Your thigh lost 10% of its size and 62% of its strength

Has your thigh shrunk in just a few days? The muscle is still there, it is the signal that is missing

September 2026 · Patient Resource

An almost intact muscle, six times weaker

You came home from hospital a few days ago. You look at your two legs side by side, and the one that was operated on already looks thinner. You try to tighten your thigh: almost nothing moves. It is normal, it is very common, and it is not what you think.

After a knee operation, we naturally assume the muscle “melts away”. It does shrink a little, that is true, without being the main cause of the weakness you feel.

Researchers measured, one month after a knee replacement, what really happens to the thigh muscle. The muscle had lost about 10% of its volume. But it had lost 62% of its strength.¹

An almost intact muscle can therefore be six times weaker, because the problem is not in the muscle, it is in the signal.

The knee that cuts the power

When a joint has been operated on, it is swollen, painful, and its internal sensors are on alert. They send the nervous system a very simple message, protect me, and the nervous system obeys by partly cutting the signal to the muscle that moves this joint. For the knee, that is the quadriceps, the big muscle at the front of the thigh.

You want to contract. The brain sends the order. But the order is held back on the way, at the spinal cord. The muscle is there, and it only switches on halfway.

Doctors call this arthrogenic muscle inhibition: “arthro” for the joint, which inhibits the muscle. It is not a weakness on your part, it is a protective reflex, and it affects almost everyone after a knee operation.²

Why it matters to know this

Because it changes what needs to be done.

If the muscle had simply melted away, it would be enough to build it back up. But a muscle you cannot command does not build up. However hard you try, the contraction stays weak, so exercise does not make it work hard enough to strengthen it. Meanwhile, it genuinely weakens. It is a cycle: the less it responds, the less it works, the more it shrinks.

This cycle sets in during the first weeks, the ones you spend at home, between two physiotherapy sessions.

Two things that switch the muscle back on

1. Reduce the swelling. The more swollen the knee, the more the muscle is cut off: the link is direct. Reducing the swelling is the first way to give the signal back to the muscle, and that is the role of cold and compression. Their effect is immediate: in patients four days after their operation, twenty minutes of cold on the knee gave 30% more strength to the voluntary contraction straight away.³

2. Make the muscle work without going through the signal. That is exactly what electrical stimulation does. Electrodes placed on the thigh send a current that triggers the contraction directly in the muscle, without needing the order from the brain to get through. The muscle contracts for real (you see it and feel it) during the weeks when it could not do so otherwise.

This is why international guidelines recommend electrical stimulation early after a knee operation: added to physiotherapy, it improves quadriceps strength, and the effect is three times stronger when you start in the first week.^4,5

What this means for you

• The weakness you feel is a signal problem before it is a muscle problem. It can be treated.
• The first weeks matter most. That is when the muscle needs to be worked in another way than by willpower alone.
• Cold and compression are not only for the pain: they are also what lets the muscle switch back on.
• Electrical stimulation makes the muscle work while the signal repairs itself. It adds to your physiotherapy, it does not replace it.