The Biology of Exercise

Discover the scientific reasons that running, cycling, and lifting make you fitter and healthier.

While it's a foregone conclusion that exercise is good for your heart, it's a mystery to most guys how working out bestows this benefit. But understanding the details can change how you view exercise forever, providing you with powerful motivation to make it a habit for life—a life that's longer, healthier, and happier.

At the risk of oversimplification, the system we're targeting when we exercise has three basic parts: a pump, pipes, and the muscular engines that burn different forms of fuel to power different forms of work. Regular exercise tunes the cardiovascular system in all these areas.

Start with the heart itself, which is actually four pumps, with the powerful left ventricle being the main one affected by training. While you may have come to believe that only aerobic exercise, or "cardio," positively impacts your heart, studies conducted over the past two decades have begun to document that resistance training also provides a multitude of benefits.

A bit of history: In 1985, Canadian researchers found that during a leg press, powerlifters registered blood pressure up to 480 over 350 mm Hg—three to four times normal. "At the time, this really shocked everyone," says William Kraemer, Ph.D., a strength-training researcher at the University of Connecticut's human-performance laboratory. "We thought, My god, this is going to kill you."

It turns out that blood pressure rises during heavy lifts in order to better pump blood against the large internal pressures that your body creates to stabilize your spine. Over time, this causes the walls of the left ventricle to thicken. A similar thickening is seen in patients suffering chronically high blood pressure. Known as left ventricular hypertrophy (LVH), this condition is a precursor to congestive heart failure. As a result, some cardiologists began to advise against lifting weights altogether, believing it led to a pathological enlargement known as athlete's heart.

"We now know athlete's heart is a healthy adaptation to the resistance overload the heart undergoes with lifting," says Kraemer. "Your heart grows stronger, and if you find yourself in an emergency situation, your whole body is just much better at tolerating everything from hormonal surges to increased work demands."

Blood pressure also rises during performance in endurance athletes, especially when they push the aerobic envelope. When a cyclist races up a mountain on a bike, for instance, it's not unheard of for his systolic blood pressure (the upper number) to spike to 250 mm Hg, twice the healthy limit for at-rest blood pressure. 

Still, what really distinguishes the Lance Armstrongs of the world is not their tolerance for the short-term "pressure overload" generated by hoisting heavy weights, but rather their capacity for long-lived "volume overload." In other words, for hours on end, top endurance athletes can circulate prodigious amounts of blood—at least twice as much with every beat as a sedentary man can muster. Not surprisingly, the hearts of endurance athletes typically undergo a different remodeling than weight lifters: The interior volume of the left ventricle expands. Last September, French researchers presented data showing that the heart of an average Tour de France rider can hold 20 to 40 percent more blood than a normal man's. This oversize pumping chamber delivers significantly more oxygen and nutrients to the muscles used in endurance sports—for instance, the type 1, or aerobic, muscle fibers that require oxygen to burn fuel.

What's more, a larger pump demands more fluid to circulate, and well-trained endurance athletes typically increase their overall blood-plasma volume, as well. A normal guy may have 4 liters coursing through his body, but a top aerobic performer can have 6 liters or more. The net effect of these and related changes is that VO2 max—a measure of aerobic fitness—increases dramatically.

Peak performance in strength athletes, on the other hand, is much less dependent on oxygen. Sports like powerlifting require short but very intense bursts of effort. The type 2, or anaerobic, muscle fibers that do this kind of work are powered by fuel sources that don't require oxygen to burn. For this reason, resistance training, especially the kind that emphasizes heavy weights, low reps, and plenty of rest in between, does little to increase VO2 max.

There is an exception, however: fast-paced circuit training that involves higher reps and less rest between sets. Over a relatively short 8- to 20-week period, circuit training has been shown to boost VO2 max by 4 to 8 percent. Traditional aerobic training for the same period provides an increase of 15 to 20 percent.

That's not to say that endurance athletes should assume weight training is a waste of their time for boosting performance. Studies of untrained, moderately trained, and highly trained endurance athletes have convincingly shown that adding weight training to a conditioning program lowers both 5-K and 10-K running times, even without a significant increase in VO2 max. The theory behind how weights enhance speed in long events is that more strength improves running economy, metabolic efficiency, and lactate threshold (the familiar point at which your muscles start to feel the burn).

"There's a bit of false separation between strength training and endurance training," says cardiologist Paul D. Thompson, M.D., author of more than 200 scientific articles on exercise training and cardiovascular health. "When you train a muscle to make it stronger, you also increase its endurance. On the other hand, when you train a muscle for endurance, you also inevitably strengthen it. There's a continuum here with a bit of crossover between these extremes of exercise."

Dr. Thompson, who is in his 60s, has run more than 30 marathons. Despite his personal predilection for endurance exercise, he's quick to endorse strength training as "absolutely beneficial" for both health and athletic performance. "If you're strong," he says, "it means that when you do a task, that task will stress you less."

Perhaps just as important, all these training adaptations- lead to less stress on your cardiovascular system when you're lying on the couch.

With regular exercise, resting heart rate typically drops, thanks in large part to increased "vagal tone." Simply put, the vagus nerve, which tells the heart's pacemaker when to beat, orchestrates a slowing of the heart rate at rest, which decreases the demands on your ticker.

A reduction in resting blood pressure is also commonly observed—typically 9 to 10 points in aerobic athletes and a more modest 4 points in resistance-trained men.

The upshot is that by working your heart hard for 3 or 4 hours a week, it won't have to work as hard the other 165 or so hours. That reduces the wear and tear on your entire cardiovascular system, in theory extending its life and yours.

