3. Using Bad Form. Don't just do the exercise; do it right, says Fabio Comana, MA, MS, certification and exam development manager for the American Council on Exercise.
Improper exercise technique not only poses a greater risk of injury to muscles and joints, it also wastes your time.
You may be thinking you're strengthening one muscle when in fact you are straining another or stressing a joint. For example, doing bicep curls with your knees hyper-extended and your back muscles shortened could do more harm to your knees and back than good to your arms.
Fitness trainers or floor assistants are on hand at most gyms to assist you with proper form. Use them. Ask for someone to walk you through the equipment, showing you proper technique with machines and free weights.