Discussion
Approximately 1 in 3 people in the USA suffer from MetS [
7]. This statistic has increased significantly in the past decades and is projected to continue in an upward trend [
7]. Similar prevalence and increasing rates have been found across the globe [
101]. A plethora of evidence exists demonstrating that physical activity and exercise reduce the risk factors of MetS [
4,
8‐
10], and therefore, exercise recommendations have been published internationally [
8,
22]. Despite this evidence, only 1 in 4 adults in the world and less than 25% of adults in the USA meet these recommendations [
22,
51], claiming the perceived barriers of “lack of time” and “lack of enjoyment” in the prescribed exercise recommendations [
52‐
54]. HIFT is a time-efficient modality of exercise that has been shown to elicit enjoyment and adherence [
55]; improve oxygen capacity, insulin resistance, and muscular strength [
58], as well as the MetS
z-score [
59] in at-risk populations; and improve cardiorespiratory fitness [
60,
61], muscular strength, and power [
62], as well as waist circumference and agility [
61] in healthy populations. This study aims to continue the investigation of HIFT as it seems to combat the barriers to regular exercise and improve cardiometabolic risk factors.
Researchers of this modality have used several terms to define it (e.g., HIIT-type, high-intensity circuit resistance training, multimodal high-intensity training, high-intensity interval neuromuscular training, functional high-intensity training); all terms trying to describe a modality of exercise that combines aerobic and functional resistance training performed in an interval fashion and completed in a time-efficient manner. These modalities seem to be trying to achieve a similar thing, to combat the barriers to exercise and optimize the physiological responses to achieve comprehensive health and fitness benefits. Recently, a paper was published synthesizing this initiative and modality into common terminology, high-intensity functional training (HIFT), in order to formalize a definition for better access and comparison of future studies [
57]. HIFT emphasizes functional, multi-joint movements via both aerobic and muscle-strengthening exercises [
57]. These exercises (e.g., squats, push-ups, pull-ups, deadlifts, weighted carry) are designed to improve the pattern of movement for activities of daily living, specific occupational tasks, and sports skills [
57,
74]. Traditionally, these exercises are prescribed in sets and reps with long recovery periods and are said to have a minimal aerobic response [
102]. But if performed in a high-intensity interval fashion, this modality can elicit aerobic and anaerobic adaptations as well as improve strength and power [
102].
Cardiovascular fitness, muscular strength and endurance, neuromotor fitness, and cardiometabolic health are all components improved if one adheres to the published exercise recommendations (150 min/week-moderate aerobic training + 2–3 days/week-resistance training + daily flexibility training) [
8,
22]. The time commitment to meet these recommendations is upwards of 250–300 min/week. For special populations, such as those with MetS, the recommendations increase the aerobic exercise to anywhere between 300 and 420 min/week [
8]. If adherence to the general guidelines is poor due to the time commitment, a larger time commitment could likely increase that barrier. HIFT protocols seem to range between 20 and 40 min of exercise time, resulting in approximately 50–60 min per session if the warm-up, rest periods, and cool-down are included [
57‐
63]. The HIFT interventions within the literature that demonstrated fitness and health improvements were performed 3–4 times per week, requiring a time commitment of 150–200 min, significantly less than the published recommendations [
57‐
63]. However, it is unknown if health benefits are seen when HIFT is performed less than 3 times per week, therefore requiring even less time commitment. Thus, the exploration of this study.
Minimal dose, or minimal time commitment, exercise protocols are currently being explored aiming to combat the “lack of time” barrier to exercise [
103,
104]. Most interventions modify modality, intensity, and/or load to therefore reduce duration and frequency, which ultimately reduces time commitment. For example, when aerobic intensity was increased to maximum, the duration of the session necessary for equivalent cardiorespiratory benefits was 67% less time than exercise at moderate intensity [
105,
106]. Equivalent improvements in the markers of muscle lipid and carbohydrate oxidation were also found between the high-intensity and moderate-intensity groups [
105]. The workload performed by the moderate-intensity group followed the general exercise guidelines of 150 min/week [
8,
105], which are the same recommendations for blood lipid and insulin resistance improvements [
107]. Although blood lipids and insulin resistance were not measured, it is possible these could be improved due to the mitochondrial remodeling seen, however with 67% less time commitment in the high-intensity group [
105]. In another study investigating HIIT frequency, when high-intensity aerobic exercise (10 × 60s cycling at 83% peak power; ~ 20 min session) was performed only two times per week, significant improvements were seen in cardiorespiratory fitness and body composition [
65].
Resistance training studies have also explored the minimal dose necessary for increases in strength and hypertrophy. To achieve gains, investigators have determined that resistance loads can vary (30–70% 1RM) as long as they are performed with high effort near volitional or momentary muscular failure [
108]. When loads reached high amounts, the repetitions to failure were reduced, therefore reducing the time commitment [
108]. These authors also determined that two of these training sessions per week was the minimum frequency to see increases in strength and hypertrophy [
108]. In another dose-response study, sedentary adolescent males performed 1, 2, 3, 4, or 5 sets of a HIIT protocol (4 × 20s all-out exercise interspersed with 10-s rests), twice weekly plus one resistance training session per guidelines [
8,
109]. Surprisingly, the group that performed 1 set (3 min 40s per week of HIIT) had equivalent decreases in visceral fat as the groups performing up to 5 sets (18 min 20 s per week of HIIT) [
109]. Maximal oxygen uptake was improved in all groups with only a 1% greater improvement in the groups performing 4 and 5 sets [
109]. These findings suggest that meaningful health benefits are gained with an extremely low dose of maximal HIIT twice weekly, plus one resistance training session [
109]. All of these protocols require much less of a time commitment than the general guidelines state, yet still seem to provide beneficial metabolic and fitness adaptations.
