Here鈥檚 a somewhat depressing question to ponder if you鈥檙e in your thirties聽or beyond: Are your muscles getting slower, or are they just getting weaker? It鈥檚 an important question, because for many functional tasks鈥攕printing up a hill, pulling yourself past the crux of a climb, or simply getting out of a plush armchair鈥攕uccess depends not just on how much force you can exert, but on how quickly you can exert it. This is the question tackled by , 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鈥檚 what enables explosive movements like jumping. The older you get, the less power you鈥檙e 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鈥檙e losing muscle and getting weaker; but it could also be that the properties of the muscles themselves are changing, so that they鈥檙e no longer able to contract and generate force as quickly. There鈥檚 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 (, the same guy who first studied VO2 max, as it happens). If you鈥檙e trying to lift something that鈥檚 near the limits of what you鈥檙e capable of, you can only do it slowly. If you鈥檙e 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鈥檙e 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鈥攁s long as they鈥檙e both working a similar relative load, like 60 percent of maximum force. That鈥檚 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鈥檙e keeping score), there was basically no relationship. You can be the Incredible Hulk, but all that extra power doesn鈥檛 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鈥檛 really matter until it drops below a critical threshold, at which point you鈥檙e 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鈥 results offer a reminder that muscle speed is, to some extent, a product of strength. You can鈥檛 be powerful unless you鈥檙e also strong, and it鈥檚 strength that wanes with age. I enjoy the hopping and bounding, but I also added some kettlebells this year.
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