A friend sometimes cracks my back when I get locked up. Is that OK?

As long as you’re not physically injured by this, you’ll be OK. What’s happening is the spinal joints are being stretched to their physiological limit, causing the cavitation described previously. Usually the most mobile joints cavitate first, leaving the more restricted joints still locked. The cavitation effect produces an endorphin release, so it might feel good.  But as soon as this wears off, the contrast between the locked and free joints becomes noticeable, prompting the urge to crack it again. It can become a habit with some people. It’s been said that the joints become unstable with constant popping, but research seems to have dispelled that as myth.

This “cracking” should not be confused with a chiropractic adjustment because it lacks specificity and integration with other body structures. The tight sensation in your back may be coming from an adaptation to a physical stress elsewhere, and serving a real purpose of balancing compression or tension forces in your body. That said, the “crack” is better termed a mobilization (in other words, a localized stretch), and if it feels good, go ahead, but you might have a chiropractor find and fix the source of the tension.