In training for BJJ I have noticed a few common misconceptions that many students and gym's fall victim to; Partially because of "old-school" mind sets but also because there is so much misinformation floating around. Warm-Ups: A warm up is supposed to do just that- Get you warm. By getting blood flowing to "cold" muscles one's risk of injury is greatly reduced. The risk of joint injuries are also reduced as the begin to produce synovial fluid, which acts as a sort of lubricant. Not to mention you begin to wake up your mind preparing it for the training ahead. Misconceptions: Many gym's have intense warm-ups; the warm-ups themselves almost become a workout which is not what they are intended to be. If intense physical exertion occurs right away when the body is cold you not only risk injury but also your Cognitive ability begins to suffer. This is why most professional athletes do not train strength before agility ( most don't even train the two skills on the same day). In short, if your warm up is crazy hard and it leaves you feeling shaky or exhausted your actual technique lesson will suffer as your body is not in the proper state to learn. Stretching: stretching is important for increasing flexibility and getting the muscle fibers ready for the real work to come; thus reducing injury etc. etc. Misconceptions: Doing static stretches ( Holding stretches) before a work out or training session- This is wrong as the muscles will loose elasticity thus decreasing explosiveness and also risking injury by deep stretching a "cold" muscle. However, not all stretching prior to exercise is wrong- Dynamic stretching: movements which mimic motions that may occur during training will positively elongate the muscle fibers without risking injury or greatly decreasing elasticity). For example controlled leg forward leg swings or arm circles lunges etc. are great ways to prepare the body for the coming workout. ( note: Balistic stretching: Bouncing movements are not the same as dynamic stretches, they are very dated and put the user at high risk for injury). In short: Do not hold stretches or deep stretch prior to a workout or training session, instead do dynamic stretches which bring the muscles and joints through a full range of motion. After training holds are fine as they reduce lactic acid thus aiding in recovery. Long post I know but many people do not understand these principles. Hope this sheds some light for some students or teachers who may be unsure or confused!