Old rule: Avoid exercising in the heat.
  New rule: Acclimatize slowly and cool your neck.
  A series of experiments shows that cooling the neck before working out in hot,   humid weather can significantly boost athletic performance. Volunteers wearing   ice-cold, strap-on neck collars could run faster on a treadmill in 87-degree   heat than when they weren’t wearing the collars. You can get similar results   by dipping a handkerchief in ice water and draping it around your neck.

Old rule: Drink before you’re thirsty.
  New rule: Drink up, but avoid fluid overload.
  For years, sports nutritionists recommended “drinking ahead of thirst”   to avoid dehydration. However, recent studies show that slight dehydration doesn’t   hurt athletic performance or health.,A study by the Sports Science Institute   of South Africa compared runners who did three two-hour workouts, in which they   either quaffed a sports beverage according to thirst (about 13 oz. per hour),   at a moderate timed rate (about 4 oz. every 15 to 20 minutes) and at a high   rate (about 10 oz every 15 to 20 minutes). There were no significant differences   in core body temperature or finishing time.

Old rule: Cardio burns the most calories.
  New rule: Weight training is a better fat-burner.
  A recent University of Southern Maine study found that 30 minutes of weight   training torches as many calories as running at a blistering six-minute-mile   pace. Along with boosting strength and chiseling muscles, weight training revs   up your metabolism for up to 36 hours. High-intensity training is another excellent   fat-burner, with a recent review in Journal of Obesity reporting that it’s   more effective than other forms of exercise for flattening the belly, while   also improving aerobic fitness.

Old rule: Work out longer and faster to boost your health.
  New rule: Aim for 30 to 60 minutes of moderate exercise daily.
  A new study found that people who jogged 10 to 15 miles per week, at a pace   of six or seven miles per hour, significantly outlived those who ran further   and faster. The low-mileage group had 27 percent lower risk of death, compared   to non-runners, while people who logged more than 25 miles weekly at higher   speeds had no significant drop in mortality. The study analyzed the medical   records of nearly 53,000 adults over an average of 15 years.

Old rule: Subtract your age from 220 to get your maximum   heart rate.
  New rule: Women should use 206 minus 88 percent of their age.
  To burn fat and improve endurance, typically, your target heart rate should   be 60 to 80 percent of your max. However, the traditional formula is based on   research in men and can result in a max that’s too high for women, according   to a study published in Circulation. The researchers developed the new gender-specific   formula based on an analysis of about 6,000 healthy women ages 35 and older.   And while the math is a little tricky, you only need to run the numbers once   a year—on your birthday.

Old rule: Stretch before your workout to prevent injuries.
  New rule: Do a 10-minute dynamic warm-up.
  A study published in British Medical Journal found no scientific evidence to   back up the notion that stretching before a workout reduces injuries or that   stretching before and after prevents muscle soreness. Other studies show that   static stretching can actually impair athletic performance, while dynamic warm-ups—such   as pairing calisthenics (like squatting and lunging) with running drills—improve   it. Dynamic warm-ups also reduce injuries, a 2006 study found.

Old rule: Prevent or treat overuse injuries with custom   foot orthotics.
  New rule: Cheaper prefab inserts are just as good.
  Custom orthotics are a billion-dollar industry, but there’s little evidence   that they deliver more benefit. A study of military recruits found no difference   in rates of stress fractures, ankle sprains, or foot problems in those given   custom orthotics versus those who wore prefabricated inserts.