Aerobic exercise vs. weight training, long runs vs. HIIT training. As quickly as science and technology change, so do philosophies on which type of exercise is best. Yet, one theory has remained the same: stretching leads to greater flexibility.
Scientists are now saying that they're not exactly sure what happens during a stretch. Although muscles and tendons are elongated, it's clear that stretching doesn't actually make the muscle longer. Muscles are connected at specific points along the bones, so they can't get permanently longer. And if one likens the muscle to a rubber band, you wouldn't want it permanently stretched out, says Jules Mitchell, a yoga instructor and a master's degree candidate in exercise science at California State University, Long Beach.
So if stretching isn't actually increasing muscle length, how does it lead to greater flexibility?
Scientists propose that it actually has nothing to do with the muscle, and more to do with the nervous system. Nerve endings throughout the muscles and tendons fire off signals when a movement doesn't feel safe. So every time you bend and touch your toes, you're actually training your nervous system to tolerate more muscle extension without firing off pain signals. This is the reason a person under anesthesia, whose nerves are quieted, can undergo a full range of motion without any resistance.
There's no doubt that athletes can deepen their stretch and increase flexibility. But it's because they've trained their bodies to quiet those nerve signals. "The body adapts to the movements you most frequently make," Mitchell said. "The corollary to that is that the body adapts to the movements you don't make: It adapts by not making those movements anymore."
And passive stretching might not be the most efficient way to increase flexibility. Instead, experts are recommending something called proprioceptive neuromuscular facilitation (PNF), where people extend their muscles and then try to contract them from a lengthened position. For instance, if you're trying to stretch your hamstrings, try contracting the hamstring muscle as you simultaneously bend forward and stretch it out. This type of exercise places a greater load on the muscle and tells the nervous system that it can handle it, leading to quicker progress.