Stretching
To maintain your strength and increase flexibility, do your stretching at the end of your workout, after your cool down. Your muscles will still be warm, making them more elastic and allowing them to stretch further than if they were cold.
Stretching before strength training can negatively affect strength. You can test this out on yourself by jumping as high as you can and touching a spot on the wall. Stretch your legs; then jump again and try to touch the same spot on the wall. You may find that you won’t be able to. Some people also like to stretch for a long period of time before beginning their strength training workout. This will slow the heart rate and drop the temperature of the muscles. If you really want to do some long stretching before your strength training, you may want to do a second warm-up to be properly prepared for your workout.
I only recommend stretching before strength training if your muscles are so tight that you can’t properly perform your exercises through a full range of motion. An example is when you perform a squat, but have difficulty getting down to 90 degrees without rounding your lower back because of poor lower-back and/or hamstring flexibility. If, after stretching, you still don’t have an adequate flexibility to perform the exercise properly, either limit your range of motion or choose another exercise for that muscle group.