Why strength fades faster than endurance as we age

HostI was watching my neighbor help move some boxes the other day, and it hit me how much he struggled with the heavy lifting, even though he walks three miles every single morning without any trouble. It feels like there's this strange split in how our bodies change, where the ability to keep going stays around, but that raw, explosive power just slips away. Does the body just give up on being strong long before it gives up on staying power?

GuestIt really does seem that way, and it comes down to the fact that we don't just have one kind of muscle. We have a mix. Think of it like having two different types of engines under the hood. You have one engine built for fuel savings and long trips, and another one built for a quick burst of speed. As we get older, the body starts to pull parts from the fast engine much sooner than the slow one. By the time we hit our seventies, we might have lost half of our quick-power muscle mass, but our steady-going muscle might not have changed much at all.

HostI always thought a muscle was just a muscle. You're saying they're not all the same stuff?

GuestNot at all. We have these fast fibers and slow fibers. The slow ones are the workhorses. They use oxygen to keep you moving for hours, and they're very hard to tire out. The fast ones are for big, sudden moves, like jumping out of the way of a car or lifting a heavy crate. These fast fibers are the ones that shrink and disappear as we age. They're expensive for the body to keep around because they take a lot of energy to build and maintain. When the body starts looking for ways to save resources, those big, hungry power fibers are the first things to go on the chopping block.

HostI don't quite buy that. If I'm using my legs to walk those three miles, my muscles are still working. Why would the body let the power part just waste away while the rest stays fine? It seems like a bad design.

GuestWell, it's not just about the muscle itself. It's about the wiring. Your brain sends signals down long nerves to tell your muscles to move. The nerves that talk to your fast-power fibers are actually thicker and use more energy than the ones for your steady fibers. As we age, those big power nerves start to break down. When the nerve dies, the muscle fibers it was talking to have nothing to do. They sit there waiting for a signal that never comes, and eventually, they just shrivel up and vanish. The nerves for your walking muscles are much tougher and tend to stay plugged in a lot longer.

HostSo it's like the wiring in an old house is going bad, and it doesn't matter if the lightbulbs are still good because the power can't reach them?

GuestThat's a great way to put it. And here is the really wild part. Sometimes, a steady nerve will see a power fiber that has been left all alone and it'll try to reach out and hook up to it. But because it's a steady nerve, it turns that power fiber into a slow fiber. You're basically losing your ability to be fast and strong because your body is trying to save what's left by turning everything into a slow, steady walker. You end up with a body that's fine for a long stroll but can't find the spark to lift a heavy bag of salt.

HostBut I see people in their eighties running marathons. If they can do that, why is a heavy lift so much harder? It feels like moving your whole body for twenty miles would be more work than lifting one box.

GuestIt's a different kind of stress. Running a long way is about how well your heart and lungs can move air and how well your muscles can use that air. That system is actually very hardy. You can keep your heart and lungs in great shape for a long time. But lifting a box is about how many muscle fibers you can turn on all at once. That takes a massive, sudden hit of electricity from those big nerves we talked about. Since those are the parts that go first, you can have the lungs of a marathon runner and still find yourself unable to get up out of a low chair without using your arms.

HostThat sounds a bit bleak. If the nerves are just quitting and the fibers are being changed over, is there even any point in trying to lift heavy things as we get older?

HostSo the endurance is almost like a gift that stays with us, while the strength is something we have to fight to hold onto every day.

GuestThe steady nerves that keep us moving are very resilient, but those quick-burst fibers will start to fade the moment we stop challenging them with real weight.

HostThe heavy boxes might always get a little tougher to move as the years go by, but the legs that carry us through a long morning walk are built to last.