Short answer: When you sit for hours, a muscle in your calf called the soleus becomes inactive. This muscle normally acts as a pump, pushing blood upward from your legs toward your heart. When it stops working, blood can pool in your lower legs, which may contribute to that heavy feeling 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 legs feel heavy, tired, or sluggish after 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. Unlike most muscles that rely on stored energy, the soleus draws fuel directly from your bloodstream.
But when you sit with your knees at 90 degrees, this muscle becomes largely inactive. The pump slows. Blood can pool in your lower legs. That pooling — combined with other factors like prolonged inactivity or reduced circulation — may explain why your legs feel heavy, tired, or sluggish in the calves after a long day of sitting.
Heavy legs are 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 heavy legs include: venous insufficiency, edema, dehydration, medications, pregnancy, obesity, or neurological conditions. If your symptoms persist or worsen, consult a doctor.
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. The Niccolini study (2020) confirmed that during soleus contraction, upward blood flow increases 4x compared to rest.
Read the full research summary →
If you have sudden swelling, pain, redness, or warmth in one leg, or difficulty breathing, seek medical attention immediately. These could be signs of a blood clot (DVT).
If heavy legs are accompanied by chest pain, shortness of breath, or fainting, call emergency services.
Educational purpose only. This information is not medical advice. If you have concerning symptoms, consult a doctor immediately.
The soleus muscle is often called the "second heart" because of its role in circulation. To understand why this muscle matters for your leg health, explore these resources: