In recent years, a modified form of high-intensity interval training (HIIT) known as reduced-exertion high-intensity training (REHIT) has grown in popularity. REHIT shortens the traditional HIIT workout by using fewer and shorter interval bouts, but how low can you really go and still get an effective workout? This article looks at the available research to answer that question and explores practical strategies for incorporating REHIT into your clients’ training programs.    

The Evolution of REHIT

Over the past decade, the concept of HIIT has captivated the attention of the scientific community and health and exercise professionals alike due to its superior ability to improve cardiorespiratory fitness and cardiometabolic health for a lesser weekly time commitment relative to the current aerobic physical-activity guideline of moderate-intensity continuous training. HIIT involves multiple (4 to 10 repetitions), brief bouts (20 seconds to 5 minutes) of high-intensity exercise [80 to 100% peak heart rate (HRpeak)] interspersed with either rest or low-intensity workloads throughout an exercise session. Another form of interval training is known as sprint interval training (SIT), which is characterized by repeated (6 to 10 bouts), 20- to 30-second all-out supramaximal sprints [>100% maximal oxygen uptake (VO2max)]. This strategy enables unfit individuals to accumulate periods of vigorous- to high-intensity exercise that would otherwise not be possible if executed continuously. 

One drawback to the protocols employed in the majority of previous HIIT/SIT studies, however, is that they are not actually time-efficient, with most HIIT/SIT protocols requiring a time commitment (about 120 minutes per week) that is similar to the current recommended aerobic physical-activity guideline of 150 minutes per week of moderate-intensity continuous training. Moreover, it is important to understand that what a person feels (pleasure/displeasure) and how a person feels (rating of perceived exertion) during exercise can impact the adoption of, and adherence to, exercise programs and future exercise participation. It has been suggested that the potential for a negative perceptual response (displeasure) intensifies with increasing repetition. In other words, as the number of intervals increase the feelings of pleasure decrease. Thus, for HIIT or SIT to be a feasible option to improve public health, it must be time-efficient and perceived as pleasant, both of which can be achieved by specifically reducing the number of bouts (intervals) performed at vigorous- to high-intensity. 

As mentioned, emerging research suggests that REHIT elicits substantial cardiometabolic health benefits. Additionally, REHIT optimizes the time efficiency of exercise sessions and permits clients to perform a more manageable intensity regimen.

The Scientific Evidence for REHIT

As a health and exercise professional, to benefit from critically evaluating scientific research you must take what you learn and apply it to your daily work with clients. Because designing and implementing exercise programs requires that multiple factors be considered (e.g., the client’s personal goals and abilities; your skills and experience level), the possible approaches to delivering fitness services are endless. For this reason, the best approach to offering quality programs is to draw from as much evidence-based information as possible. With this in mind, here are the findings from two ACE-commissioned REHIT studies our team of researchers in the High Altitude Exercise Physiology Program at Western Colorado University conducted in recent years:  

  1. Our first REHIT study in 2018 looked at the effectiveness of an eight-week REHIT program for improving cardiorespiratory fitness and positively modifying cardiometabolic health in the workplace environment. Thirty-two participants were divided into two groups that performed either an eight-week REHIT program or a moderate-intensity continuous training program. Cardiometabolic risk factors and cardiorespiratory fitness were measured at the start and conclusion of the study. The REHIT sessions, which used a CAR.O.L cycle ergometer, were performed three to four days per week. We found that eight weeks of REHIT elicited more potent and time-efficient improvements in cardiorespiratory fitness and cardiometabolic health when compared to traditional moderate-intensity continuous training. In fact, the REHIT group saw nearly twice the level of improvement in cardiorespiratory fitness (12.3%) relative to the moderate-intensity continuous training group (6.9%). In addition, the reductions in systolic blood pressure in the REHIT group were three times what they were in the moderate-intensity continuous training group. We also found that participants were able to stick to their REHIT programs, as they completed 89% of their assigned workouts.
  2. Our second REHIT study in 2019 examined whether three weeks of REHIT workouts was enough to overcome the negative cardiometabolic effects of a sedentary lifestyle. We recruited 10 volunteers who were sedentary for eight or more hours per day and had two cardiometabolic risk factors for metabolic syndrome. Participants were randomly assigned to either a REHIT group or a control group for the first three weeks of the six-week study. After three weeks, the participants switched groups. The REHIT group rode the CAR.O.L bike three times each week for 8 minutes and 40 seconds, including a warm-up, cool-down and two all-out sprints separated by a three-minute recovery period. The bike’s software coaches the participant through each workout with written and verbal cues of when and how long to sprint and offers tips on proper breathing and recovery techniques. After just three weeks of REHIT, cardiometabolic health improved. Given the widespread prevalence of sedentary behavior, these findings are quite promising and suggest that as little as 25 minutes per week devoted to REHIT could offset the health problems linked to excessive sedentary time.    

