Here’s the latest research on what’s behind increases in flexibility.
There’s probably only one thing that everyone can agree about when it comes to stretching: if you stretch diligently and frequently over a prolonged period of time, you will get more flexible.
The consequences of that increased flexibility have been hotly debated over the last decade. Will it reduce your risk of injury, or increase it? Allow you to run more efficiently, or turn you into a floppily inefficient Gumby?
But we’ve paid less attention to the causes of that increased flexibility. What is it about stretching that increases the range of motion in your joints?
A new review in the Scandinavian Journal of Science & Medicine in Sports, from a team of researchers at the University of Lisbon and elsewhere, takes a look at two competing theories.
The “mechanical theory” is the one I’ve always intuitively assumed was true – that repeated stretching alters the mechanical properties of the affected muscles, tendons, or joint, either by lengthening them or making them less stiff and stretch-resistant. If that’s true, then sticking to a stretching routine will eventually allow you to bend your joint further without applying more tension (or, conversely, to apply less tension to reach the same angle).
The other possibility is the “sensory theory”, which posits that stretching simply teaches you (and your muscles and tendons) to tolerate more tension in a stretch. You can bend or reach further because you’re willing to, not because you’ve made any change in the mechanical properties of the joint.
To investigate this question, the researchers pooled the results of 26 studies that measured the mechanical properties of the joint, muscles, or tendons after stretching programs lasting between three and eight weeks. The participants in each study stretched at least twice a week, with average total active stretching time of a little less than 20 minutes per week.
The results appear to support the sensory theory. Participants did indeed get more flexible and were willing to tolerate greater tensile force while stretching, but only trivial changes were seen in the mechanical properties measured.
There are, of course, plenty of caveats. The most obvious is that eight weeks may simply be not long enough. As the authors point out, studies of weight training over similar durations tend to find neuromuscular strength gains but no appreciable change (i.e., growth) in the muscles themselves. The same may true for stretching: neural changes come first, but physical changes take longer.
There are also plenty of questions about the type, duration, and intensity of stretching required to elicit physical changes. Of the studies in this analysis, 23 looked at static stretching, three at dynamic stretching, and three at a technique called proprioceptive neuromuscular facilitation.
It’s worth noting that the authors are not anti-stretching zealots by any means. In fact, the lead author, Sandro Freitas, previously published a small study that did see muscle lengthening after an intense eight-week stretching program, in which the subjects held their stretches for 7.5 minutes (!) at a time without a break.
In the end, most of us care more about the outcomes than the mechanisms. These days, the pendulum has swung away from static stretching as a pre-run ritual, and those who advocate regular stretching tend to do so for its long-term rather than acute effects. Does it work, either for injury prevention or performance? I simply don’t know. I haven’t seen any convincing evidence that it’s helpful, but the relevant questions are so complex and difficult to study that I certainly don’t rule out the belief – which, perhaps tellingly, is almost universal among elite athletes and coaches – that it helps.
For now, if there’s one thing to take away from this meta-analysis, it’s that there’s no point being a stretching dilettante. If you really want to change the properties of your muscles and tendons, you’ll need to stick with your stretching program for many months.