basically imagine how your biceps feel when you do curls after taking a NOS supplement then apply that same feeling to those 2 rather large muscles of the lower back. It is a constantly contracted feeling like they just will not relax. It also makes you walk around with your back arched like you have something stuck up your arse. The best way to avoid it is to drink plenty of water and take taurine. Doing crunches and other ab excercises between sets of deads or squats also helps to stretch it out and tone the muscles which have to fight it.