When you hit your arm, at the bicep, beside your knuckle slightly frozen, you go and get a touch bulge. What is it?

It lasts for similar to 3 seconds. It of late bulges and then disappears. Anyone know what cause this? You're causing the muscle to spasm by hitting it. If you hit it low adequate (near the bend of the elbow) you'll feel the bicep muscle contract.
Muscle spasm.