We asked 3,000 ACE-certified professionals to give us their take on the biggest mistakes they see people making in the gym during their workout. Some of these mistakes can mean the difference between an effective and an ineffective workout, while others may lead to injury.

  • NOT STRETCHING ENOUGH. The best time to stretch is immediately following an aerobic activity while your muscles are warm and pliable. Flexible muscles are far less likely to be strained or pulled than tight ones.
  • LIFTING TOO MUCH WEIGHT. The best way to guarantee yourself an injury is to try to lift more weight than your muscles can handle. Gradual, progressive resistance is a far more effective — and safe — way to increase muscle strength.
  • NOT WARMING UP PRIOR TO AEROBIC ACTIVITY. Muscles need time to adjust to the increased demands of aerobic activity. Rather than hitting the treadmill running, for example, take a few minutes to walk, build up to a light jog, and then hit your stride.
  • NOT COOLING DOWN AFTER ANY TYPE OF WORKOUT. Too many people wrap up their workouts and head straight to the showers. Instead, take a few minutes to lower your heart rate and stretch your muscles. Not only will you improve your flexibility, but you will be better prepared for your next workout as well.
  • EXERCISING TOO INTENSELY. This is the primary vice of the weekend warrior, the individual who tries to fit a week's worth of exercise into a Saturday afternoon. For weight loss, it's more effective to sustain a moderate workout for longer periods of time than to exercise intensely for only a few minutes.
  • NOT DRINKING ENOUGH WATER. This cannot be said — or heard — too many times. Unless you're exercising for more than two hours per day, water can fill all your fluid needs.
  • LEANING HEAVILY ON THE STAIRSTEPPER. Supporting your weight on the handrails while cranking up the intensity defeats your purpose and puts excessive strain on your wrists and elbows. Try lowering the intensity to the point at which you can maintain good posture while lightly resting your hands on the rails for balance.
  • NOT EXERCISING INTENSELY ENOUGH. You want results, but are you working hard enough to get them? While it's true you don't want to overdo it, you should exercise intensely enough to work up a light sweat, get your heart beating up into that training zone, and feel a sense of satisfaction for having completed your workout.
  • JERKING WHILE LIFTING WEIGHTS. This goes along with number 2 — lifting too much weight. The most effective way to train is to control the weight — the weight shouldn't control you. If you have to jerk the weight, it is probably too heavy, which can lead to strain and injury.
  • CONSUMING ENERGY BARS AND SPORTS DRINKS DURING MODERATE WORKOUTS. Unless you're working out for longer than two hours per day, you really don't need to supplement with high-energy bars and drinks. High energy is often a code word for high calorie, which is the last thing someone who is trying to reduce their body weight needs. A well-balanced diet and plenty of water are sufficient to meet most exerciser's needs.