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Also not a biologist - just an enthusiastic layman.

The Soleus muscle (Soleal pump) is partially responsible for helping to return blood from your legs back up to your heart while upright. [1] This is a fairly critical process so it would make sense that it would be able to metabolize multiple energy sources.

[1] https://www.physio-pedia.com/Soleus

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If anything, the above article says that the soleus can’t utilize multiple energy sources, it relies mostly on the blood stream (which makes sense for better endurance at running/walking).

Also, pumping the blood back is a purely mechanical process, the same is true for your arm muscles, I don’t think it has a relevance here — its a very important process that circulates lymph and helps circulate blood.

Interesting. And the logical second conclusion is that this is an evolutionarily costly process, otherwise it would be common across our muscles. Maybe the muscle has a higher risk of injury, degeneration, cancer than othe muscles...?

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