Recent researches have broken the age-old myth of prior warm up before going ahead with workout sessions. However the exact reasons are still unknown.

Recently, two studies were assessed to understand the actual reasons for evading stretching exercises. A study published in the Journal of Strength and Conditioning Research highlighted that you end up feeling lethargic and fatigued by doing stretching workouts before weight regimes.
Another study from Croatia that was published in The Scandinavian Journal of Medicine and Science in Sports also expressed similar views.
The conclusion drawn from both the studies was that static or stretching exercises are quite irrelevant and unnecessary before starting a rigorous workout regime.
In a study conducted at the University of Zagreb, a number of volunteers did stretching exercises and then did heavy workouts such as jumping, weight lifting and dunking.
The Croatian scientists included warm-up exercise without jogging or any other active warm up.
Another study, on the other hand, supported warm up exercises before any workout regime. Dr. Goran Makovic, the professor of kinesiology at the University of Zagreb, favored warm up before doing heavy workout sessions to enhance performance and reduce the risk of injury.
Source-Medindia