For any exercise program to be recommended for improving or maintaining optimal health, the benefits of the exercise program need to be at least as good as those associated with current aerobic physical-activity recommendations. As such, our findings offer solid evidence that you can confidently substitute traditional moderate-intensity continuous training with REHIT and not sacrifice overall health benefits. 

Is REHIT Safe?

Safety is a paramount issue when designing and implementing a HIIT program. It is important to note that, overall, exercise is safe for most individuals, and exercise alone does not incite adverse cardiovascular or other untoward events. Nevertheless, the risk of an acute myocardial infarction or sudden death during exercise is higher in adults compared to their younger counterparts. Additionally, the greatest risk exists for those individuals with underlying or diagnosed cardiovascular disease (CVD). Overall, the absolute risk of sudden death during vigorous-intensity physical activity has been estimated to be one per year for every 15,000 to 18,000 people. 

A systematic review of the safety of HIIT for patients with CVD found no adverse cardiac or other life-threatening events secondary to the use of HIIT. Our experience with REHIT in sedentary, inactive and risk factor−burdened individuals are consistent with these findings. A key to minimizing complications during and after exercise is to identify participants who may be at an increased risk of adverse symptoms through appropriate preparticipation health screening. (Note: ACE Certified Professionals have exclusive access to preparticipation health screening forms, which are accessible through the link at the top of this page.) Individuals with diagnosed cardiovascular, metabolic or renal disease and/or the presence of signs or symptoms of cardiovascular, metabolic or renal disease should receive medical clearance prior to participating in REHIT. Given the “all-out” nature of REHIT, careful preparticipation health screening prior to the commencement of REHIT for previously inactive and/or risk factor-burdened clients is recommended.

The Science of REHIT

You may be thinking that REHIT, which consists of less than 10 minutes per workout, sounds a little too good to be true. For the curious, here is a more detailed explanation of the mechanisms that underpin how REHIT rapidly leads to benefits.

REHIT creates a sudden and substantial increased energy demand, which perturbs the body’s internal metabolic homeostasis. Specifically, this increased energy demand is in the form of a tremendous increase in adenosine triphosphate (ATP—the body’s energy currency) regeneration, which can be 100-fold greater during REHIT when compared to rest. An individual’s body may grudgingly respond to the initial, sudden and unfamiliar energy demand of a first REHIT workout, but in an effort to be better prepared for future episodes of increased energy demand, the body undergoes rapid adaptations.

Mitochondria are responsible for making most of the ATP. Accordingly, it makes sense that to better prepare for future episodes of increased energy demand, the body needs to make more mitochondria. More mitochondria equate to higher levels of cardiorespiratory fitness, but how does this happen? The process of making more mitochondria is known as mitochondria biogenesis. The “master regulator” of mitochondrial biogenesis is a protein called peroxisome proliferator-activated 1a receptor gamma coactivator (PGC-1α). This master regulator can be upregulated by an enzyme known as AMPK, which is activated in an energy crisis, such as REHIT. In fact, research has shown that AMPK activity is maximally activated with just a couple of “all-out” 20-second sprints and does not increase further with additional sprint repetitions. In other words, the severe disruption of homeostasis associated with REHIT instantaneously “saturates” the signaling response mechanism (i.e., AMPK) required to make more mitochondria and regularly performing just two brief REHIT sprints on two to three days per week is sufficient to gain the desired health benefits. Talk about a solid return on a minimal investment of time!

