Muscle Types: Voluntary vs. Involuntary

Your biceps and your stomach muscles are running on completely different operating systems.

The skeletal muscles you use to lift and run are voluntary. A nerve signal releases calcium,

which binds to a protein called troponin, kicking off a fast, powerful contraction. This is why you

can move quickly, but also why you get tired. But the smooth muscle in your organs,

like your stomach and blood vessels, is involuntary. It works on autopilot. Its contractions

are slow and sustained. Here, calcium binds to a different protein, calmodulin,

to get things moving. This is why your gut can work all day without getting tired,

while your arms need a break after a few reps. Two different jobs, two different muscle