You don't need to be a scientist to know that fresh air and open spaces are preferable to artificial lighting and air-conditioning, but proponents of the "biophilia theory" believe contact with the natural world is not just beneficial to our wellbeing - it's essential. "Study after study shows that people feel better both emotionally and physically if they look at, or experience, certain types of natural setting," says biophilia enthusiast Roger Ulrich, a professor at Texas A&M University. "There's quicker recovery from stress, reduction in blood pressure, people's muscles relax and even the brain's electrical waves alter."
It's more engaging
Eye-candy apart, there's not much chance of the scenery changing when you exercise in a gym. "Outdoors, not only is there fresh air, but there are lots of things to look at and to occupy your mind," says Jeremy Wormington, operations manager for British Military Fitness, an organisation that runs outdoor fitness sessions at 22 venues nationwide (britmilfit.com). "Even in the same park, or running the same route, there is always something new to catch your eye or imagination. Being out in the real world keeps all your senses alive and takes your mind off the 'how much longer do I need to do this?' factor." Recent research on the Green Gym initiative (greengym.org.uk), in which volunteers do outdoor conservation work, concluded that it was a viable alternative to gyms and sports centres, improving participants' physical fitness and mental wellbeing.
It won't make you sick
"Colds spread in crowded, indoor areas and via contaminated surfaces," says Professor Ron Eccles, director of the Common Cold Research Centre. What better breeding ground than a gym packed with sweaty, scantily-clad people handling everything from dumbbells to weight pegs, locker keys to buttons on cardio equipment? Air-conditioning can also increase the chance of catching a cold because it extracts moisture from the air, drying the protective mucous layer in the nose and predisposing you to infection. Athlete's foot, E coli, strep-bacteria and the influenza virus have all been identified in gyms.
It burns more calories
You'll get a tougher workout outdoors compared with a similar one indoors. "Softer surfaces demand greater extension at the hip, knee and ankle, increasing energy demand," says Keith Anderson, director of TrailPlus (trailplus.com), which operates trail running and adventure training camps in the Forest of Dean. "The softer, or soggier, the surface, the greater the increase in effort." Research in Belgium found that running on sand requires 1.6 times more energy expenditure than on a hard surface. Other research found that workouts in cold weather increase calorie expenditure by up to 12%, because the body has to work harder to maintain core temperature.
Outdoor exercise tends to mimic more what we do in daily life, making it "functional". "On the treadmill, you are just bouncing up and down on a stable, shock-absorbable surface," says Jason Anderson, movement and injury specialist with GIM training. "Running outside, when the foot hits the floor it is forced to adapt to the surface, which alters the relative position of the rest of the body, effectively meaning you never train in the same position twice." Whether you are walking, cycling or running, the outdoors offers an unpredictable environment - uneven terrain, unexpected obstacles, varied surfaces. This helps to build skills such as balance and faster reflexes.
It reduces stress
A US survey found 70% of people rate outdoor activities more effective than indoor activities at reducing stress. Researchers are still investigating why being outdoors makes us feel so good, but it may be at least partly due to the greater abundance of negative ions in the air in natural surroundings (especially around running water, mountains and forest). These make us feel alert and energised, and strengthen the immune system, while positive ions, the kind that linger in built-up and polluted areas, can make us feel tired.
It makes you smile
Sunlight is our primary source of vitamin D (it is synthesised in the body after exposure to UV rays). "This fat-soluble vitamin is well known for its role in preserving bone health," says Pete Williams, founder of health clinic Health Dept. "Recent research also suggests it may play a part in maintaining a healthy immune system and even in cancer prevention." Williams uses light therapy in some clients' exercise sessions: "It helps facilitate production of vitamin D in those who don't get enough sun exposure." Ten to 15 minutes of sun exposure at least twice a week to the face, arms, hands or back without sunscreen is usually enough to provide adequate vitamin D, according to a 2002 report in the journal Current Opinion In Endocrinology And Diabetes.