[porpoisemonkey]

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

[kaba0]

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.

[lock-the-spock]

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...?