Is Stretching Necessary??

This video on stretching is directed to those who lift weights in the gym, not to those who perform yoga exercise.
There are 2 types of stretching, dynamic and static. Are either necessary when you're trying to lift weights and build muscle mass? Maybe NOT!!
But, if you insist on stretching, how should these two very distinct stretching methods be applied? Before, during and/or after your weight lifting routine??? And what the heck is the body's "stretch reflex"? Why does it cause pain when you stretch??
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