Stiffening up?

B

Anyone else have a baby who stifens up for what appears to be no reason? He's super happy, but locks out his legs, hips, and back often - no crying, no gas, no spit up. He also likes to arch his back to look at things upside down. During tummy time he will lift his feet off the ground - almost like a super man exercise. He has amazing neck strength.

Just seems weird... Thoughts?