Practical Strategies for Incorporating REHIT

Strategy #1: Know the F.I.T.T. for REHIT 

The F.I.T.T. formula—frequency, intensity, time (duration) and type of exercise—is commonly used to design exercise training programs, and the same approach can be applied to REHIT. Table 1 outlines F.I.T.T. recommendations for REHIT.

Table 1.  F.I.T.T. for REHIT

Program Component

Component Recommendation(s)


  • 2 to 3 days per week



  • All-out effort
  • Rating of perceived exertion (RPE) = 9 or 10 (0-10 scale)

Time (duration): 

  • 2 interval bouts
  • 20 seconds per bout
  • 3 minutes of active recovery at a light intensity between bouts


  • Choose a modality the client prefers
  • Appropriate modalities include running, cycling, jumping rope and elliptical cross trainer
  • Dropout rates are lower with cycling-based interventions when compared with running/walking as the REHIT exercise modality

A quick word about the frequency of REHIT workouts. Apart from the REHIT interval duration (20 seconds) and the number of interval repetitions (two), another overall strategy to optimize the time-efficiency of exercise sessions is to reduce the training frequency. A recent study  examined the effect of training frequency (two, three or four sessions per week) on changes in cardiorespiratory fitness in response to six weeks of REHIT in inactive individuals. Here are two key findings from that study:

  • Changes in cardiorespiratory fitness were not significantly different among the training frequency groups (two sessions/week: +10.2%; three sessions/week: +8.1%; four sessions per week: +7.3%). 
  • Researchers concluded that reducing REHIT training frequency from three or four to two sessions per week did not diminish improvements in the key health marker of cardiorespiratory fitness.

Strategy #2: REHIT Periodization

Over my past two decades of training clients, one of the most challenging aspects has been to carefully undulate programming throughout the calendar year to keep things fun, effective and safe. While this article has focused on the effectiveness and time-efficiency of REHIT, don’t make the mistake of thinking that you and your clients should forgo all aspects of your current training regimens and replace everything exclusively with REHIT. On the contrary, consider periodization of REHIT. Periodization is an organized approach to training that involves the progressive cycling of various aspects of a training program during a specific period of time. Periodization is frequently used by athletes to avoid overtraining and to achieve peak performance at desirable times of the year. Taking a periodization approach to REHIT can help clients avoid boredom while maintaining cardiometabolic health throughout the calendar year as the various stresses of life ebb and flow. The next section highlights some specific ways to periodize REHIT workouts.

Strategy #3: When do I recommend REHIT?

I commonly recommend clients perform REHIT during their summer vacations and holidays, particularly during the weeks between Thanksgiving and New Year, when it can be a challenge to stick to a regular workout routine. Excess caloric intake combined with interruptions to a regular exercise routine can make maintaining fitness a daunting task. In fact, some of our previous ACE-commissioned research shows that even one- to two-week interruptions to regular exercise coupled with too much sedentary time can quickly result in lost fitness and declines in cardiometabolic health. Given that most of us—health and exercise professionals included—wish we had more time during the holidays, REHIT can be a great solution for simply maintaining one’s health and fitness. 

REHIT also is a great option for shift workers. Healthcare workers and first responders frequently work 12- to 24-hour shifts and are unable to get away for a lengthy workout. REHIT can be performed in a short 10-minute block of time before a lunch break, for example. Finally, with many people avoiding gyms and fitness centers during the current COVID-19 crisis, a great alternative is to design at-home REHIT workouts that are time-efficient and incorporate the space and equipment available within the household of each client.    

Final Thoughts

REHIT workouts are more potent and more time-efficient than traditional moderate-intensity continuous training. And given that “lack of time” continues to be the most often cited reason from clients for not exercising regularly, REHIT workouts are particularly effective for improving both fitness and increasing adherence. By offering your clients alternative programming in the form of REHIT, they can remove the obstacle of time and achieve their health and fitness goals.