Thirty minutes of moderate-intensity exercise most days of the week can work wonders for the human body. Committing to a fitness program can:
- Lower blood pressure
- Decrease the risk of heart disease
- Help prevent type 2 diabetes
- Reduce the risk of certain types of cancer
- Add to bone strength
- Assist in weight control
Regular exercise also improves the chances of living longer with more stamina. But how much exercise is enough? This is where fitness comes in.
We often use the term “fit” to describe someone who looks healthy and trim. But the more technical definition refers to cardiovascular fitness. The level of cardiovascular fitness is determined by how efficiently the heart, lungs, blood vessels, and red blood cells supply muscles with oxygen during sustained exercise, along with the ability of the muscles to use that oxygen. It is also known as cardiorespiratory fitness or aerobic fitness.
Defining fitness more precisely means taking some measurements of bodily functions. In special fitness laboratories, technicians can get the most accurate assessments by measuring maximum oxygen consumption (VO2 max for short) during strenuous exercise. This requires breathing through a mask that measures the volume of air inhaled and the concentration of oxygen in the exhaled air while cycling on a stationary bike or running on a treadmill. It's a complicated and expensive procedure, and so is usually limited to research or training elite athletes.
Using METs to gauge fitness
An easier way to gauge cardiovascular fitness is to measure it in metabolic equivalents (METs). One MET is the amount of oxygen used when you are completely inactive, such as sitting still or sleeping. Average healthy but non-athletic middle-aged men and women have peak exercise capacities in the range of 8 to 10 METs; marathon runners can have values as high as 18 to 24.
The easiest way to find out where you currently stand is to go to a gym and ask to get on an exercise machine that displays your MET level. Many models of treadmills, elliptical trainers, and rowing machines have built-in MET calculators. If an exercise machine with a MET display is not available, your exercise intensity level can be estimated in METs according to the type of physical activities you do. For example, walking at 3 miles per hour puts your level at about 3.5 METs. Jogging at a pace of 5 miles per hour (12-minute miles) brings the intensity level to 8 METs.
Tables listing different physical activities and their associated intensity levels are readily available.
What is the right amount of METs for me?
Similar to maximum heart rates adjusted for age, exercise researchers have developed a simple calculation to find out target MET levels:
- For women, MET level = 14.7 - (0.13 x your age in years).
- For men, MET level = 14.7 - (0.11 x your age in years).
- For example, a 45-year-old woman has a target MET level of almost 9 METS.
Hitting your target METs or higher indicates very good to excellent cardiovascular fitness. Falling under 100 percent is associated with diminished health status.
In the August 4, 2005, issue of the New England Journal of Medicine, researchers from Rush University Medical Center in Chicago reported results of an eight-year study on 5,721 women healthy enough to walk on a treadmill.
They recorded each woman's exercise capacity in METs. Women whose exercise capacity at the start of the study in 1992 was less than 85 percent of the predicted value for their age were twice as likely to die over the next eight years compared with those who achieved 85 percent or better.
High fitness—hitting your target METs or higher—is clearly good. Medium—greater than 85 percent but less than 100 percent—is OK, but there is room for improvement. Low fitness, or less than 85 percent of your target, means exercise needs to be increased.
Raising your METs
Unlike some measures of health, fitness and exercise capacity are not set in stone. Virtually every person’s fitness level can be boosted with regular physical activity that challenges the body. That means working hard enough to speed up the heartbeat and breathing.
A recent University of Florida study suggests some answers to the questions of duration and intensity. Researchers recruited almost 500 sedentary women and men for an exercise experiment. All were asked to walk for 30-minute sessions at different intensities and frequencies:
- Moderate intensity—45 percent to 55 percent of their maximal heart rate
- High intensity—65 percent to 75 percent of their maximal heart rate
- Moderate frequency—three to four times a week
- High frequency—five to seven times a week
After two years, all the subjects who followed their exercise prescriptions improved their cardiovascular fitness. Those who walked at moderate intensity for 30 minutes five to seven times a week and those who walked at high intensity three to four times a week had substantially greater fitness improvement over the low-intensity, low-frequency group. No surprise, the group that stuck with high intensity and high frequency did the best.
Thirty minutes of dedicated exercise at moderate intensity most days of the week is likely to achieve the target MET level. Doing higher-intensity workouts on fewer days also works, but people stick with this one less often. To know for sure that you are hitting your target, measure your METs now and track your progress during the coming weeks.
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