A 1981 study demonstrates the effect that our mental-emotional state can have on the outcome of exercise. Previous studies found, for instance, that anger and fear both raise heart rate and blood pressure while at rest, but didn't show what happened during exercise.
The study itself involved college-level acting students, who would be relatively comfortable accessing different emotional states and displaying them in public. They accessed different states through visualization. The mind-body connection's power is clearly seen: in all cases the visualization produced physical changes. Before beginning their exercise, the subjects all had elevated heart rate and blood pressure levels while experiencing anger, fear, and happiness. Their heart rate and blood pressure were lower in both relaxed and sad states. All of the test subjects then did the same exercise sequence (stair-climbing) in each of the emotional states.
There are a few interesting observations corresponding with different emotions, and they are relevant to our ability to access sympathetic and parasympathetic states. (Quick review: sympathetic is the fight or flight response to stress that mobilizes our body for immediate action, and parasympathetic is the relaxation response in which our body recovers, repairs, and regenerates). Performing the exercise sequence in a relaxed state, the subjects saw an increase in heart rate and blood pressure that seemed relatively appropriate for exercise. We do need an increase in both of these markers, after all, to get blood to our working muscles. In fearful, angry, and happy states, heart rate and blood pressure rose much higher, indicating an exaggerated sympathetic state. In a sad state the subjects actually had too little increase in heart rate and blood pressure, indicating an overly parasympathetic state in which they could not mobilize their energy stores at all.
The takeaway here is that there is an optimal level of emotional-intellectual stimulation during exercise. If we want the long-term health benefits of exercise, then we should use our training as an opportunity to learn to handle more and more stimulation without having to dip into our sympathetic reserves. Under real stress, our bodies release adrenaline and other stress hormones to help us deal with the situation at hand. And when we have genuinely stressful situations, that is a good thing. The problem is that most people in our society are dipping into their adrenaline stores too often, which is the cause of chronic stress-related diseases. Proper training can actually help them learn to stay more relaxed and only go to their sympathetic reserves when absolutely necessary, which will have a dramatic effect on long-term health.