How stretching helps prevent injuries and why you’re running slower
TO STRETCH OR NOT TO STRETCH? A decade ago, not many people would have dared to even ask that question. But a series of careful studies have upended assumptions that endured for decades, leading to major changes in how scientists view the link between flexibility, injuries, and athletic performance. In particular, it turns out that traditional “static” stretching before exercise not only doesn’t help you—it actually reduces your strength, speed, and endurance. Instead, new research suggests that the best way to warm up involves “dynamic” stretching.
Beyond traditional stretching, many people turn to activities like yoga and Pilates to enhance flexibility and build core strength. Researchers are only now beginning to study these activities and draw conclusions about what benefits they can and can’t offer.
Will stretching help me avoid injuries?
If there’s one topic that perfectly illustrates the divide between sports scientists and athletes of all levels, it’s stretching. From peewee to the pros, almost everyone does it—despite the publication of study after study suggesting that stretching won’t prevent injuries, won’t prevent soreness, and in some circumstances will make you slower and weaker. Hockey players, for example, are “pathologically obsessed with stretching their hamstrings and groin,” says Mike Bracko of the Calgary-based Institute for Hockey Research. The pre-game ritual is so ingrained that the players don’t really care what the evidence says.
The evidence, to be fair, is still full of contradictions and unanswered questions even after decades of study. This is partly because there are so many different ways to stretch and because people expect so many different benefits. For clarity, we’ll first examine the general question of whether a stretching program can help prevent injury. Then we’ll take a separate look at whether pre-exercise stretching helps you produce your best performance, and whether post-exercise stretching helps you recover and avoid soreness.
It seems intuitively obvious that you’re most likely to pull a muscle if it’s tight. That’s why we stretch. The simplest and most common form of stretching is “static” stretching, which involves holding a position at the very edge of your range of motion for, say, 30 seconds at a time. There’s no dispute that this form of stretching increases your range of motion, both in the short term (during the subsequent workout) and in the long term (producing lasting changes).
The weak link in the chain of logic is the assumption that being more flexible protects you from injury. Most muscle injuries occur within the normal range of movement during “eccentric” contractions (while the muscle is lengthening, for instance when you’re lowering a biceps curl). In other words, you’re most likely to pull your hamstring while sprinting or changing direction, not while trying to do the splits (unless you’re a ballerina or a hockey goaltender, in which case static stretching is in fact important). “If injuries usually occur within the normal range of motion,” McGill University sports doctor Ian Shrier asked in a widely cited editorial in the British Journal of Sports Medicine in 2000, “why would an increased range of motion prevent injury?”
Literally hundreds of studies have tried to answer that question, and the Centers for Disease Control and Prevention reviewed 361 of them in 2004 in search of an answer. “Stretching was not significantly associated with a reduction in total injuries,” they concluded. Moreover, “use of stretching as a prevention tool against sports injury has been based on intuition and unsystematic observation rather than scientific evidence.” Subsequent reviews, most recently in 2008, have reached the same conclusion. One example of the “unsystematic observation” used to justify stretching is that several influential early studies compared one group that warmed up and stretched to another group that did neither. The problem with this study design is that warming up—for example with a gentle jog and some movement drills within the expected range of motion—reduces injury risk whether you stretch or not.
It’s important to note that the lack of proof that stretching prevents injury isn’t the same as proving that it doesn’t work. It may be that stretching programs have to be tailored to the individual needs of each person or each activity, so studies of a generic stretching program are doomed to produce ambiguous results. Imagine testing your prescription eyeglasses on 1,000 random people and using the results to judge whether eyeglasses help people see more clearly. So if, like Bracko’s hockey players, you’re deeply attached to your stretching routine, there’s not enough evidence to compel you to give it up—but there are some compelling reasons to think very carefully about when and how you stretch.
Could stretching before exercise make me slower and weaker?
Until the debate about whether stretching prevents injuries is settled, many people will keep it as part of their exercise routine—so it’s worth considering whether stretching has any other effects that might influence when you choose to stretch. One major change that has already taken hold is the realization that you shouldn’t stretch a “cold” muscle. Experiments with leg muscles from rats at the University of Michigan, for example, have shown that even mild stretching is enough to damage unprepared muscle fibers. Since stretching is typically incorporated as part of a pre-exercise warm-up routine, experts now recommend that you first start with a gentle jog (or bike or swim) to warm up your muscles and make them more compliant. You should also avoid overstretching to the point of pain.
