1. How Much Do Ankles Grow After 8 Hours of Sitting?
The experiment involved 9 healthy men with an average age of 22.6 years. They sat continuously for 8 hours in a chair similar to an economy-class airplane seat, and researchers measured their leg thickness every hour. [2] The results showed that calf circumference increased by about 2.4%, and ankle circumference increased by about 2.7%. For someone with a 35 cm calf, this is about 8 mm (calculated for this article). Additionally, the water outside cells in the shin and calf muscles increased by about 10%. [1] The increase was not due to more muscle or fat, but likely because of extra fluid inside the legs. This state of excess fluid collecting in the feet is called "edema" (swelling).
Since this result comes from only 9 men, it cannot be directly applied to women, children, or older people. However, it is an experiment that showed with numbers that leg measurements change just from sitting. So, why does fluid collect in the feet even though you are not moving?
2. Why Does Fluid Collect in the Feet from Gravity and Lack of Movement?
If you tie a bag filled with water and hang it up, the water settles at the bottom. In the human body, too, it is said that fluid tends to collect in the feet because of gravity. Whether standing or sitting, the feet are lower than the heart.
Normally, every time you move your legs, the calf muscles contract and help send blood back toward the heart. This acts like a pump. It is thought that if you sit without moving, this help weakens, making it easier for fluid to collect in the feet.
In another experiment where 6 healthy women sat for 6 hours, ankle circumference became 103.3% of what it was before sitting. The authors stated that a seated posture is the most likely cause of lower leg swelling and noted that gravity plays a role in edema. [3] This was a preliminary study with few participants, and the measurement methods and people differed from the previous experiment, so the size of the numbers cannot be simply compared.
This "collecting" does not seem to happen only to specific people. The MSD Manual explains that mild swelling may be seen as normal in people who are overweight, middle-aged, or elderly at the end of the day due to blood pooling, and it usually disappears by morning. [4] Fluid collects in the feet. So, what happens to the flow of blood itself while sitting?
3. How Does Sitting Reduce Blood Flow in the Legs?
In this experiment, blood flow in the artery on the top of the foot was also measured. If the flow before sitting is set at 100, it was about 61 after 8 hours, meaning blood flow dropped by about 40%. The oxygenation level of the calf muscles also decreased after sitting. [2]
It is important to note that this experiment only showed that "blood flow dropped and swelling occurred" at the same time while sitting. It is not clear from these results alone whether the lack of muscle use caused the swelling, or if reduced blood flow caused it. However, it has been confirmed with numbers that blood flow in the legs slowed while sitting.
So, where does the increased fluid come from?
4. Where Does the Fluid That Collects in the Feet Come From?
Water makes up about half to two-thirds of body weight. According to the MSD Manual, it is about 60% for adult men, 52-55% for women, and about 70% for infants. The percentage is lower for older people and those who are overweight. [5] For a 70 kg adult man, this is about 42 L, of which less than 3.5 L is in the blood. There is a large amount of water in the body.
Water is not fixed; it goes out every day through urine, sweat, and breathing, and comes in from drinks and food. The amount going in and out changes depending on age, activity, and temperature.
Looking at this, it seems natural to think that the fluid in swelling does not fall from outside, but that some of the water already inside the body collects in the feet. The experiment measured the increase in leg circumference and extracellular water, but did not investigate where the water came from.
The mechanism of how water collects is becoming clearer. So, can moving prevent this?
5. How Much Can Movement or Pressure Prevent Foot Swelling?
In the experiment, one leg wore elastic stockings (low compression), and the difference with the other leg was compared. In the leg wearing stockings, the increase in ankle and calf circumference and the increase in extracellular water were suppressed. [1] The drop in blood flow in the foot artery also stayed at about 15% instead of about 40%, and muscle oxygenation levels did not change. [2]
This is not a recommendation to wear stockings. It is an experimental result showing that changes became smaller when pressure was applied from the outside.
According to government explanations, the Ministry of Health, Labour and Welfare states that sitting for a long time in narrow seats without moving your feet can cause poor circulation and make blood clots more likely to form. They list prevention methods such as doing light exercises or stretches occasionally, drinking water frequently, raising and lowering heels, lightly massaging calves, and raising legs when sleeping. [6] This explanation is about blood clots (Economy Class Syndrome) and does not discuss swelling itself. Swelling and blood clots are different phenomena, so do not confuse them.
By the way, how long should you wait to see if swelling goes away?
6. When Is Mild Foot Swelling Okay to Watch, and When Should You See a Doctor?
The MSD Manual explains that mild swelling in the evening usually disappears by morning. [4]
On the other hand, the main causes of swelling in only one leg are blood clots formed in veins and infection. It is said that sudden swelling requires early medical attention. [4] If swelling appears on only one side or appears suddenly, do not judge it yourself; tell a family member and consult a doctor. The same applies to swelling caused by sitting.
7. How to Measure Your Ankle Circumference in the Morning and Evening
Compare morning and evening: With a family member, use a soft sewing tape measure (or mark a string and measure with a ruler) to measure around the narrowest part of the ankle right after waking up and in the evening. Do not pull it tight, and measure the same spot every time. Whether or not there is a difference in numbers, recording it has meaning.
Try heel raises: In the evening, while sitting, slowly raise and lower your heels, then measure again immediately after. The experiment subjects were adults, so children may not get the same numbers. Compare with your own records. If there is pain or numbness, stop and tell an adult.
Read materials: The Tokyo Medical University press release [2] is available in Japanese. The Ministry of Health, Labour and Welfare explanation [6] also includes diagrams and is in Japanese.