I find outdoor exercise to be more enjoyable. First of all, you've got better visual stimulation - particularly when you're exercising in pleasant surroundings, such as in a park or in the countryside. You also get the benefit of fresh air and sunshine, which always makes me feel more cheerful and energetic than working out indoors. And exercising in the gym can expose you to a number of negatives: stuffy air, loud noise, and competitive vibes.
You might be interested to know that researchers have asked this very question - I've found a few scientific studies looking at the pros and cons of exercising outdoors versus in the gym. One compared performances of the 1,800 meter run done outdoors on a wooded track with runs performed indoors on treadmills. Results showed that the participants ran faster on the outdoor course. Another study found that athletes' heart rates were higher during an outdoor run than when they ran the same distance on a treadmill. A 1995 Australian study found that trained runners felt less anxious, depressed, angry, and hostile, and more invigorated after an outdoor run than they did when they ran the same distance on a treadmill.
Some time ago, I came across an interesting article about exercise motivation by a group of researchers led by John P. Porcari, Ph.D., a professor in the Department of Exercise and Sports Science at the University of Wisconsin, in La Crosse. His team had been testing virtual reality (VR) exercise machines as a means of keeping people motivated to exercise. These machines were equipped with interactive CD-ROM systems to create a computer-generated virtual environment. For example, while exercising on a VR step machine, you could "pilot" a WWI biplane equipped with water cannons and try to shoot down hot air balloons and destroy enemy sites to earn points. The faster you step, the faster your "plane" flies, and the more points you earn. The VR recumbent bike gave users a number of options, including cycling through the Alps or crossing a Caribbean island to the sounds of island music and exotic birdsongs. Not surprisingly, the volunteers who took part in testing the VR exercise equipment preferred the interactive VR equipment to non-VR devices.
In Porcari's view, outdoor exercise is preferable to working out indoors because of the distraction and the visual stimulation. You're focused on your surroundings, and if you're enjoying yourself, time flies and you feel more invigorated and less stressed or tired than you would if you did the same amount of exercise indoors. So take advantage of the warm weather months to exercise outside. Just be sure to apply sunscreen first, and take along a bottle of water to replace fluids you lose when you perspire.
Andrew Weil, M.D.