Roger Fielding runs a muscle physiology lab at Tufts University. In a study he co-authored with Walter Frontera and colleagues, the team biopsied muscle from the same group of older volunteers twice, 8.9 years apart, and measured whole-muscle strength alongside the contractile properties of individual muscle fibers. Whole-muscle strength and specific force were measurably lower at the second visit. According to a Tufts University news release describing the same research, Fielding said the individual fast-twitch and slow-twitch fibers themselves were generating about the same force as they had roughly a decade earlier. This is a report on research in humans, not a training plan, and nothing below should be read as medical advice for anyone managing a specific condition.
That finding sat oddly with something everyone already knows: older adults lose muscle power. If the fibers themselves weren’t getting weaker, something else had to be going on.
What actually changes
The answer researchers keep landing on is that people aren’t losing fiber quality so much as losing fibers, and losing a particular kind disproportionately. Skeletal muscle is a mix of slow-twitch fibers, built for sustained, low-intensity effort, and fast-twitch fibers, built for quick, forceful contractions. With age, fast-twitch fibers atrophy and disappear faster than slow-twitch ones do.
Fielding’s group put numbers to part of this in an earlier, separate 12-year longitudinal study of older men, led by Walter Frontera and published in the Journal of Applied Physiology. Isokinetic strength in the knee and elbow fell somewhere between 20 and 30 percent, at both slow and fast movement speeds, over that period, alongside measurable shrinkage in thigh muscle cross-sectional area. Losses like that compound over a lifetime, and they don’t land evenly across every type of movement a person makes.
It’s mostly the muscle, not the signal telling it what to do
A 2024 paper in the Journal of Applied Physiology, led by David Wrucke and Christopher Sundberg’s group at Marquette University, tried to work out how much of the power loss traces back to the muscle itself versus the nervous system’s ability to call it into action, across old and very old adults of both sexes. Their answer leaned toward the muscle. Voluntary activation, roughly how fully the brain can recruit the muscle during a maximal effort, stayed close to its ceiling across every age group in their sample and explained only a small, inconsistent share of the difference in power, and only in women. What tracked closely with power loss instead was the muscle’s own contractile behavior: how quickly an electrically stimulated twitch develops force, a property of the tissue itself rather than the strength of the signal reaching it.
One detail from that same study is worth sitting with on its own. The researchers measured daily step counts with accelerometers, as a rough stand-in for how physically active each person was in ordinary life. Step count explained only about 3 percent of the variation in peak power. Simply being generally active, on its own, wasn’t doing much to preserve this specific capacity.
A separate line of research points at the wiring
That still leaves an open question: if the muscle’s own contractile properties are what’s changing, why are they changing, and why disproportionately in fast-twitch fibers? A different line of research, including the master athlete comparison described below, has pointed at motor unit remodeling, the gradual loss of the specific, larger motor neurons that control fast-twitch fibers, as a plausible upstream driver. Fast-twitch fibers that stop reliably receiving a signal are fibers on a path toward disappearing or being taken over by neighboring slow-twitch neurons, whatever their own tissue-level properties look like in the meantime.
What separates people who keep their fast fibers
A separate study, led by Tiril Tøien and colleagues and published in the Journal of Applied Physiology, compared lifelong strength-trained and lifelong endurance-trained master athletes over age 70 against both recreationally active older adults and younger active adults, and found a clear split. The strength-trained group had a distribution and grouping of fast-twitch fibers that closely matched the younger comparison group, along with maximal strength and rate of force development in the same range. The endurance-trained and recreationally active older groups did not; they showed a higher proportion of slow-twitch fibers, more grouping of those fibers, and more fibers that had visibly atrophied.
The researchers behind that comparison concluded that strength training specifically, not general activity and not endurance training, appeared to be what preserved the innervation of fast-twitch fibers into old age in their sample. That’s a single comparison of relatively small athlete groups, not a randomized trial, so it says more about what’s possible than about what any individual should expect from a given routine.
Why this might matter day to day
Power, in the physics sense, is force multiplied by how quickly that force is produced, and a lot of ordinary movement depends on the second half of that equation more than people realize. Catching yourself on a stumble, standing up quickly from a low chair, and stepping off a curb without stumbling all call on the ability to produce force fast, not just eventually. Researchers studying mobility in older adults have repeatedly found that power output predicts these kinds of functional tasks more closely than strength alone does.
None of this is a reason to self-diagnose or to start a new exercise program without talking it through with a doctor or physical therapist first, especially for anyone with an existing injury, joint condition, or balance concern.
An open question
What isn’t settled yet is how much of this trajectory is fixed by the time someone reaches their sixties or seventies, and how much genuinely responds to changing what a person asks their muscles to do. The master athlete comparisons are suggestive rather than conclusive, drawn from small numbers of unusually committed lifelong exercisers rather than a general population trying something new later in life. Whether the same neural preservation shows up in people who take up a different kind of training at 65 rather than 25 is the more useful question, and it’s one the current research doesn’t fully answer yet.