I would say don't use it on a regular basis. Use it if you have muscle spasms associated with your training. According to my nursing drug guide full effect my not be felt for 1-2 weeks taking it on a daily basis. I did not know that little fact.
I'm looking at the drug guide as I type so I'll let you know what I see:
Action: reduction in muscle spasm and hyperactivity without "loss of function".
-may cause constipation, so increase your fluid intake.
-may cause urinary retention, my arch enemy.
-may cause dry mouth, drowsiness and dizziness (so it's good that you would take it when you went to sleep)- these are the main side effects.
My PCP once told me to take B vitamins when I was a heavy weight lifter, he said these would help muscles to relax. I tried it and did not have muscle problems. So, maybe it worked and maybe it didn't. If you want to know if your low on B vitamins, take one and the next time or two you go pee, look at the color of your urine. If it is straw yellow or clear, you were low on B vits, if it's orange/bright yellow, you were not low on B vits. I don't know if this wil help you or not, but you might give B vitamins a try before you use the Flexaril. After having been on so many drugs for cancer, I'm a firm believer in taking as few (none) meds as possible.
Just don't get addicted. Almost any drug has the potential for addiction.