Until recently, exercise program design was guided largely by the principle of training specificity—the idea that the most effective program closely matches the desired outcome. More recent research, however, suggests that two methods, blood flow restriction (BFR) training and high-intensity sprint training (HIST), may produce meaningful cardiorespiratory and performance benefits in less time than traditional endurance-based approaches. BFR training involves performing exercise while specialized cuffs or bands partially restrict blood flow to the working muscles. HIST consists of short bursts of near-maximal effort separated by brief recovery periods. Rather than relying solely on longer-duration exercise, these methods appear to take advantage of distinct physiological responses to stimulate adaptation efficiently. In many cases, they can be completed in as little as 20 minutes while producing benefits comparable to training sessions that are substantially longer. In this article, you’ll learn about the methods, potential benefits, and risks of BFR and HIST, and gain practical guidance on how they may be applied and which populations may benefit most.

Limiters of Cardiorespiratory Performance

For endurance exercise to be performed effectively, the cardiovascular system must be able to circulate blood efficiently between the heart, lungs and working skeletal muscles. This capacity is referred to as cardiac output, and it is one of the primary factors limiting aerobic exercise performance. As a result, most aerobic training strategies aim, directly or indirectly, to improve the body’s ability to deliver oxygen and energy substrates to active tissue.

To support sustained exercise, the heart must first pump blood to the lungs for oxygenation and then recirculate that oxygen-rich blood to the muscles. Aerobic performance can therefore be improved by increasing heart rate, increasing the amount of blood pumped with each beat (stroke volume) or enhancing the muscles’ ability to produce energy. HIST appears to improve energy production largely through increases in mitochondrial density, whereas BFR training may improve blood delivery by elevating heart rate and stroke volume.

Blood Flow Restriction

BFR is an exercise and rehabilitation technique commonly used with patients returning from injury. It involves placing inflatable cuffs around the upper arms or legs, typically near the armpit or groin, and inflating them to approximately 40 to 80% of arterial occlusion pressure, or the pressure required to completely stop blood flow. By partially restricting circulation, BFR reduces oxygen availability in the working tissue and makes exercise feel more demanding at a given intensity.

Although BFR has traditionally been used in resistance training, recent research suggests it may also provide meaningful benefits for endurance training. The restriction limits venous return and reduces the delivery of oxygen and nutrients to the tissues, creating a physiological state that resembles high-intensity exercise. In response, the sympathetic nervous system increases cardiorespiratory activity, largely through the release of norepinephrine. This response raises heart rate and blood pressure, increases cardiac output and places greater demand on the cardiovascular system. Over time, that added demand may stimulate beneficial adaptations.

The FITT formula—frequency, intensity, time (duration) and type of exercise—is commonly used to design exercise training programs. The same approach can be applied to designing a program for BFR training. The following FITT recommendations are based on guidelines published elsewhere and from research completed in our High Altitude Performance Laboratory at Western Colorado University.  

Frequency: Schedule BFR training two to three times per week. Research shows this frequency can improve a range of performance markers.

Intensity: Set both the occlusion pressure and the exercise intensity carefully during BFR training.

• Use an arterial occlusion pressure (AOP) equal to 40 to 80% of the individual’s full AOP, which is determined by the BFR device. You can find the device used in our research here.
• Keep cardiorespiratory exercise intensity below 50% of VO₂max.
• Keep muscular training intensity at 20 to 30% of one-repetition maximum (1-RM).

Time: Limit each BFR session to 10 to 20 minutes total, whether the client is performing cardiorespiratory training, resistance training or a combination of both.

Type: Choose a BFR device that can measure and apply personalized AOP values. Many commercial devices are available, but you should prioritize tools that support individualized pressure settings to maximize both effectiveness and safety.

Additionally, the increase in blood pressure associated with BFR can place significant stress on the heart. Use this technique cautiously, especially in clients with compromised cardiovascular function. This includes, but is not limited to, individuals with hypertension, diabetes, chronic kidney disease or an elevated risk of thrombosis. If any concern exists regarding a client’s safety, consult a physician before initiating BFR training.

High-Intensity Sprint Training

As mentioned above, mitochondria play a central role in aerobic metabolism, and low mitochondrial density—particularly within skeletal muscle—can limit the body’s ability to meet energy demands during exercise. One of the classic adaptations to traditional endurance training is an increase in mitochondrial number. During prolonged aerobic exercise, fuel stores such as muscle glycogen are gradually depleted, and with repeated exposure over time, this stimulates the body to produce more mitochondria. HIST may accelerate this process because repeated supramaximal efforts deplete fuel much more rapidly, leading to a similar mitochondrial adaptation in less total training time.

Additionally, running economy, or neuromuscular efficiency, is another limiting factor to endurance exercise performance. To run efficiently, the brain must be able to accurately and sequentially recruit muscle fibers at the appropriate force to perform movements like running or cycling. This task requires a balance of needing enough force to complete a movement, but not so much force that energy is wasted and muscles are prematurely fatigued. Proper neuromuscular recruitment is important for endurance performance; HIST training has been found to offer improvements for this variable.