To our knowledge, a minimal dose of exercise to see improvements in blood lipids, insulin resistance, and endothelial function has not been explored. In clinical practice, the current recommended exercise dose for blood lipid and insulin resistance is in line with the general recommendations, noting that higher volumes lead to further improvements [
107]. Minimal dose hypotheses are worth testing, however, based on the demonstrated acute physiological responses, as insulin, glucose, and lipid changes have shown to last as long as 48 h after exercise [
96,
110,
111]. The acute physiological responses to one session of exercise are the stimulus that leads to chronic adaptions when repeated and therefore should not be viewed in isolation [
112].
Willis et al. quantified the intensity and energy expenditure of a typical HIFT workout in healthy men and women. During this 44-min exercise bout, the participant’s average HR was 80% of HR
max (range 69–100%), and the average energy expenditure was approximately 485 kcal (range 418–552 kcal) [
63]. Although these authors did not measure any cardiometabolic markers, this has been acutely explored in other exercise interventions of the individual components of HIIT and resistance training. In healthy men performing an exhaustive cycling bout (~ 8 mins) at a high aerobic intensity of 93.5% ± 5% VO
2max yet lower energy expenditure (141 kcal ± 64.9 kcal) than a HIFT bout, saw significant decreases in TC and LDL-C immediate and 1-h post-exercise [
113]. Also, in healthy men, when running intensity was lower (70% VO
2max), immediate decreases in TC and LDL-C were not seen until an energy expenditure of 1300 kcal was reached, which took approximately 94 mins [
114]. As the aerobic intensity and energy expenditure of a HIFT workout falls between the thresholds of these two studies, perhaps there is a sweet spot where beneficial acute blood lipid improvements can be seen. The research on acute blood lipid responses to resistance training is quite limited. One study exploring VLDL-TG kinetics the morning after evening exercise in untrained but healthy men, the authors found a 28% decrease in VLDL-TG concentration and a 30% increase in particle clearance rate after a 90 min resistance exercise bout expending ~ 400 kcal [
115]. This demonstrates that resistance training has a potent acute effect on blood TG and could provide additional benefits when combined with acute lipid effects from high-intensity aerobic exercise, as is done in HIFT [
8,
57].
Acute blood lipid responses seem to differ however in hypercholesterolemic (HC) men and women. Acute improvements in HDL-C and TG are commonly seen [
95,
111,
116]. However, TC and LDL-C have shown an immediate decrease [
95,
116], then return to baseline at 24 h [
95] or rose above baseline at 24 h [
116]. In another study of a 350-kcal cycling session at 80% VO
2max, TC and LDL-C were elevated at 24 and 48 h after the session in HC men [
96]. It is still unknown how hypercholesterolemic individuals may acutely respond to higher intensity intervals, as each of these studies explored steady-state exercise. “Exercise snacking” is a term to describe very short bouts of exercise (≤ 5 min) performed intermittently throughout the day and is often studied as a method to interrupt sedentary behavior [
117]. The authors exploring this minimal dose approach found that 3 min of simple resistance like exercises (half-squat, calf raises, gluteal contractions, knee raises) performed every 30 min (total of 36 min/day) attenuated postprandial blood glucose, insulin, and triglyceride responses in type 2 diabetics [
118].
The exercise-mediated improvements in cardiometabolic health are often not fully explained by the traditional risk factors of blood lipids, glucose, and insulin, but rather can be attributed to improvements in vascular health [
45]. In untrained individuals with metabolic syndrome, one session of high-intensity aerobic training (4 × 4 min at 90–95% HR
max with 3 min rests between intervals) improved endothelial function from 5 to 11% immediately after, with a lasting effect for 72 h [
119]. Perhaps the aerobic intensity, energy expenditure, combined modality, and time to completion of HIFT will be an ideal combination of factors to elicit acute lipid, glucose, insulin, and endothelial changes that could lead to chronic adaptations if performed frequently. The minimal dose of this repetition is unknown though. With the most commonly stated barrier to exercise as “lack of time,” minimal effective exercise dose investigations are highly warranted.
Not only is HIFT time-efficient, but HIFT is translatable to various physical settings. The equipment required is minimal, portable, and affordable compared to sophisticated cardio and resistance training machines found in most fitness facilities [
57,
120]. Most HIFT protocols involve exercises with bodyweight, kettlebells, medicine balls, dumbbells, resistance bands, suspension bands, slam balls, stability balls, and other portable weighted equipment. The use of these types of “free weights” was concluded as #4 in the top 20 exercise trends of 2020 [
56]. HIFT is popular [
56], enjoyable [
55], time-efficient [
57], translatable to various settings [
57], and elicits health and fitness benefits [
57‐
63]. The results of this study will provide insight into the minimal dose of HIFT necessary for cardiometabolic health improvements in people with MetS. In turn, these findings will aid in the development of new exercise programming guidelines for MetS and related disorders.