The Science of Intraworkout Rest
Resting for Heath Health, Endurance, and Fat Loss
Resting for HIIT and Muscle Building
Resting for Speed and Power
Exercise takes work. But by itself, work is no guarantee of success. Whether you’re seeking improvements in strength, endurance, athleticism, or body composition, it’s the interplay of work and rest during your exercise sessions — known to exercise physiologists as intraworkout rest — that determines both the short-term effect of each workout and the long-term results you ultimately achieve.
“We need to shift away from this idea that fatigue is the goal,” says conditioning expert Joel Jamieson, CSCS. “That’s not the goal. Improving by resting long enough is the goal — and rest is the key ingredient.”
As a rule of thumb, easier workouts require fewer and shorter breaks; harder workouts call for longer, more frequent ones.
While this sounds intuitive, exercisers often fail to put this principle into practice.
“Many people make their easy workouts too hard and their hard workouts too easy,” explains Jessie Syfko, ND, CSCS, Life Time’s vice president of group training. As a consequence, she says, every workout regresses toward a generalized medium-hard.
That’s not necessarily bad: Such workouts can still spur progress — especially if you’re a beginner. “The lower someone’s level of fitness is, the less it takes of any type of training for that to improve,” says Jamieson. “If you’ve only been training a few months, there’s such vast room for improvement [that] you can put everything together in one bucket and they’re all going to get better.”
That honeymoon period — when all your workouts net you noticeable improvements in all aspects of fitness — is short-lived, explains Jamieson. “Once you’ve reached a certain level, the body needs a [change in] stimulus for each particular thing to improve.”
When you’re seeking a higher rung on the fitness ladder — be that greater levels of strength, power, and endurance, or more muscle mass and less fat — you need to pay greater attention to your rest periods to effectively target those metabolic and muscular functions.
“This is what’s known as the SAID principle — specific adaptation to imposed demands,” says Life Time senior personal training leader and performance coach Michael Middleton, MS, CSCS. “The body adapts to whatever stimulus you throw at it … so you better get specific with your work and your rest.”
The Science of Intraworkout Rest
To understand the nuances of rest, you need to consider how the body makes and stores the energy necessary for movement.
As you may remember from biology class, all physiological functions, including exercise, are powered by adenosine triphosphate. Because ATP is so essential to life — we’d die instantly without it — the body has several ways to produce it. One of them, oxidative phosphorylation, is aerobic, meaning it requires oxygen.
The other two — the glycolytic system and the ATP-CP energy pathway (also known as the phosphagen system) — are anaerobic, meaning they require no oxygen. (Learn more about these metabolic energy systems at “All About Your Metabolic Energy Systems.”)
Slow and steady aerobic metabolism is the one you use most of the time, resting or moving. For fuel, it burns carbohydrates, protein, and fat, in combination with oxygen. It calls primarily on the dark-colored, high-endurance slow-twitch muscle fibers distributed throughout your body.
The aerobic system isn’t terribly fast, but it can hum on for extended periods. That’s why it’s possible for the average person to walk for hours without a break. As long as your energy needs remain low — you don’t move too fast or work too hard — the aerobic system can keep you going for a long time.
If you run a little faster or lift a little heavier, your aerobic system can quickly become overwhelmed. The slow-twitch muscles aren’t strong enough, and your aerobically powered ATP-production line isn’t fast enough to keep up with the energy demands of intense activity.
That’s when your glycolytic metabolism — along with your larger, lighter-colored fast-twitch muscles — springs into action. Glycolytic metabolism burns carbohydrates and requires no oxygen; this allows it to produce energy faster than your aerobic system. (Visit “What is the Difference Between Fast-Twitch and Slow-Twitch Muscle Fibers?” to learn more)
Push your limit — with an ultrafast sprint or an ultraheavy lift — and you’ll call on the potent anaerobic engine known as the ATP-CP metabolism. It burns fuels stored in your muscles to produce the fastest, most powerful bursts of energy your body can produce.
