The researchers had them answer questions about their psychological state, and they measured their heart rate, and heart rate variability (the variation in time between heartbeats), and their levels of the stress hormone cortisol before and after the walks. Why are nature workouts so valuable for health? This doesn’t mean indoor or urban workouts are bad. De Dominicis stresses this: The findings don’t suggest that indoor or urban workouts aren't worth your time. "Exercise regularly and, if you can move some of it to the outdoor setting, you may get extra heart health and mental benefits from it," he says.