Even if you follow this advice, a series of studies in the last few years suggests that you’ll still suffer from temporary after-effects that make you weaker, slower, and less efficient. This phenomenon isn’t yet fully understood, but some experiments have found that it can last up to two hours after stretching—not exactly the ideal preparation for a workout or competition where you want to be at your best. Consider, for example, a 2010 study by researchers at the University of Milan, published in the Journal of Strength and Conditioning Research. A group of 17 volunteers performed a series of vertical jumps from different squatting positions, with or without stretching their leg muscles beforehand. The height of the jumps, the peak force they exerted with their legs, and the maximum velocity attained were all significantly lower after stretching, in agreement with similar results from a half-dozen earlier studies.
Why does this happen? There are various theories—for example, it’s possible that “looser” muscles and tendons aren’t able to transmit force to the bones as effectively, much like the ropes on a sailboat need to be taut in order to trim the sails. Or, on a microscopic level, it may be that individual muscle fibers are able to exert more force when they’re shorter. There is also evidence for “neuromuscular” effects, in which the after-effects of stretching disrupt the signal between brain and muscle calling for a contraction. Most likely, it’s a combination of these factors—that would explain why researchers at McMaster University found decreased strength related to neural signaling that lasted for about 15 minutes, along with weakness in the muscles themselves that lasted for up to an hour after a session of calf muscle stretching.
Of course, observations in the laboratory don’t always translate to the real world. So scientists and coaches at Louisiana State University’s NCAA-champion track team put the theory to the test with 19 of their star sprinters in 2008. Each athlete performed three 40-meter sprints in two different sessions separated by a week; they performed a dynamic warm-up before both sessions and added four static stretches of the calf and thigh muscles before one of the sessions. The result: they were significantly slower (5.62 versus 5.72 seconds) when they stretched, with most of the deficit coming in the second half of the sprint.
All of this evidence, along with related studies that show endurance may also be compromised, makes a compelling case that you should avoid stretching before workouts or competitions, says Jason Winchester, the lead author of the LSU study, whose consulting clients include National Football League teams and the U.S. national track team. Instead, incorporate light stretching after your workouts or on rest days. If you really prefer stretching before workouts, the slight loss of strength and speed is unlikely to make much difference, but if it’s a competition, leave as much time as possible between stretching and the start.
Do flexible runners run more efficiently?
Even if we can’t agree on what the “perfect” running form is, most of us do recognize when we see a runner who looks particularly smooth and effortless. Seeing it is one thing, but imitating it is another thing entirely—no one is quite sure how to develop a more efficient running style. One of the most common ways people try to do this is to increase flexibility with a regular stretching program, so that they won’t be held back by tight hamstrings or locked-up hips. This makes sense intuitively, but the evidence doesn’t back it up.
When researchers tinker with running form, they’re less interested in subjective outcomes like how smooth you look than in more objective measures like “running economy”—a concept similar to fuel economy in cars. Running economy tells you how much energy you have to burn to run at a given pace; the less energy you burn, the longer you’ll be able to continue at that pace. (It’s usually determined by measuring very precisely how much oxygen you breathe in and how much is successfully transported to your working muscles.)
Researchers have been sticking volunteers on treadmills for decades, trying to figure out which factors lead to good or bad running economy. As far back as 1990, studies were starting to suggest that the runners with the most trunk and lower-body flexibility had the worst running economy. A rigorous study of 34 world-class British distance runners in 2002 reached the same conclusion, using a simple sit-and-reach test, which involves sitting on the ground with your legs straight in front of you and reaching as far as possible toward your toes. Another study, at Nebraska Wesleyan University in 2009, found the same connection between better sit-and-reach score and worse running economy.
This effect likely stems from the remarkable ability of your muscles and tendons to store energy like coiled springs, providing an estimated 40 to 50 percent of the energy you use for each step. “If you decrease the stiffness of the muscles and tendons, then you can’t store and reutilize energy as well,” explains Jacob Wilson, an exercise physiologist at Florida State University. The connection is less pronounced in women, who are generally more flexible to start with. And the initial studies only measured correlations, rather than showing that increasing your flexibility actually causes lower running economy.
To address this gap, Wilson and his colleagues asked 10 male runners to complete a pair of one-hour tests consisting of 30 minutes running at a predetermined pace to measure running economy, followed by 30 minutes as fast as possible. Before one of the tests, the subjects did 16 minutes of “static” stretching—the most common technique, which involves stretching a muscle to the edge of its range of motion, then holding for 30 seconds. Sure enough, the non-stretchers burned about 5 percent fewer calories in the first part of the run and ran 3.4 percent farther in the second part. Though it was already well established that static stretching causes a temporary decrease in strength and power, the 2010 study marked the first time the effect had been observed in an endurance activity.
That doesn’t mean you should leap into exercising with no warm-up—it just means you should rethink how you warm up. New results from a follow-up study by Wilson and his colleagues suggest that, unlike traditional static stretching, an alternative approach called “dynamic” stretching doesn’t hurt running performance.