HIST involves highly intense, maximal anaerobic efforts with longer periods of rest. This might look like intervals lasting 10 seconds to 1 minute interspersed with three to five minutes of rest and repeated one to six times. These highly intense intervals push the cardiovascular and muscular systems to their limits and have the potential to create a significant number of adaptations with very little time commitment required. The most important adaptations that HIST training provides are increases in mitochondrial density and an increase in the ability to contract and relax through improvements in calcium pathways, which are necessary for muscle contraction. These changes result in more available energy for the working tissue and an increase in the ability to contract both skeletal and cardiac muscle fibers. HIST has been widely studied, and many protocols have been developed and tested over the past few decades.

A common protocol for incorporating HIST intervals is to perform four to six repetitions of 10- or 30-second all-out efforts. Depending on the protocol and population, HIST has been shown to increase VO₂max by roughly 9 to 14% within two to six weeks, while lower-volume REHIT protocols have produced gains of about 10 to 13% after six weeks. Although the total exercise time is brief, the supramaximal intensity can produce meaningful and lasting physiological adaptations. HIST may also reduce neural fatigue by improving neuromuscular control, which can enhance performance in both short- and long-duration aerobic exercise as well as strength training.

When selecting what exact protocol to do, leave the final decision up to what the client will find most enjoyable and is most likely to want to repeat. Additionally, it is important to recognize that many endurance training–focused clients may not have much experience with exercise that is this intense. When beginning a training program that involves HIST, be sure to screen your client for their previous history with sprinting and personal interest in this style of training.

Additionally, introduce sprint training gradually in clients who are new to it, progressing over several sessions or even several weeks as needed. This style of training tends to work best for moderately trained clients (i.e., those meeting the minimum physical activity guidelines for Americans) with limited time, but it may also carry a higher risk of injury in that population if introduced too aggressively. Be extra cautious with maximal sprint efforts, and screen clients carefully to confirm that they are healthy and free of underlying medical conditions. If any concerns arise, consult a physician before beginning training and obtain medical clearance when appropriate.

Practical Strategies for Incorporating BFR Training and HIST

Both BFR and HIST can improve cardiorespiratory and muscular performance while requiring relatively little time. However, each method can increase injury risk or provide little benefit if applied incorrectly. With BFR, exercise must be performed at very low absolute workloads to reduce injury risk, the cuffs must be inflated to 40 to 80% of the individual’s AOP, and cuff fit and placement must be correct. HIST, by contrast, must be performed at a maximal or near-maximal intensity—typically an all-out effort corresponding to a rating of perceived exertion (RPE) of 9 or 10 on a 0–10 scale—to produce its intended benefits. At the same time, the intensity should be introduced and monitored carefully so that the client does not exceed a level that increases the risk of muscular injury. This balance requires close supervision and programming to ensure proper intensity. Yet, when utilized correctly, BFR and HIST offer worthwhile training strategies that can make a significant difference to clients who are looking to improve their endurance performances. Be sure to utilize a warm-up protocol with both techniques that prepares your client for intense exercise. A minimum of 10 minutes of easy aerobic exercise, dynamic muscle activation exercise, and gentle movement prep is a good place to start to ensure your client is prepared.

What Clients Will Most Benefit From These Techniques?

Clients who are short on time and those who are looking to improve their cardiorespiratory performance for recreational activities or sports may be the best candidates for these novel training strategies. Indeed, many individuals enjoy recreational activities that require a degree of cardiorespiratory fitness (e.g., skiing, golf, dancing) but may not have the time or interest in traditional aerobic-exercise training. Utilizing one or both training strategies may offer an approachable, interesting or tolerable way of increasing a client’s fitness. Additionally, clients who are looking to challenge themselves in new ways or add additional training to their existing plans may be interested in these strategies. Endurance-focused clients who are bored with their regular workouts may find HIST to be a new and entertaining challenge. On the other hand, clients who are looking to add a bit more volume to their week may be interested in BFR.

Additionally, consider that clients who do not have a significant interest in aerobic training may also significantly benefit from these strategies. Both BFR and HIST provide significant and positive cardiorespiratory adaptations that are important for any individual's overall heart health. However, these two modalities of training are not meant to replace traditional aerobic training and should be treated as a way to switch up programming for a short period, or as the “dessert” on top of an already balanced training program. The table below provides an “at-a-glance” summary of these techniques for your reference.

In this video, Dr. Dalleck explains how BFT and HIST work, the physiological adaptations they may produce and how you can apply them in your work with clients.

Final Thoughts

BFR and HIST are novel approaches to training that can promote significant cardiorespiratory fitness adaptations in a variety of clients. They can serve as useful additions for clients seeking to further improve their cardiorespiratory fitness. Although both approaches involve some risk, you can reduce that risk by confirming that clients are injury-free and have been cleared by a physician for intense exercise, and by adhering to the guidelines outlined in this article.