Besides having a direct effect on the heart, regular exercise appears to rejuvenate our pipes as well: It helps them become more pliable and elastic, thanks to enhanced production of nitric oxide, the same chemical that drugs like Viagra indirectly target. Maybe it shouldn't have been a surprise when James White, Ph.D., a researcher at the University of California at San Diego, discovered that sedentary middle-aged men who took up aerobic exercise saw a quantum leap in their libidos. Besides feeling better about their bodies, the men almost certainly benefited from an improvement in the arterial relaxation upon which erections depend.

There are numerous other vascular changes that can occur. Endurance training, for example, increases the number and density of capillaries supplying muscles with blood—and may even spawn the growth of new large blood vessels, a process called angiogenesis.

In lab animals, angiogenesis has been shown to create additional circulation in the heart arteries themselves, an insurance policy of sorts that can help guarantee bloodflow to the heart muscle even after the main route is blocked. Whether the same process can take place in humans remains controversial, but a study last summer by researchers at Germany's Leipzig University showed that heart-failure patients who exercised on a bike for up to 30 minutes a day for 4 months did indeed develop more small vessels in their muscles, compared with patients who did not exercise. The mechanism: The cycling regimen helped patients produce additional stem cells in their bones, which in turn circulated to where they were needed to repair and strengthen tissues. There are currently no drugs available that increase stem cells, leading the scientists to conclude that exercise may be the only way for such patients to rebuild a broken heart.

To be sure, if you start early enough, exercise can help prevent you from reaching the broken-heart state altogether. Numerous studies have shown that, for at least some individuals, regular workouts can help normalize the blood-fat bugaboos so often cited as the building blocks of arterial plaque. Most men who begin even a moderate exercise program will enjoy significantly lower triglycerides, and many will also see at least a modest reduction in their LDL ("bad") cholesterol. Exercise can also boost "good" HDL, though this usually requires the equivalent of 3 or more miles of running per week.

Even if none of your lipids change significantly, a growing number of researchers now believe that physical activity also offers some protection (albeit hardly foolproof) against a wretched American diet. One possible reason is that our bodies may simply burn junk better. Regular exercisers, it seems, usually enjoy enhanced insulin sensitivity. This has the effect of reducing diabetes risk by driving blood sugar into our muscles, where it can be readily used for fuel or stored for later use.

Strength training in particular boosts your body's percentage of lean mass, which is much more metabolically active than fat. It also increases the amount of storage space in your muscles for sugar, decreasing the likelihood of it being converted to fat. Aerobic training, on the other hand, raises the level of oxidative enzymes in muscle cells, so you become a more efficient fat burner. And both forms of activity multiply the intracellular powerhouses called mitochondria. The net result is that as long as you exercise, you can burn significantly more calories than your sedentary self would be capable of using.

All these facts ultimately bring us to what may be the most significant long-term benefit of all: a kind of de facto slowing of the aging process. If you're in your 20s, chances are you're not overly concerned about creeping decrepitude. But take it from a middle-aged guy who's reached the Continental Divide: One day—and sooner than you think—you will be middle aged, too.

In terms of what happens to the body's muscle, sedentary living leads to a 10 percent decline in total body muscle mass from ages 30 to 50, and 1 percent every year after that. The good news here: It's not unusual for a septuagenarian who trains with weights to be stronger than a 30-year-old guy who doesn't. You might want to keep this in mind before picking a bar fight with Clint Eastwood.

Cardiovascular capacity left to rust fares just as poorly. "Aerobic capacity in sedentary people declines 8 percent to 10 percent per decade after age 25," says Brian J. Sharkey, Ph.D., author of Fitness & Health. "People who remain reasonably active, however, cut this decline in half. And people who train can cut it in half again."

The research is nearly unanimous: Working out regularly, vigorously, and steadily over the years triggers a host of cardiovascular and muscular adaptations that together make you a better, stronger, more indefatigable version of yourself. It won't make you immortal, but it will nudge you in that direction.

Scientists have long attempted to quantify what it is about exercise that improves our health. Among the numerous biological benefits, the central theme remains this: Getting into shape conspires, in a host of grand and subtle ways, to make us more robust—that is, more capable of enduring life's inevitable physical and emotional stresses.

"Exercise lets us take a physiological hit and bounce right back," says sports medicine physician Geoffrey E. Moore, M.D., director of the Cayuga Center for Healthy Living, in Ithaca, New York.

One practical consequence of this resiliency: A study in the New England Journal of Medicine found that in-shape guys are at little extra risk of dying of a heart attack during strenuous activity. That's not the case for out-of-shape men, who are up to 100 times more likely to expire while shoveling snow, sprinting for a bus, or undergoing other sudden, heavy exertion.

And consider two long-term studies of Harvard University alumni. Both show that regular exercise has a protective effect against "all-cause mortality." The more walking, stair-climbing, and sports participation reported, the lower the odds of dying for any reason. Indeed, those people simply expending, on average, 1,500 calories a week through exercise—the equivalent of walking 3 miles five times a week—had three-fourths the risk of dying compared with couch-bound guys their age. And the more vigorous this exercise, the greater the risk reduction. The key, of course, is to find an exercise routine that you can stick to—not for weeks, but for decades.

"There is no medicine or magic food that can make you tougher and stronger," says Dr. Moore, who twice competed in the U.S. Olympic marathon trials. "If you want to become robust in this sense, the only way to do it is by exercise.