There’s a catch to calling on either of these anaerobic metabolic systems, though: Both produce massive amounts of heat, lactate, and metabolic byproduct, which inhibit muscle contraction and cause muscles to burn and, ultimately, fail. Even though they don’t use oxygen, you’ll still breathe heavily during and after anaerobic work as your aerobic system struggles to cool you off, clear the metabolites, and replenish necessary fuels.
“The anaerobic side is disruptive to our internal environment,” says Jamieson. “It’s like slamming the gas pedal on a car: Keep it there, you burn everything out. It’s a harsh environment for your body.”
That’s why you can’t sustain anaerobic work for long, and it’s why, when you’re doing bursts of repeated high-intensity activities, like sprints or strength training, you need to intersperse frequent breaks.
“Rest is a byproduct of intensity,” says Jamieson. “The higher intensity you go, the more you’re going to have to rest.”
“Rest is a byproduct of intensity,” says Jamieson. “The higher intensity you go, the more you’re going to have to rest.”
In practice, your aerobic and anaerobic systems usually work in tandem, with your aerobic system going as hard as it can and your anaerobic systems making up the difference.
“Let’s say you ride a bike at 8 miles an hour aerobically,” Jamieson offers. “You can sustain that pace for a long time.” But if you increase your pace to 10 miles an hour, he says, “now you have to make up that 2-mile-per-hour difference anaerobically.” The further you go up that scale, the higher the contribution of your anaerobic metabolism and the less time you can exercise at that same intensity.
Different activities require different relative contributions from your metabolic engines. Learn to rest and work right, and you can get better at any of them.
Resting for Heart Health, Endurance, and Fat Loss
Cardiovascular Endurance:
If your goal is to improve your cardiovascular health, to cultivate calm and focus in your life, or to complete a 5K or marathon, then you’re seeking mainly cardiovascular endurance. Keep your effort low and use rest intervals (either half as long or as long as the duration of your effort) to help maintain it.
To begin, use a 2:1 or 1:1 work-to-rest ratio. With time and experience, your endurance will improve, and you’ll be able to maintain the slow, steady effort with little to no rest.
“Let’s say you find yourself working at too hard a pace for your aerobic system,” says Syfko: You feel your muscles begin to ache, and your heart rate is climbing. “Focus on your breathing and maintain your form. Do your best to hold yourself accountable to the work interval, even if it means slowing your pace.”
If you know your heart-rate zones and use a heart-rate tracker, zone 2 — that low- to moderate-intensity level of 60 to 70 percent of your maximum heart rate — is a good range to aim for. (Learn more about zone 2 training at “How to Use Zone 2 Training to Optimize Fitness Performance.”)
Muscular Endurance:
If you’re after greater stamina, performance in team or endurance sports, or muscle mass for health, injury prevention, longevity, or body composition, then you’re going to focus on muscular endurance. Perform sets of 15 to 30 reps of strength-training moves at a steady pace, focusing on excellent technique, up to and past the point when your muscles start to burn.
Between sets, rest for 30 to 60 seconds — just enough to give your slow-twitch muscle fibers a chance to relax and reset, but not so much that you recover fully — before you repeat the same exercise.
You can also perform circuits of exercises — one move for the chest, one for the legs, one for the core, and then repeat, for example. Between exercises within the circuit, rest only as long as needed. Between rounds, rest up to 30 to 60 seconds. Because maintaining good form is key, you might not be able to do the same number of reps in subsequent sets of each individual exercise, and that’s OK. Some of that muscular fatigue and burn is desirable if muscle endurance and mass is your goal.
Improved Metabolic Health and Fat Loss:
People seeking improved metabolic health and fat loss should also emphasize aerobic activities in their training.
You might think that the best way to lose fat would be to work as hard as possible to burn the optimum number of calories during your workout. That’s how many people seeking fat loss organize their training: Each session is an all-out, heart-pounding, sweat-pouring frenzy that requires lots of rest. But that’s a mistake.
