Well, symptoms vary a little. Mainly, it's like getting popped with a nail gun everytime you move.
In my case, it's like geting popped with a nail gun in my left hip, with the pain concentrating down the sciatic nerve of my left leg.
In a straight muscle spasm, it'd be like having a charlie horse from your shoulder blades to your waist, then it gets worse as you try to move.
Other times, it's like someone slapping you in the kidneys with full round-house swing with an eight-pound sledge.
As I said, symptoms vary, none of them are even close to fun.
FWIW
Scott