It's easy to fix - you just have to squeeze the baby's limbs a lot so that they learn their body's boundaries and challenge them to use their whole body by tactically putting toys out of reach.
Unfortunately all this takes time, so being there to do it is invaluable.
Apparently this is what babies do when they're not stimulated properly in the limbs and have built-up tension in the muscles and compensate for it.
Long term effects include one's default posture being like that of Mr. Burns from the Simpsons - with wrists in a sort of T-rex position:
https://i.pinimg.com/736x/ed/e3/b6/ede3b668d617f03927aca5ca5...
It's easy to fix - you just have to squeeze the baby's limbs a lot so that they learn their body's boundaries and challenge them to use their whole body by tactically putting toys out of reach.
Unfortunately all this takes time, so being there to do it is invaluable.