Recent research shows that when at rest, most people burn only small amounts of fat. “It’s between 0.2 and 0.4 grams per minute,” says Jamieson — about one-fifth the weight of a paper clip. And, in general, once you start exercising, you’re actually burning even less fat per minute compared with when you’re at rest.
That’s because most people don’t have an efficient aerobic engine, which means their anaerobic engines kick in almost instantly. Consequently, he adds, “they end up burning almost all carbohydrates the whole time they’re working out.”
Don’t focus on caloric burn if you’re training for fat loss, Jamieson advises. Instead, think of exercise as a way to tune up your fat-burning metabolism so that you’re better at burning fat 24 hours a day. And the way to do that is to improve your aerobic engine with long, slow, no-frills, low-intensity cardio sessions.
Resting for HIIT and Muscle Building
Hard Anaerobic Activity:
If you’re doing high-intensity interval training (HIIT), then your goal is improved work capacity — or the ability to do lots of hard anaerobic activity in a short period of time. In that case, rest no less than 90 to 150 seconds between sets. That may seem like a long time, but work-capacity training, which consists of all-out 30- to 60-second work sets of intervals, sprints, or other high-intensity cardio work, is extreme exercise.
“Your rest intervals are designed to optimize recovery and help your body find a new balance, which permits the next high-intensity interval to be at a peak output again,” says Syfko.
Let’s say you’re doing 200-meter sprints on a track, and on your first set, you complete that distance in 40 seconds. You need to rest long enough to complete your next sprint in 40 seconds or less. At first, you might have to estimate an appropriate rest period, so you might try resting for two minutes. If you match or improve on your 40-second time, you guessed right. If you’re slower, you’ll need to wait longer before your next effort.
Err in the direction of resting more — not less — between rounds so you can give your all to each sprint. “Your goal is to improve your performance,” says Jamieson.
Sufficient rest between sets is essential for maintaining that high intensity, which, if you’re monitoring your heart rate, will likely translate to zones 4 and 5. (Learn how to calculate and train according to your heart-rate zones at “The Active Metabolic Assessment: The Most Accurate Test For Cardio Training.”)
Muscle Mass:
If your goal is building muscle, you’ll be focusing on basic, compound strength-training exercises, like deadlifts, squats, rows, and presses, using weights that allow you to perform about five to 12 reps per set.
Here, you’ll rest anywhere from 90 seconds to three minutes between hard sets. This approach requires that you have a firm enough grasp on exercise technique such that your form doesn’t fall apart as you fatigue.
“Do your best to find a weight that challenges your ability to finish the total rep count, and adjust accordingly to maximize each set,” says Syfko. “For example, if you choose a weight to perform 12 reps but you could really perform many more than that, increase your weight until you find a challenging finish to every set.”
As with HIIT training, there’s some trial and error required with relatively lower-rep strength training. A recent meta-analysis found that one minute or slightly more of rest was, on average, about right for people seeking muscle growth.
When in doubt, though, rest longer. You want to avoid a steep drop-off in your reps from one set to the next.
“If you are training for maximal strength-and-power gains but don’t allow adequate time in between sets, you are not actually achieving maximal strength,” Middleton says. “You are training endurance.”
Resting for Speed and Power
Maximum Power:
Explosiveness and short-burst speed are important attributes in athletics and everyday life. To build power, practice heavy basic strength moves — think squats, deadlifts, cleans, and the like, performed at weights that you can lift for one to five reps — along with maximal-force jumps, medicine-ball throws, and short sprints. These pure anaerobic moves are as strenuous as it gets: all-out blasts of speed and strength that last no more than a few seconds.
Because these are top-effort activities, your rest periods should be even longer than the ones you take for high-intensity interval training — at least three minutes.
