A new study suggests that the more intensely you exercise, the less likely you are to suffer a heart attack. Researchers followed 403,681 U.S. adults for an average 10 years and found that those who spent a greater proportion of their exercise time exercising intensely had a significantly lower risk of death from heart attacks than those who exercised for the same amount of time but at lower intensity (JAMA Intern Med, Nov 23, 2020). Those who exercised more vigorously also had a lower risk of death from cancer and lower all-cause mortality during the study period.

Other studies show that people who exercise intensely are significantly less likely to die prematurely than casual exercisers (JAMA Intern Med, 2015;175(6):970-977), because vigorous exercise is more effective in:
• preventing weight gain (Prev Med, 2014;60:131-133),
• preventing heart disease (Am J Cardiol, 2006;97(1):141-147),
• preventing diabetes (Int J Epidemiol, 2012;41(4):1132-1140), and
• promoting fitness and the ability to process oxygen (Med Sci Sports Exerc, 2002;34(1):152-157).
The HUNT study from Norway found that the more fit people are, the less likely they were to develop a first heart attack (JAMA, April 19, 2019). The SUN study from Spain found lower heart attack rates in those who exercised intensely compared to those who exercised less vigorously (Am J of Cardio, Dec 1, 2018;122(11):1871–1878).

Any Level of Exercise is Healthful
A regular exercise program of any intensity is associated with reduced death rate (Int J Epidemiol, 2011;40(5):1382-1400), because a regular exercise program is associated with reduced rates of:
• cancer (JAMA Intern Med, 2016;176(6):816-825),
• diabetes (Endocrine, 2016;52(2):226-230), and
• heart disease (Br J Sports Med, 2019;53(22):1405-1411).
Spending long periods of time sitting or lying in bed increases risk for heart disease (Prevent Med, May 02, 2019).

Benefits of Intense Exercise
Intensity makes all muscles stronger, including your heart muscle. All people lose heart muscle as they age, which increases risk for frailty and heart failure. Strengthening your heart muscle helps you to live a more vigorous lifestyle and to protect you from heart failure. Intense exercise stabilizes plaques in arteries and widens heart arteries to help protect you from a heart attack. One study showed that men with the highest levels of VO2max (a test of oxygen use that is a measure of fitness) were least likely to have high blood pressure, high HBA1C (a test for diabetes), high fasting blood sugar levels, obesity, an abnormal treadmill exercise test, and a high 10-year risk for heart attacks (American J of Cardiology, March 2012;109(6):839-843).

Starting a New Exercise Program
I think everyone should have a regular exercise program, and it is never too late to start. First check with your doctor. If you are not a regular exerciser, you should spend your first few months exercising in your chosen activity at a very casual pace. Stop when you feel tired, your muscles feel heavy, or you feel any discomfort, even if you have only exercised for a few minutes. When you can exercise every day for about 30 minutes at a casual pace, you are ready to try exercising at a more intense level. See my article on How to Start an Exercise Program.

Increasing the Intensity of Your Current Exercise Program
All exercise training is done by stressing and recovering. To increase the intensity of your exercise program, start by taking a harder workout on one day and expect to feel sore the next morning. This is called delayed onset muscle soreness, or DOMS. Go slow and easy for as many days as it takes for your muscles to feel fresh again. You should not take your next hard workout until the muscle soreness is gone. You may need to take from one to five or more easy recovery days before you do your next intense workout.
• When you are training properly, your muscles may feel sore every morning. If they don’t feel better after a 10 minute warm-up, take the day off.
• If you feel pain in one spot that does not go away after you slow down, stop that workout immediately for that day. Otherwise you are likely to be headed for an injury.

Your Intense Workout Days
Always warm up your muscles before you exercise more intensely. Start out by going very slowly for the first 10 or more minutes of your workout. To make a muscle stronger, you have to exercise intensely enough to feel a burning or tightness in your muscles. For non-competitive athletes, you should slow down immediately when you feel this discomfort. Most people will start out by picking up the pace for only about 10 seconds. If you are a runner, cyclist or skater, pick up the pace for a few strides or pedal strokes. Then go slow and easy, and when your muscles feel fresh again, pick up the pace and then slow down. Repeat these alternating bursts of intense exercise (called “intervals”) until your muscles just start to feel heavy and tired, then slow down. When your muscles continue to feel heavy and tired after you slow down, you are through for that day. In the beginning, you may be able to do only a few intervals in a workout. However, with practice you will improve until you can do lots of intervals, perhaps 15-20 or more. Then you can extend the time that you stay in each hard interval, gradually going from a few seconds up to about 30 seconds. If you are not a competitive athlete, there is no reason to stay in an intense interval longer than 30 seconds. Most healthy exercisers will be able to work up eventually to 10-20 repeats of 20-30 second intervals. For non-competitive athletes, there is no need to do more than that.

Your Recovery Days
Expect your muscles to feel tired and sore when you get up the morning after an interval workout. You can try to exercise that day, but you should do so at a slow pace and stop when your muscles start to feel heavy and fatigued. If your muscles don’t feel better after a five-minute warmup, or if you feel pain in any area that does not go away when you slow down, you should stop your workout for that day. Do not take your next intense workout until your muscles feel fresh after you warm up for 5-10 minutes. Most people who exercise for fitness will follow each hard day with one or two recovery days. Competitive athletes are likely to take such intense workouts on their hard days that they may require up to five recovery days before their next hard day. See Recovery: the Key to Improvement in Your Sport

CAUTION: Intense exercise can cause a heart attack in a person who has blocked arteries or heart damage. Check with your doctor before you start a new exercise program or increase the intensity of your existing program.