What Happens During Exercise at 90% of Maximum Heart Rate?

Knowing your maximum heart rate can help you develop an exercise program that meets your fitness goals. Exercising at lower percentages of your maximum heart rate burns a greater percentage of fat calories -- but not more calories overall -- and allows for recovery, while exercising at higher percentages burns more calories overall and increases the strength of your heart muscle. Many exercise protocols warn against exceeding 90 percent of your heart rate maximum – some cardio exercise machines even shut off if they detect you have gone past this “zone.” Exercising at 90 percent of your maximum heart rate can offer some benefits when used as part of a complete fitness program.

The common formula of determining your maximum heart rate, which involves subtracting your age from 220 is unreliable, notes a 2001 article in the “New York Times.” The results can be off by 15 to 20 beats in either direction – meaning you may be over- or under-exerting yourself. You can get a more accurate determination of your maximum heart rate by using a newer formula, published in the March 2001 issue of “The Journal of the American College of Cardiology,” that was established using data from more than 19,000 people. This formula determines maximum heart rate by subtracting .7 times your age from 208. The newer formula is especially useful when applied to older adults, who find the 220-minus your age formula yields too low numbers for challenging training. In labs, scientists estimate maximal heart rate by having participants undergo a 10- to 15-minute test on a treadmill or other piece of cardio equipment. Heart rate is measured as the level of difficulty of the work increases; the highest reached is assumed to be close to the maximum. Also note that your heart rate maximum may be different depending on the activity you do. Running usually produces a higher max heart rate than cycling, while swimming produces an even higher heart rate than running, notes a 2001 article in “Northwest Runner.”

Regardless of what your heart rate monitor says, you will likely experience very clear symptoms when you are at about 90 percent of your heart rate max. This zone, known as the anaerobic threshold, is when your body switches to using primarily glycogen – or sugar – stored in the muscles for fuel rather than oxygen supplied by the bloodstream. For example, when you go for an all-out sprint, you become short of breath to the point of not being able to continue. You may feel a burn in your muscles as lactic acid and other waste products build up. When working at such a high intensity, your energy stores are used up quickly and your blood flow is inadequate to remove the waste matter, so you have to stop. Work at 90 percent of max heart rate usually uses fast-twitch muscle fibers, which are good for short, explosive power but cannot endure for longer periods like slow-twitch muscle fibers do. Anaerobic drills at 90 percent of your heart rate max can only be sustained for a few minutes at the most. In untrained people, these symptoms may occur at a level far below 90 percent of maximum, note University of New Mexico professors Len Kravitz, Ph.D. and Lance Dalleck, Ph.D.

Performing intervals at 90 percent of your maximum can help you build better aerobic capacity. By training at these levels, you may be able to increase the amount of time you can tolerate working at such a high intensity. This can help athletes such as sprinters last longer at a faster speed. Testing your limits makes your body more efficient and your heart stronger – making you a better athlete. Exercising for short bouts at 90 percent of max heart rate followed by one-minute recoveries for eight to 10 repetitions can also improve the health of your heart, concluded a review published in the June 2012 issue of “Sports Medicine.” Regular high-intensity interval training performed for at least eight weeks improved people’s aerobic fitness and some cardiometabolic risk factors when compared to continuous moderate-intensity exercise.

If you are new to exercise or returning after illness or a hiatus, build up a base of aerobic fitness before adding in interval training that involves reaching 90 percent of your heart rate max. You should also consult a physician to make sure that such high-intensity training is appropriate for you. To find the number that is 90 percent of your max heart rate, roughly estimate your heart rate maximum using one of the formulas from section one or by participating in a maximal heart rate test performed by a certified trainer or your physician. Multiply that number by 0.9 to find your target. To preform drills at 90 percent of your max heart rate, warm up, then run one minute fast enough to reach 90 percent of your heart rate max and one minute at an easy recovery pace for a total of 20 minutes. Because of the stressful nature of the workout, perform it only two to three times per week, leaving at least one full day between sessions. You can fill in the other days with more moderate-intensity training or strength work.