You know the feeling. Every weekend warrior knows the feeling. You spend time building up your exercise capacity by biking, running, swimming, hiking, or going to the gym. Then for some reason (illness, injury, other obligations) you have to stop exercising for weeks or months. When you return to your activity, you are more short of breath, you don’t have the same exercise capacity, and you feel “out of shape”. What happens to the heart during deconditioning (the medical term for “out of shape”)? Does deconditioning occur with elite athletes or astronauts? Does it happen on weekdays to weekend warriors?
The heart is a muscle. With training athletes can build up their arm and leg muscles. The same process builds up the heart muscle as well. The cardiac effects of long-term exercise include increases in the size of the heart, the thickness of the heart’s muscle and the cardiac mass. (The heart enlarges to accommodate the increased blood flow during exercise. The walls of the main pumping chamber, the left ventricle, thicken and become more muscular to pump the excess blood. Cardiac mass is the weight of the heart and represents the long-term effective of blood pressure on the heart). A larger, thick walled heart is called an athlete’s heart. Unfortunately, a thick muscular athlete’s heart resembles a form of cardiac pathology, hypertrophic cardiomyopathy (a congenital abnormality where the heart muscle is very thick and there is a risk for arrhythmia). Congenital hypertrophic cardiomyopathy is the most common reason for sudden cardiac arrest in the athlete. Athletes are screened to see if they have hypertrophic cardiomyopathy and, if present, vigorous exercise is prohibited. Unfortunately, it is difficult to differentiate between pathologic hypertrophic cardiomyopathy and an athlete’s heart. One way to tell the difference is to have the athlete stop exercising for a period of time, a method called detraining. If the heart muscle reverts to normal thickness during detraining, then it is an athlete’s heart. Studies on Olympic athletes have shown that after about 12 weeks of detraining left ventricular thickness rapidly decreases (thickness goes down by 15-33%). In addition, cardiac mass decreases quickly as well, within 4 to 8 weeks. For full regression of left ventricular thickening, detraining should last 6 months.
The healthy heart needs exercise, but it also needs gravity. Both of those items are in short supply to astronauts on a long-term space mission. The lack of gravity has the same effect as prolonged bed rest on the heart, worsening the deconditioning that takes place in space. Studies performed on astronauts who spend months in space show that space flight causes significant cardiac atrophy. After only a few weeks in space, there is a reduction in the heart’s muscle mass. It is estimated that cardiac muscle mass decreases about 1% per week in flight. This deconditioning has obvious consequences for prolonged space missions. A recent study followed astronauts on the International Space Station. The astronauts were in flight for many months and spent 2 hours each day doing endurance and resistance training. Cardiac work load was 12% lower in space than on Earth due to zero gravity and the confines of the station. Despite the lower work load, the astronauts’ heart muscle mass stayed intact. It seems that exercise can preserve the heart’s structure and function offsetting space flight deconditioning.
Most people don’t have to worry about their heart becoming too thick from exercise or losing cardiac muscle mass on a space flight. However, deconditioning occurs in the average person, with changes similar to the detrained athlete or the astronaut in space. Deconditioning is defined as the adaptation of the body to a less strenuous environment and the decreased ability to function with physical exertion. The body resets to a lower level of functioning so that when it is asked to increase physical activity, it is unable to meet the demand. Deconditioning occurs when people stop exercising; the most extreme example is bed rest. Many studies of patients during bed rest show that skeletal muscle strength is lost rapidly (10-20% in a week, 50% in 3 to 5 weeks). Skeletal muscle mass also decreases 3% within a week of bed rest. In addition, bone density decreases as well. From the cardiovascular standpoint, blood volume goes down and heart rate goes up with bed rest. This means that the body cannot compensate when going from a supine to a standing position; blood pressure drops on standing and patients can pass out. Lastly, bed rest causes cardiac atrophy. With 2 weeks of bed rest, there is a reduction in cardiac muscle mass of 5%, similar to what happens with astronauts in space and the detrained athlete.
Can the “weekend warrior”, someone who only exercises one or two days per week, become deconditioned during the work week? Likely this is not the case as loss of cardiac structure and function occurs after weeks to months of inactivity. In fact, a recent study showed that as long as weekend warriors exercised for 150 minutes per week or more, they had similar reductions in their risk for heart attack, stroke, atrial fibrillation and heart failure as people who exercise daily.
You worked hard to build up your exercise capacity. Don’t stop now and let your heart become flabby and deconditioned. Keep active and keep exercising a minimum of 150 minutes per week, even if it is only two days a week.
