Why your legs ache by four o'clock, and what compression actually does about it

It usually arrives the same way. You are fine in the morning, fine through lunch, and then some time in the middle of the afternoon your legs start to feel like they belong to somebody heavier. Shoes are tighter. Socks leave a ring. Standing up takes a second longer than it should.

Most of us file this under being tired, or being a bit older, or having had a long day. Sometimes it is exactly that. But there is also a plain mechanical explanation, and once you know it, the feeling stops being mysterious.

Blood leaves the heart under pressure, which is why the journey down to your feet is the easy half. Getting it back is the hard part. From the ankle upwards the return trip runs against gravity, through veins that are softer and far lower pressure than arteries.

Those veins have one-way valves to stop blood sliding back down, and they rely on something outside themselves to push it along: your calf. Every step you take squeezes the deep veins in your lower leg and moves blood upwards. It is called the calf muscle pump, and it is doing more work than most people realise.

Six hours at a desk, eight hours behind a till, or four hours wedged into a seat at 35,000 feet all have the same effect on that pump. It stops running. Fluid settles in the lowest part of you, the ankles swell slightly, and by the evening your legs feel heavy and tight.

This is the gap compression is designed to fill. Not to replace movement, but to keep some pressure on the system during the hours when you cannot move.

Compression is not a treatment and it is not for everybody. But there is a fairly clear list of situations where the calf pump is switched off for long stretches, and those are the situations where people tend to notice a difference.

If you recognise yourself in more than one of those, you are the person compression was made for. If you recognise yourself in none of them, you probably do not need it, and we would rather say so than sell you a pair you will never wear.

The first pair tends to surprise people twice. The first surprise is that they take a moment to get on. Twenty to twenty five millimetres of mercury is real pressure, and a compression sock is meant to be a firm fit rather than a loose one. Roll them up the leg instead of hauling on the cuff and it is straightforward.

The second surprise is how quickly you stop noticing them. Most people describe the sensation as supported rather than tight, and after an hour it disappears into the background entirely.

The part people talk about comes at the end of the day, when you take them off and notice what is missing: the ache across the top of the foot, the heaviness in the calf, the ring where an ordinary sock dug in.