what does 'が入らず' mean?


全身に力が入らず- what does '力が入らず' mean? I know 力 means strength, but what does
'が入らず' mean?

I've done some googling and it either means without strength or filled with
strength. That's why I'm very clueless right now.

If possible, can I get the meaning of 'が入らず' and the meaning of 'が入らず' in this
sentence's concept if it contains another meaning?

Thanks guys!

