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The smoking gun, for me, is the hill sprint at the end of the workout. I can still keep up with some of my younger training partners in tempo runs and mile repeats, but when it comes to going up an incline, I find myself in the dust. This kind of explosive muscle power was once in my wheelhouse, but now that I’m approaching 40, those speeds are lacking.

I’m hardly alone. The decline in muscle power is one of the characteristics of aging from midlife onwards. This is a problem because power is one of the best indicators of your ability to handle typical activities of daily living, like climbing stairs and pulling yourself out of a chair, in your later years. But scientists don’t really understand why power declines so quickly and inexorably with age. A new study in the Journal of Applied Physiology looks into this mystery and its findings offer some clues on how to combat this loss of power.

Why muscle power is important

Let’s start with a definition: power is equal to force times speed. When we talk about muscle strength, we are referring to the force that a muscle can exert. When we talk about muscle power, we’re talking about the combination of how much force we can exert and how quickly we can exert it. To climb a hill, jump on a box, or get off the couch, you must exert force explosively rather than incrementally. And of course, research over the years found that power is more important than strength in predicting whether older adults are able to successfully manage activities of daily living.

The problem is that power starts dropping earlier than strength, and continues to drop faster. A typical estimate is that you lose 0.5 to 1 percent of your muscle mass per year once you’re on the wrong side of 40. Strength generally follows a similar trajectory. In contrast, muscle power declines by 2 to 4 percent per year. We don’t know exactly where the additional losses come from: it could be that our brain sends weaker signals to the muscles; signals can be disrupted during their transmission through the nervous system; or something in the muscles themselves might change the way they contract.

What the new study reveals

Researchers at Marquette University, led by Christopher Sundberg, tested a group of young adults with an average age of 23; a group of elderly people with an average age of 70 years; and a group of very old adults with an average age of 86 years. The tests measured the maximum power the subjects could produce with their quadriceps, sitting with one knee bent and then trying to straighten it as powerfully as possible. Using magnetic brain stimulation and electrical muscle stimulation, researchers were able to determine the contribution of the brain and nervous system versus the muscles themselves.

The resulting data are quite complex and have some nuances (the study is free reading online if you want to dig deeper), but the big picture is simple. Here’s how peak strength and power (divided by body weight) compare in three groups, for men and women:

No surprise here: we lose muscle power with age, and as expected, the decline in power is greater than the decline in strength. But there appears to be no appreciable decline in the ability of the brain and nervous system to signal a powerful contraction, even in the “very old” group. Instead, it’s the muscles themselves that no longer contract as powerfully as before.

There are many possible reasons why muscles may lose some of their contraction. It could be that the muscle tissue is now mottled with fat and scar tissue, which interferes with the effectiveness of contractions. Tendons may be less elastic and therefore less able to transmit force quickly. There may be changes in the chemistry of the muscle fibers themselves. The most likely candidate, Sundberg and colleagues suggest, is that we tend to lose fast-twitch muscle fibers more quickly than slow-twitch fibers, leaving us with weaker, less explosive muscles overall.

What you can do to combat declining muscle power

As with most studies on aging, there is a chicken and egg question here: am I slower during hill sprints because my muscle composition is changing, or is my muscle composition changing because that I don’t do hill sprints and other explosive activities as often as possible? I used to do that? For aerobic fitness, a study that I wrote about last year It is estimated that only about half of age-related declines are the inevitable result of aging, with the other half resulting from changes in exercise habits. For muscle power as well, the answer probably lies somewhere in between.

However, it is not just a general decline in activity. Subjects in Sundberg’s study wore an accelerometer to measure their daily step counts and provide a rough estimate of their physical activity level. There was virtually no relationship between daily physical activity and maximal power: step count explained only 3% of the variance in maximal power. I’m a big fan of aerobic exercise, but staying fit doesn’t seem to be enough to keep those fast-twitch fibers active.

I’m venturing beyond what the study found, but the message I take away is that if you want to maintain as much explosive power as possible, you need to move and train explosively. Plyometric exercises, which include things like box jumps and skips, are one approach. Another option is resistance training with lighter weights, less than about 60% of the one-rep max, depending on the American College of Sports Medicine guidelines– where you focus on performing the movement as quickly as possible.

With the specific goal of hanging on to your fast-twitch fibers, it may also be a good idea to do intense resistance training, with sets of six reps or fewer. If the weight is heavy enough, you’ll need to recruit your fast-twitch fibers to lift it. It’s worth remembering, after all, that power equals force times speed. Increasing the force you are able to provide is therefore a key part of the equation. There is evidence that once the force falls below a critical levelusable power drops off a cliff.

It remains to be seen which of these different approaches is the most effective. For now, my plan is simple: no more hill sprints.

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