Light stretching is done anytime you need to use a motion, but can feel that you are a bit tight for the movement. This type of stretching is not for improved flexibility. It is for functional ROM to perform a movement/activity right now.
Stretching for improved flexibility should be done after a period of at least moderate intensity activity. If you have just finished an actual workout, this is a great time to stretch for the purpose of improved flexibility. For stretching in this manner, I recommend at least 10 minutes of moderate intensity activity, so including the warm up you are looking at 15 to 20 minutes of pre-stretching activity.
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