Why it's bad to stretch before training:

It's common practice to stretch your muscles before you train. I'm not talking about "warming-up" by jogging or jumping rope, I'm talking about passively stretching. You've probably heard that stretching prevents injuries by "preparing" your muscles for hard work. Intuitively, it makes sense, but research is mounting to refute the notion that stretching before a workout provides any real benefit. In fact, passive stretching before a workout might impede your performance.

Flexibility training, or passive stretching, is relaxing to your muscles. That's great for recovery, but it's also relaxing to your nervous system. For example, you'll temporarily lose strength in your hamstrings if you passively stretch them by weakening the signal between your nerve endings and muscles.

You of course know that you want the nervous system to work at its peak while you're training in order to recruit as many motor units as possible. By reducing the nervous system transmission between your motor neurons and muscles, you won't be able to produce as much force as if you skipped the passive stretching.

After a workout, however, it's beneficial to relax your muscles and nervous system since it was revved up during your workout. When your nervous system is stimulated, it's very difficult, if not impossible, to get your muscles to the point of relaxation that's necessary for recovery.

This is why you typically get a stiff neck when you're stressed. Therefore, I recommend flexibility training, or passive stretching, at the end of your workouts for two reasons. First, it'll help your muscles and nerves relax. Second, it's a time when a temporary loss in strength isn't detrimental to your performance.
 

Mobility training, if performed correctly, can enhance your nervous system and prepare your joints for a workout. When you actively move your muscles through a full range of motion, you stimulate the motor nerves that innervate your muscles, thus allowing you to recruit more motor units.

If you've ever watched Olympic swimmers right before they dive into the water for a race you'll notice that they do big, fast arm circles to prepare their shoulders. This is a smart move. By actively moving their shoulder joints through a full range of motion, they're enhancing the neural drive to their muscles and stirring up synovial fluid that reduces joint friction.

This information doesn't end with the nervous system and joint fluid, though. Remember that fascia that I was referring to? Well, flexibility and mobility training help keep that fascia supple. By doing so, you'll receive three big benefits. First, you'll be able to train your exercises through a greater range of motion.

Second, you'll potentially reduce your risk for joint injuries. When one area of your body is stiff and restricted, another area has to compensate. Compensation is a big set-up for injury since it makes a specific area function differently than it was designed.

Third, you'll give your muscles room to grow by "unbinding" them.