Why your legs swell when you sit or stand still

Your shoes fit fine at 8am. By six, the sock line is pressed into your ankles, your laces feel a size too tight, and your legs feel heavy in a way that's hard to explain to anyone who hasn't had it.

That end-of-day swelling isn't random, and it isn't something you're imagining. It comes down to how fluid gets back out of your legs, and what happens to that drainage when you spend the day sitting or standing still.

Where the fluid in your legs comes from

All day, a small amount of fluid seeps out of your tiniest blood vessels into the tissue around them, carrying oxygen and nutrients to your cells. That's normal. It's how your tissues get fed.

Most of that fluid then has to find its way back. Some is drawn back into the blood vessels, and the rest is collected by your lymphatic system, a network of fine vessels that gathers excess fluid from your tissues and returns it to your bloodstream. That process is called lymphatic drainage.

When more fluid seeps out than gets carried away, it builds up in the tissue. That build-up is swelling, and the medical name for it is edema.

Your legs are the hardest place to drain

Getting blood down to your feet is easy, because gravity helps. Getting it back up is the hard part. Blood returning from your legs has to travel up through your veins against gravity, and so does the fluid in your lymphatic vessels.

Your veins manage it with one-way valves that stop blood slipping back down, plus help from the muscles around them. Your lymphatic vessels have no pump like the heart at all. They contract gently on their own, but they rely heavily on the movement of the muscles and tissue around them to keep fluid moving.

So how well your legs drain depends a great deal on one thing. Whether you're moving.

The calf muscle pump, and why stillness switches it off

Every time you take a step, your calf muscles tighten and relax. As they tighten, they squeeze the deep veins running through them and push blood upward. As they relax, the valves stop it falling back. This is known as the calf muscle pump.

The same squeezing helps push fluid along your lymphatic vessels too. Walking is quietly doing a lot of drainage work you never notice.

Now stand at a counter for an hour, or sit through a run of back-to-back meetings. Your calves barely contract, the pump slows right down, and pressure builds in the veins of your lower legs. More fluid seeps out into the tissue and less of it is carried away, and gravity settles it at the lowest point, which is why it shows up around your ankles and feet first.

That's why the swelling follows your day so closely. It isn't the hours on your feet as such. It's the hours your calves spend doing nothing.

Why standing still can be harder on your legs than walking

This is the part that surprises people. A nurse walking the wards and a retail worker standing at a register can both finish the day with heavy legs, but standing in one spot is generally the harder of the two for drainage, because walking keeps the pump running and standing still doesn't [SOURCE TO VERIFY: prolonged static standing and leg swelling, e.g. occupational health literature on standing work].

Sitting has its own version of the same problem. Your calves sit idle for hours, and a long flight or a long drive packs a lot of those hours into one block. It's why so many people notice puffy ankles after landing.

Warm weather tends to make it more noticeable, because your blood vessels widen in the heat. Worth knowing with summer on the way.

Why the swelling is gone by morning

When you lie down for the night, your legs are no longer hanging below your heart. Gravity stops working against the drainage, and fluid that pooled through the day has a chance to move back. That's why the sock lines have usually faded by the time you wake up.

It's also a useful check. Swelling that comes from being still usually goes down while you sleep. Swelling that's still there in the morning is a reason to see your GP, and there's more on that below.

What helps your legs drain during a long day

The idea is simple. Give the calf pump some work to do, and give gravity less to work with.

• Get up and walk for a couple of minutes regularly, even if it's only to the kitchen and back.
• Do slow calf raises while you stand: up onto your toes, then back down.
• Under a desk, circle your ankles and point and flex your feet.
• Stuck behind a counter? Shift your weight and walk on the spot when you can.
• At home, rest with your feet up higher than your heart for a while.

Some of this is harder than it sounds. Nobody on a busy shop floor gets to wander off whenever they like, and after a long day, getting your feet properly above your heart takes more arranging than flopping on the couch. Do what your day allows. If you're at a desk, these daily habits for desk workers are an easy place to build movement in, and if you're on your feet all shift, we've written about foot pain from standing all day as well.

Where compression fits

Compression works from the outside. By squeezing the leg, it takes on some of the job your calf muscles do when they're moving, pressing on the tissue and veins so fluid is encouraged back up rather than left to pool.

Compression socks apply a steady pressure that's usually firmest at the ankle. Compression boots use a different approach called sequential wave compression, where chambers inflate one after another in a wave rather than squeezing the whole leg at once. Many boots stop at the knee. Swelling and heaviness don't always.

Compression isn't right for everyone. People with some circulation, heart or skin conditions are told not to use it, so if you have a medical condition, or swelling you don't have an explanation for, talk to your GP before you start. It also works alongside moving, not instead of it.

When swelling needs a doctor

Swelling from a long day of sitting or standing is common. But swelling can also be a sign of something that needs checking, and a blog post can't tell you which kind yours is. See your physician if:

• only one leg is swollen
• one calf is painful, warm, red or tender
• the swelling came on suddenly, or keeps getting worse
• it's still there in the morning after a night's sleep
• the skin over it is broken, weeping, shiny or changing colour
• you have a heart, kidney or liver condition, or the swelling started after a new medication
• you're pregnant and swelling comes on suddenly, particularly in your face or hands

Call emergency straight away if swelling comes with chest pain or shortness of breath. And don't use compression of any kind on a leg that's painful, hot or swollen on one side until a doctor has checked it.

The short version

• Fluid seeps out of your blood vessels into your tissues all day, and your veins and lymphatic system carry it back.
• From your legs, that return trip is uphill against gravity.
• Your lymphatic vessels have no pump like the heart, and your leg veins rely on your calf muscles squeezing as you walk.
• Sitting or standing still switches that calf muscle pump off, so fluid pools around your ankles and feet by evening.
• Regular movement and time with your feet up give drainage a hand, and compression can take on some of the squeezing when you can't move.
• Swelling in one leg, swelling that doesn't go down after a night's sleep, or swelling with pain, heat or breathlessness needs a doctor.

The Vibit FlowBoots are our compression boots for heavy, swollen legs at the end of the day. They're wireless and thigh-high, and they pair sequential wave compression with heat. Their thigh compression chamber carries the compression above the knee, where calf-only boots stop, so the squeeze doesn't end halfway up your leg.