Runners and weightlifters occupy different ends of the athletic spectrum, and often have equally opposing views of what it means to be “fit.” But if there’s one thing they generally agree upon, it’s this: Easy runs amount to little more that “junk miles,” producing no specific physiological benefit. And in their agreement, they are both utterly wrong.
“Low intensity aerobic exercise in any form—running, cycling, rowing, etc.—can have a profound effect on blood flow within muscles, increasing the network of capillaries surrounding each fiber,” says Trevor Thieme, C.S.C.S., Beachbody’s fitness and nutrition content manager.
If you’re an endurance athlete, the advantages of improved blood flow through “capillarization” might seem obvious. But it can have implications for strength training as well. “Increased capillary density can benefit all because it improves the delivery of oxygen and nutrients to your muscles and allows you to clear waste products faster,” says Tristan Rice, a performance specialist at EXOS, an elite training facility in Arizona.
In short, it doesn’t matter whether your workouts skew more towards pounding asphalt or pumping iron—junk miles are misnamed. And as you’ll discover in a moment, they aren’t the only way to enhance the microvascular network feeding and cleaning your muscles.
The Capillarzation Process
The body’s capacity to perform work is largely an issue of supply and demand on the cellular level. As long as the energy required by a muscle (i.e., the demand) is less than or equal to the muscle’s ability to produce it (i.e., the supply), physical effort can be sustained. But when production falls short of demand, fatigue and ultimately failure occur. “Fortunately, the repeated stress generated by consistent aerobic activity stimulates two major adaptations to muscle fibers that increase their ability to generate energy and thus perform physical work,” says Rice.
The first adaptation is an increase in mitochondria, which are the energy-producing structures within cells. Indeed, research shows that aerobic exercise can boost mitochondrial volume by up to 40 percent. “You can think of mitochondria like tiny motors,” says Rice. “They take fuel [in the form of glucose] and turn it into energy [in the form of ATP].”
The more motors you have, the more energy you can produce—if you can deliver enough fuel to them to support an uptick in production. The body solves this infrastructure problem with the second major adaptation: An increase in the number of capillaries surrounding each muscle fiber.
Capillaries are the end of the line in the body’s elaborate vascular network, and their thin walls facilitate the diffusion of nutrients and oxygen into cells. As the capillary network around each muscle fiber grows, so too does the amount of oxygen and nutrients delivered to the cell, and thus mitochondria. But the benefit of this microvascular proliferation also works in reverse. “Not only can more oxygen and nutrients reach the muscle fibers, but also more waste products can be removed from them,” says Juan Delgado, C.S.S.C., a sports scientist and certified biochemist at the Sports Science Lab in New York. The faster those waste products can be removed, the faster the repair and recovery processes can occur, and the less time you need between intense workouts.
This recovery benefit extends to strength training as well, further debunking the myth that cardio and resistance exercise interfere with one another. “Efficient removal of waste products is essential for top performance [and recovery] in any form of exercise,” says Rice. But if you don’t like the idea of taking a page from Rocky’s training manual and weaving long slow runs into your strength training program, you have other options for increasing capillary density.
How to Boost Capillarization
There are two primary ways to do it. The first is through long slow distance (e.g., easy running or cycling). Research published in the Journal of Atherosclerosis and Thrombosis shows that several hour-long, low intensity cardio workouts per week can increase capillary density by more than 25 percent. A second study in The Journal of Physiology found interval training to be equally effective. Participants who performed four to six 30-second all-out sprints on a bike three times per week increased “microvascular volume” (AKA capillary density) as much as those who performed an equal number of 40 to 60-minute rides.
So which type of training should you do? Ideally both. Steady state cardio appears to increase capillary density primarily in slow twitch fibers while HIIT appears to increase capillary density more in fast twitch fibers, according to researchers at the University of Missouri, but either one will do the trick. “We’re always looking for that one mythological perfect way to create adaptation, but [the best way is to try to] check all the boxes,” says Rice.
One box you don’t want to leave unchecked is strength training. Contrary to popular belief—and what’s suggested in many personal training textbooks—heavy resistance exercise likely does not decrease capillary density. Indeed, strength training likely has no effect—positive or negative—on capillary density, according to a study in PLos One. And when combined with endurance training, strength training might even enhance the process of capillarization, providing one more reason for runners and cyclists to hit the weights, and weightlifters to run or cycle.
“There is also some evidence that doing muscular endurance training [e.g., higher rep sets with lighter weights] can increase capillary density,” says Thieme. “That supports what smart trainers have advised all along—that you should include a variety of rep ranges in your workout program.”
Just don’t confuse capillarization with vascularization, which is the veiny look prized by many bodybuilders. Those large, obvious veins snaking down their arms and bulging from their legs indicates little more than good muscle development and remarkably low body fat. “As muscles grow larger, they push veins to the surface, and as body fat diminishes, there’s less tissue to hide them,” says Rice, adding that such “vascularity” offers no performance benefits in and of itself. “But it means you’re lean, which can have a huge impact on performance!”
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