While some sagging of the skin on your face is just part of aging, it's true that running can have a negative impact on skin elasticity. Contrary to popular belief, the effect of gravity from the constant pavement pounding isn't to blame, but rather some experts believe that "cardio and running can cause more oxygen or free-radical damage, which can break or damage the skin's supportive fibers (collagen and elastin)," says Dr. Annet King, director of training and education at Dermalogica.
But don't retire your running shoes just yet. According to a 2008 study published in Free Radical Biology and Medicine, only very strenuous exercise leads to signs of cell damage, while moderate exercise—defined as exercising at 40 to 60 percent of your maximum heart rate three to five days per week—has a rejuvenating effect on skin. And don't worry: HIIT is in the safe zone. The detrimental effects don't kick in until after 90 minutes of running at 70 to 80 percent of your maximum heart rate, the scientists say. Keep your workouts short and sweaty and you'll reap all the fat-burning and anti-aging benefits!