Do you normally do stretching exercises before a workout? That’s a NO-No because it’s like pulling a cold taffy when you do — it can break down muscle fibers and reduce their strength capacity. Stretching should be down between sets or after doing a cardio routine. Studies show that people who used this approach saw a 32 percent boost in strength and a 29 percent improvement in muscular endurance. The theory behind this, according to experts is that once the muscles are warmed up, stretching increases blood flow to the area, making the fibers more responsive to activity.