Why You Feel Slower But You're Not Weaker

Your lifts are fine. Better than fine: most of them are the best they've been.

But you're half a step late to the wide ball. You're not first to the loose ball anymore. Off the blocks, out of the turn, into the first two strides of a sprint, something that used to be there isn't.

The strange part is that nothing in your training data reflects it. The squat went up. The bench went up. You're running the same weekly volume. And yet there's a quality you can feel the absence of and can't name.

It has a name. It also has a number, and it's one of the most trainable things we measure.

The spring you stopped noticing

When your foot hits the ground running, or when you plant to change direction, something slightly counterintuitive happens. Your muscles and tendons absorb force on impact, store part of it elastically, and return it as you push off.

That stored-and-returned energy is free. You didn't generate it with a contraction. You caught it and gave it back. The more efficiently you do that, the less work your muscles have to do to produce the same movement.

This is the stretch-shortening cycle, and it's happening thousands of times in any run and constantly in any court or field sport. It's largely a property of your tendons and nervous system working together, and it operates in timeframes too short to feel: a good ground contact and a poor one are separated by around a tenth of a second.

That's what "springy" actually describes. Not a mood. A measurable mechanical property.

Why it's the first thing to go

Maximal strength is remarkably durable. Keep training it and it holds up well into your forties, fifties and beyond. The research here is more encouraging than most people expect.

Reactive and rate-of-force qualities don't hold up the same way. They decline earlier, and more quietly.

And here's the problem with "quietly": because nobody measures them, there's no baseline to notice the decline against. There's no moment where a number drops and you investigate. There's nothing in your training log that captures it, because your log records how much you lifted, not how fast you produced it.

So the decline arrives as an impression. A general sense of having slowed down. And the available explanation is the obvious one (you're getting older), which fits, requires nothing of you, and can't be falsified without a measurement.

Two things that feel identical from the inside: getting older, and being untrained in one specific quality. Only one of them responds to training.

It's not trained, either

The second half of the problem.

Walk into almost any gym intervention written for an adult who isn't a competitive athlete and you'll find sets of eight to twelve at a controlled tempo. That's sensible training and it builds force production well.

It asks essentially nothing of the stretch-shortening cycle.

Reactive quality is trained with short ground contacts, fast eccentric-to-concentric transitions, and strict quality criteria rather than accumulated repetitions. It's low-volume, highly specific, and it barely resembles conventional gym work, which is a large part of why it rarely appears in an intervention unless somebody has a reason to put it there.

So the quality that fades first is also the one that almost never gets trained. Not through anyone's negligence. Because nobody measured it, so nobody had a reason to prioritise it.

How it's measured

A drop jump. You step off a 30cm box, land, and jump again as quickly and as high as you can.

Two things are measured: how long your foot is on the ground, and how high you go. Divide height by contact time and you have the Reactive Strength Index: a single number describing how efficiently you absorb force and return it.

Five attempts, the middle three averaged. The whole thing takes about ninety seconds.

What makes it informative is precisely that you can't feel it. You have no sense of whether your contact time was 0.19 seconds or 0.31. Both feel like jumping. The difference between them is substantial and completely invisible from the inside.

What the number means, and doesn't

This is where most writing on RSI goes wrong, so let me be direct.

You'll find benchmarks online. Ignore them. The published figures (above 2.0 being excellent, and so on) come from elite and professional athletic populations. Holding a 42-year-old who plays tennis twice a week against them tells you nothing useful, and grading someone that way is both discouraging and meaningless.

Your number means something against your number. The question isn't where you sit relative to a professional athlete. It's where you sit relative to yourself eight weeks ago, under identical conditions: same box height, same footwear, same protocol.

Small changes aren't real changes. Between-day reliability for RSI is good but not perfect, with typical variation around 4 to 6% in trained populations. That sets a floor on how small a difference can honestly be called an improvement rather than noise. Anyone reporting a 3% gain as progress is over-reading their data.

It doesn't predict injury. It describes a current neuromuscular property. Nothing more.

The good news

Of everything in the assessment battery, this is among the most trainable.

Reactive quality responds well to specific work, and it responds relatively quickly: a change over eight to sixteen weeks is realistic in a way that large shifts in maximal strength over the same period usually aren't.

One important qualifier. Low reactive strength is not automatically an argument for plyometric training. If force production is also low, jumping asks a system to produce force rapidly when it can't yet produce it slowly, which is the wrong order and not a trivial mistake. In that case the base gets built first and reactive work is introduced later.

Low RSI with good force production is the opposite situation, and that person is exactly who reactive training is for.

Which of those two you are isn't something you can determine by feel. It's the reason this test sits alongside strength testing rather than on its own.

What it changes

If there's a number on it, there's something to train and something to check.

The vague complaint, "I feel slower", becomes a specific, measured quality with a plan attached and a retest date. And at eight weeks you find out whether the work did anything, rather than whether it felt like it did.

Most people never get that. They notice the step they've lost, file it under age, and train harder at the thing they were already good at.

Common questions

What is the reactive strength index?

Jump height divided by ground contact time during a drop jump. It describes how efficiently you absorb force and return it: the stretch-shortening cycle, mostly a property of tendons and the nervous system.

Why do I feel slower if my strength hasn't changed?

Strength and reactive quality are different things, trained differently and declining on different timelines. You can be stronger than you were at thirty and noticeably less springy, because the second was never measured or specifically trained.

Can reactive strength be improved after 40?

Yes. It's one of the more responsive qualities we measure, and meaningful change over eight to sixteen weeks is realistic. The caveat is sequencing: if force production is also low, that base is built first.

What's a good RSI score?

Published benchmarks come from elite athletes and don't transfer usefully to recreational adults. The number worth comparing against is your own, retested under identical conditions.

How is it tested?

A drop jump from a 30cm box, five attempts, middle three averaged, measuring jump height and ground contact time. It's part of the FORMA performance assessment in London, alongside strength symmetry, force-velocity profiling and recovery testing.