VO2max is associated with a 9% reduction in overall mortality, and each 3.5 mL/kg/min improvement is linked to 15% reduced risk of premature death from cardiovascular disease and cancer. The average blood pressure reductions (−12/−9 mmHg) correspond to a 22% reduced risk of coronary heart disease and 41% reduced stroke risk per 10/5 mmHg decrease. The practical take-home message is that clinicians can feasibly prescribe LOW-HIIT intensity using a submaximal LT-based exercise test when patients cannot or will not perform maximal cardiopulmonary exercise testing. The very low exercise volume (14 min/session, twice weekly) makes this approach particularly attractive for patients who cite time constraints as a barrier to regular exercise. However, for pure weight loss, higher-volume exercise modalities may be more effective, as the low-volume protocol did not substantially impact daily energy balance beyond dietary restriction alone.