Forgetting to stretch after a workout, if stretching after is what you normally do, is probably a lot less of a problem than failing to stretch before a workout, if that's what you normally do. Here's why: What matters more than when you stretch is being consistent about when you stretch, because your muscles get used to whether or not you stress them in that way before doing something else.
So, to clarify, there's no significant difference in likelihood of injury between those who stretch before running and those who stretch after. The danger increases, however, if someone normally does one and then tries to switch to the other. The body is unprepared for the unusual activity or lack thereof before the run, and injury can result.
That being said, my opinion is that, if you usually stretch before you work out, failing to do so can have quite an impact on what happens while you work out and how you feel after. However, if you normally stretch after you work out, the activity it could've impacted is over if you forget or don't have time to stretch, so the biggest problem you'll have is probably that you'll be disappointed that you missed out on a part of your routine. But if you stretch regularly, missing every once in a while won't likely cause you any harm.
One way to make sure you stretch is to do it in between sets of whatever strength training you're doing. That way, you always make time, and you make sure to stretch whatever muscle you're strengthening.
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