Scientists disagree about the underlying reason for our loss of power. (Photo: Trinette Reed/Stocksy)
Sweat Science

Why Older Athletes Lose Explosive Power

Scientists have been debating whether muscles contract more slowly as you age, but new data suggests the real problem is a loss of strength


Heading out the door? Read this article on the new Outside+ app available now on iOS devices for members! Download the app.

Here’s a somewhat depressing question to ponder if you’re in your thirties or beyond: Are your muscles getting slower, or are they just getting weaker? It’s an important question, because for many functional tasks—sprinting up a hill, pulling yourself past the crux of a climb, or simply getting out of a plush armchair—success depends not just on how much force you can exert, but on how quickly you can exert it. This is the question tackled by an interesting new study in PLOS ONE, from a research team at Manchester Metropolitan University led by Hans Degens.

The combination of strength and speed is what we call power. Mathematically, power is force times velocity, and it’s what enables explosive movements like jumping. The older you get, the less power you’re capable of generating, which translates into reduced athletic performance and, beyond a certain point, difficulty in carrying out the daily activities needed to live independently.

Scientists disagree about the underlying reason for our loss of power. It could simply be that we’re losing muscle and getting weaker; but it could also be that the properties of the muscles themselves are changing, so that they’re no longer able to contract and generate force as quickly. There’s evidence on both sides, so Degens and his colleagues designed a study to explicitly test the question.

They recruited 20 men and women in their twenties, and 20 men and women in their sixties and seventies. The key test was a countermovement jump, which simply means bending your knees and then leaping as high into the air possible. This is a standard test of muscular power, because you have to be both strong and fast to produce an explosive jump. The twist: the subjects also performed jumps wearing sandbags that added 15 percent to their body weight, and while wearing a counterweighted harness hanging from a pulley that effectively reduced their body weight by 15 percent.

If you simply compare young and old jumpers, it seems obvious that the older jumpers have slower muscles, as measured by their take-off speed from the ground. But the speed of a muscle contraction depends on how heavy the load is (an equation derived in the 1930s by A.V. Hill, the same guy who first studied VO2 max, as it happens). If you’re trying to lift something that’s near the limits of what you’re capable of, you can only do it slowly. If you’re trying to lift a feather, you can whip it up very rapidly. Since the older subjects are weaker (as measured in a static test of leg strength pushing against an immovable barrier), they’re lifting a relatively heavier object when they try to propel their bodies into the air. Hence the sandbags and pulley: by making the younger jumpers heavier and the older jumpers lighter, you can test them at a similar place on that force-velocity curve.

Crunch the resulting data, and you find that the older subjects have muscles that contract just as quickly as the younger subjects—as long as they’re both working a similar relative load, like 60 percent of maximum force. That’s the good news. The flip side of the coin is that this means the loss of power that accompanies aging is entirely a result of lost strength.

Degens and his colleagues also put their subjects through a timed up-and-go (TUG) test, which involves getting up from a chair, walking around a cone ten feet away, then sitting back down in the chair. The older subjects were a little slower on average than the younger ones: a little over five seconds compared to a little over four seconds. But the interesting pattern was the relationships between TUG time and jump power. Above a certain critical power (23.7 watts per kilogram of bodyweight, if you’re keeping score), there was basically no relationship. You can be the Incredible Hulk, but all that extra power doesn’t help you get out of a chair any faster. But if your max jump is below that critical power (which was true for about half the older group), times drop off a cliff. For activities of daily living like the TUG test, in other words, muscular power doesn’t really matter until it drops below a critical threshold, at which point you’re in trouble.

I suspect there are some useful insights here for older athletes, too. For athletic performance, particularly in endurance sports like running, explosive power seems to be more useful than raw strength. Plyometric exercises, for example, are thought to improve the neuromuscular connections between brain and muscle, enabling you to move more efficiently. I include some box jumps and one-legged hops in my own routine. But Degens’ results offer a reminder that muscle speed is, to some extent, a product of strength. You can’t be powerful unless you’re also strong, and it’s strength that wanes with age. I enjoy the hopping and bounding, but I also added some kettlebells this year.

For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.

Lead Photo: Trinette Reed/Stocksy