“On power, it’s short work, long rest,” says Syfko. This might seem counterintuitive: The moves are so brief in duration that you may think you don’t need much rest. But remember, your goal in performing them is not cardiovascular endurance but raw speed and power. And your ATP-CP system — responsible for generating speed and power — takes a long time to recharge.
“If you’re training yourself to get faster, work on explosive movements at your max effort with long rest ratios,” says Syfko. “Work to bring the best version of yourself back to the sprint, effort, or movement in order to facilitate the muscle memory of what you seek to obtain. Tired muscles and low energy don’t produce the optimal environment to make you faster.”
Mind Your Mindset
If you’re used to training for aerobic fitness, taking long breaks during a strength or power workout can feel like an uncomfortable leap of faith. If you’re used to training strength, taking short rests during an aerobic workout may leave you feeling rushed and out of breath.
“Converting to a different approach takes a change in mindset,” says Life Time’s Michael Middleton. “So I explain to my clients what the goal is and make sure they understand the benefit of resting shorter or longer.”
A few tech options can help. A phone with a stopwatch app or a wristwatch with a second hand can be valuable, particularly as you make the leap from one training goal to the next. After a few workouts, you’ll be able to sense when 90 seconds has passed and you’re ready to go.
For aerobic work and HIIT, a heart-rate monitor can be useful. During aerobic efforts, aim to maintain a heart rate in zone 2. During intervals and sprints, Middleton says, your heart rate will likely climb higher, into zones 4 and 5, approaching your max.
Between rounds, though, he adds, “you’re looking for that point when your heart rate comes down at least 20 beats per minute, and ideally closer to 40.” So, if you finish a round at 160 beats per minute, don’t start the next sprint until your heart rate drops to about 130.
Middleton stresses that these are basic guidelines, not strict rules: “Everyone is different, and everyone changes day to day based on stress, sleep, hydration, and other factors.” So let the numbers — rest periods and heart rate — inform you, but allow your intuition to dictate ultimately when you’re ready to go.
“Completing an active metabolic assessment will ensure that your efforts and rest periods are spot-on,” he says.
Bored during rest periods? Middleton suggests stretching or foam rolling between sets. “You can accomplish a lot by being smart with your rest periods,” he says. “I might have a client perform chest stretches between sets of back exercises.” Or they might foam roll the quadriceps (front thighs) between sets for the hamstrings (rear thighs).
The idea is to relax and release the muscles opposite the ones you’re working: Stretch your lats while working shoulders, your biceps while working triceps. “This can make your workout feel a lot better,” says Middleton. “Plus, you’re not spending an extra 15 minutes stretching and foam rolling at the end of your workout.”
Alternatively, boost your brain power with low-impact neurobic exercises geared to improving your coordination and focus. (Find the moves at “The Neurobic Workout.”)
Whatever you do, don’t stress about making the rest periods too active or productive. After all, the key to resting is, well, resting. Conditioning expert Joel Jamieson prefers a relatively passive approach. “Maybe walk around and listen to music, but that’s about it.”
Whether you choose to stretch, roll, or rest fully during your downtime, be sure to bring attention to your breath. “The breath is the key that unlocks any optimal health, fitness, or well-being goal,” says Life Time’s Jessie Syfko. A hard set, she says, “is like climbing a fitness mountain: You want to get off that mountain as fast as you can so you can get to the next mountaintop faster.”
Between sets, she explains, “deepen the breath and feel the spherical expansion of your torso. Breathe low into your abdomen — not up into your chest. The better you breathe, the faster you’re going to recover.”
Do it consciously, she advises, and the breathwork will spill over into the workout. “Suddenly, you’re breathing better while you lift, while you run on the treadmill, or while you’re doing a high-intensity workout.”
Along with those appropriately timed rest periods, that better breath means you bounce back faster from your workout — and make more progress. Says Syfko: “That’s the Holy Grail of optimal.”
This article originally appeared as “Right Rest, Right Time” in the November/December 2024 issue of Experience Life.
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