The Exercise Sweet Spot

 

The eyes of the world are on Milan-Cortina in Italy (pictured above, the Duomo, symbol of the city of Milan). The world’s best athletes are competing in some of the toughest sports in the Winter Olympics. As we watch the events, we know there is a difference in the energy a speed skater expends racing around a track compared to the average person walking on a treadmill. What is that difference and how can it be quantified?  Is there an optimal amount of time spent in our favorite exercises to maximize the benefit? Is there an exercise sweet spot?

 

Exercise is quantified using the metabolic equivalent (MET). The MET is the energy cost of doing any activity such as walking, running, playing a sport or doing household chores. One MET is the energy used just sitting and doing nothing. An activity that costs 4 METS is four times as strenuous as sitting still. Light activity is considered less than 3 METs, moderate activity is 3 to 6 METs and vigorous activity is greater than 6 METs. The higher the METs, the more exercise energy is required and the more calories burned. The table below displays the METs of some common activities and sports: 

Activity/Sport	METs
General bicycling	7.0
Bicycling 14-16 MPH	10.0
Jogging 2.6-3.7 MPH	3.3
Running 4 MPH	6.5
Marathon Running	13.0
Walking the dog	3.0
Walking 2.8-3.4 MPH	3.8
Swimming laps	5.8
Tennis- singles	8.0
Tennis- doubles	4.5
Free weights	6.0
Squats/Pushups	3.0
Exercise classes	5.5
Yoga	2.3
Zumba	6.0
Pilates	2.8
Snow blower	2.5
Snow shoveling	5.0-7.5
Mowing lawn, power mower	5.0
Mowing lawn, hand mower	6.0

 

How does this compare to the energy required for some Olympic sports?

 

Winter Olympic Sports	METs
Ice dancing	14.0
Speed skating	13.8
Cross country skiing	15.0
Biathlon	12.8
Downhill skiing	8.0
Slalom skiing	9.3

 

As we can see, winter sports are very high energy. Also keep in mind that these METs are for the average exerciser. Olympic competitors are working at an even higher rate. The website https://pacompendium.com/adult-compendiun/ has a list of many more activities and sports.

 

What is the optimal number of METs per week? What is a good amount of exercise? It is well known that the largest benefit of exercise occurs when going from doing nothing to doing a moderate amount of physical activity. People doing 150 minutes of moderate exercise per week lower their risk for heart disease and death by 14% compared to people who are sedentary. It is also well known that increasing the exercise volume doesn’t give much more benefit; the mortality benefit plateaus. In other words, exercising more and more doesn’t mean that you will live longer and longer. Mortality isn’t the only thing that reaches a plateau, heart disease, respiratory disease and cancer all hit a limit. Does this relationship hold for various types of exercise? A new study examined this question. The study followed 111,000 participants for 30 years. The study found a similar relationship for multiple different types of activities including walking, jogging, running, bicycling, swimming, tennis, rowing and weight training. All of these exercises lowered the risk of dying and all plateaued as the volume went up. The study used MET hours per week to quantify exercise volume. MET hours per week is derived by multiplying the MET associated with an activity by the hours per week engaged in doing the activity. For example, walking 3 MPH (3.8 MET) for 2 hours each week yields 7.6 MET hours per week. With some exercises (jogging, swimming), mortality actually went up as the volume of exercise per week increased. Is there an exercise “sweet spot”? It turns out that exercising about 5 MET hours per week for any activity in the study gives the maximum mortality benefit. The 5 MET hours per week is not hard to achieve. The time needed to reach the sweet spot for each activity is:

Walking                      90 minutes/week

Jogging                       45 minutes/week

Running                      30 minutes/week

Bicycling                    50 minutes/week

Swimming                  45 minutes/week

Tennis                         45 minutes/week

Rowing                       60 minutes/week

Weight training           90 minutes/week

 

Of course, there may be other reasons to exercise above this rate (for example, to lose weight, to keep in shape, to train for a competition, to control blood pressure, to improve diabetes, to reduce the risk for rhythm problems, etc), but the additional work load won’t reduce the risk of dying further. In addition, engaging in different types of exercises gives additional health benefits. 

 

Why is there an exercise plateau? Why doesn’t mortality continue to decline with more activity? Most studies have shown that elite athletes live longer than the general population. On the other hand, studies have shown that high volume exercisers do experience cardiac abnormalities. Men who do high volume training (50 MET hours/week) have a higher burden of heart artery calcium than nonathletes. Higher coronary calcium levels are consistently found in avid exercisers. Calcium is associated with heart artery blockage and this could certainly limit life span. Similarly, high blood levels of troponin are found in athletes after exercise. Troponin is a measure of damage to the heart muscle and elevations are diagnostic of acute heart attacks. More than 80% of marathoners have troponin concentrations that are above normal cut offs. Troponin in athletes could be a normal physiologic finding or it could be due to transient damage of the heart from over exercising. Lastly, fibrosis or scarring of the heart muscle has been found in endurance athletes. Fibrosis is associated with malignant arrhythmias. No one really knows why these phenomena happen in high volume exercisers or its significance, but it could explain why there is a plateau. 

