It is not a traditional recognition that do not stretch before work-out.
The main concept is that it is like extending a rubber ban to its limit so when people stretch to the maximum, they are more likely to pull a muscle.
So it would be good to do exercise after a workout or at the end of the day.
http://www.usatoday.com/news/health/2010-04-15-stretching-muscles_N.htm
