That is another issue. I always spend time warming up the old body before class. Sometimes I feel like the tin-man from the wizard of oz. "OIL-CAN,Oil-can, oil-can." I realize that if I don't stretch out before we begin, I will break! But my darling daughter never stretches out. I encourage her to but she says she doesn't need to. Oh the wonders of youth
Modern sports medicine shows there is NO benefit from stretching before working out and, in the case of cold muscles, stretching may make them more prone to micro-tears. It is recommended that the pre-workout regimen be general warming up (slightly raising heartbeat while doing gross muscle movement - like what rehab nurses and PTs call range of motion) followed by additional movement training related to the sport/activity one will be doing (so will vary) - for those of us doing aikido, for example, 10 minutes of the warm ups from Tohei Sensei are excellent preparation of the body for doing aikido. It is generally recommended that stretching is of benefit after the conclusion of the working out.
BTW, as with the women and ACL injury research - I cannot cite specific papers because if I kept either the papers or the url links from 33 years of nursing and 17 years of aikido there would be no time or space to have a life. The info is out there for those who google.