Short answer: When you sit for hours, a muscle in your calf called the soleus becomes less active. This muscle normally acts as a pump, helping push blood and fluid upward from your legs. When it slows, fluid can pool in your lower legs and ankles, which may contribute to swollen ankles in some people.
Most people know their heart pumps blood to their entire body.
Few people know there is another muscle that helps push that blood back upward from your legs.
It's called the soleus. Researchers call it your "second heart." →
When you sit for hours, this muscle becomes almost silent.
That silence may be why your ankles swell or feel puffy after a long day of sitting.
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The soleus is a muscle in your calf, located beneath the larger gastrocnemius. It is composed of slow-twitch fibres designed for endurance — meaning it can work for hours without tiring. One of its key roles is to act as a venous pump. The soleus is part of what is often called the calf muscle pump, which helps push blood upward from your lower legs toward your heart.
But when you sit with your knees at 90 degrees, this muscle becomes largely inactive. The pump slows. Fluid can accumulate in your lower legs and ankles — a condition sometimes called dependent edema. This may explain why you notice swollen ankles after sitting for long periods, or why your legs feel tight and puffy by the end of the day.
Long flights and travel: Have you ever noticed swollen ankles after a long flight? The same mechanism is at work. Prolonged sitting in a cramped seat reduces calf muscle pump activity, leading to fluid pooling in your lower legs. This is why many travelers experience swollen ankles after flying.
Leg swelling is just one example of a broader area we call Soleus Health — the study of how soleus activity influences circulation, metabolism, and health during prolonged sitting. Explore sitting physiology →
Other possible causes of swollen ankles include: venous insufficiency, heart conditions, kidney problems, medication side effects, pregnancy, or injury. If your swelling is persistent or worsening, consult a doctor.
The Niccolini study (2020) confirmed that during soleus contraction, upward blood flow increases 4x compared to rest. The Houghton study (2021) found that people with reduced soleus and calf muscle pump function were twice as likely to experience serious health events over time, including conditions related to poor circulation.
If you have sudden swelling in one leg only, accompanied by pain, redness, or warmth, seek medical attention immediately. These could be signs of a blood clot (deep vein thrombosis or DVT).
If you have swollen ankles after a flight that do not improve or are accompanied by chest pain or difficulty breathing, seek medical attention.
If swelling is persistent, worsening, or affecting both legs, consult your doctor to rule out underlying conditions like heart failure, kidney disease, or venous insufficiency.
Educational purpose only. This information is not medical advice. If you have concerning symptoms, especially sudden swelling in one leg, consult a doctor immediately.