 

Next, we’ll make use of our MET knowledge and tackle a favorite winter sport, snow shoveling. Shoveling snow exerts a significant workload on the heart. This begs the question, at what age should you stop shoveling snow?  One expert says to stop at age 65. Another expert, citing a study showing that 85% of adults over 50 years of age already have atherosclerosis, says to stop at age 45! Clearly the recommendation must be individualized. If you have known heart disease or do not regularly do an equivalent amount of exercise (for example running at 4 MPH, 6.5 METs; snow shoveling 5.0-7.5 METs) then put down the shovel. If you regularly run on a treadmill at 4 MPH without difficulty at home or in the gym, then go for it. The same thought process should occur during the summer with lawn mowing (power mower 5 METs, hand mower 6 METs).

 

Once you are done with snow shoveling, put on fire, sit back and enjoy the Winter Olympics. Now you can appreciate the difference in energy expenditure between competing in an Olympic speed skating event (13.8 METs plus) versus walking on a treadmill (3.8 METs). However, if your goal is to live longer and not winning an Olympic medal, you can still hit your exercise sweet spot by walking only 13 minutes per day.

 

How to Do CPR in Space (and How to Prevent Sudden Cardiac Arrest on Earth)

                                                    Photo courtesy of Nick Nikolaides
 

Becoming an astronaut is a highly selective process. Only the best, the fittest, are chosen to be astronauts and to go on missions. In 2024, 8,00 people applied to become an astronaut, only 10 were chosen. To be selected, the candidates have to be in perfect physical health. There are vision and height requirements, they need to have excellent heart health and no high blood pressure. Once chosen, the process continues. Astronauts go through one of year of intensive physical training including wilderness survival, underwater training and learning to cope with low gravity. Despite being in tip top shape and low risk, it is still possible that an astronaut could suffer sudden cardiac arrest (SCA) while in space. If that happens, could they be resuscitated? How would cardiopulmonary resuscitation (CPR) be performed in zero gravity? How can astronauts (and those of us on Earth) lower our risk for SCA?

 

Sudden cardiac arrest is a common problem and is often the first manifestation of heart disease. It is an abnormal heart rhythm most often caused by ventricular fibrillation (an irregular heart rhythm from the lower chambers of the heart).  When the heart’s ventricles are fibrillating, they cannot pump blood to the brain and vital organs. If not treated promptly, this leads to death.  Sudden cardiac arrest affects more than 300,000 people in the US each year with a death rate of about 90%. Treatment of SCA involves prompt initiation of CPR and performing defibrillation. Defibrillation is an electric shock to the heart that restores the heart to normal rhythm. The shock is usually provided by an Automatic External Defibrillator (AED). Cardiologists can determine which patients are at high risk for SCA including those with prior heart attack, previous SCA, congestive heart failure and low ejection fraction (below 30%). However, these high-risk patients only account for a small proportion of the SCA total (about 10% of the total number of arrests). In contrast, the majority of SCA occurs in asymptomatic, low-risk people in the general population (more than 50% of the total). It is very hard to predict, or prevent, these events in the general population. 

 

By any measure, astronauts would fall into a low-risk category. Still, the risk is not zero and SCA could occur while on a mission. If SCA happened in space, how would CPR be performed? Traditional CPR relies on gravity to be effective and weightlessness poses a problem. NASA recommends that a rescuer stand behind a victim, wrap their arms around the person (in a bear hug) and squeeze (almost like the Heimlich maneuver, but applying continuous compressions to the chest not the abdomen).  The rescuer and the victim can be moved to a room with medical equipment and the victim strapped down. The rescuer then does compressions in a handstand position, with hands on the patient’s chest and legs braced against a wall. This method can’t provide the compression depth that is recommended, so it would not be as effective as CPR on Earth. One way around this would be to use a LUCAS automated chest compression system, a machine used in emergency medicine with a plunger that automatically does chest compressions to the right depth and at the right rate. Current space vehicles do not have this device (due to size and weight considerations) but do have AEDs.

 

If you are an astronaut or a low-risk citizen of Earth, what can you do to lower the risk for SCA? Over the past twenty years, the incidence of SCA has gone down, but there is room for improvement in preventing it. The CARES (Cardiac Arrest Registry to Enhance Survival) network is a national registry tracking SCA. It currently covers about 40 states. CARES showed that from 2021 to 2024, overall survival improved from 9% to 11%, public AED use increased from 10% to 13%, but bystander CPR remained steady at 41%. However, we can do better. A large study of SCA was published this year covering 500,000 participants, average age 56 and 50% were women. The study showed that the American Heart Association’s Life’s Essential Eight were strongly related to risk for SCA. The eight risk factors are:

Diet- high in fat, low in fruit and vegetable intake

Activity- sedentary lifestyle

Smoking

Sleep- less than seven hours per night

Obesity- high body mass index (BMI), high waist circumference

High Cholesterol

Diabetes

High Blood Pressure

In addition, the study found several novel risk factors for SCA. These are depression, social isolation, low educational level, increased arm fat, reduced grip strength (a marker of frailty) and air pollution. The study concluded that SCA could be reduced by 40% to 60% if these risk factors were controlled or eliminated.

 

With this information, we can now respond to SCA anywhere in the universe and begin to prevent its occurrence. So, if you are planning on being a space tourist (or if you want to reduce your risk for SCA here on Earth), work on controlling these risk factors. And maybe bring your own LUCAS device on board, just in case.