Fitness Flaw #1: You Are Married to the Machine
Don't rely solely on weight machines because typically you can work more muscles and build greater practical strength with free weights…or even no weights.

The Fix:
To get the most benefit from each workout try to perform a mix of all kinds of exercises—ones using machines, body weight, free weights and dumbbells. Jay Dawes, instructor of kinesiology and health studies at the University of Central Oklahoma, suggests including two to three sets of a move like the lunge-and-press several times a week.

To do the lunge-and-press, hold a dumbbell in each hand at shoulder height with your arms bent close to sides and palms in. Keep your feet hip-width apart with one foot about a stride ahead of the other and your back heel lifted. With your abs contracted, bend your knees so the front knee aligns with your front ankle and the back knee points toward the ground. At the same time, straighten your arms to press the dumbbells overhead. Straighten your legs and lower the dumbbells to the starting position, and repeat for 12 to 15 repetitions.

Fitness Flaw #2: You Speed Through Your Workouts
Moving quickly through your sets could also be a sign you're misusing momentum, executing the exercise incorrectly, or using too much or too little weight.

The Fix:
With any strength move, go at a slow, controlled pace in the lifting and lowering phases—two seconds up, two seconds down. "There should be no point at which you pick up speed," says California trainer Jay Blahnik, author of Full-Body Flexibility. With crunches, hold dumbbells or a medicine ball on your chest to slow you down. For lunges, hold a dumbbell in each hand as you perform your reps or try doing them with your front foot on a Bosu balance trainer.

Fitness Flaw #3: You Use the Same Setting He/She Just Did
You wouldn't drive without adjusting the seat and mirrors, so don't do sets on exercise machines in somebody else's seat settings.

The Fix:
In general, the rule is this: "You should be able to go through the full range of motion and maintain good posture with back and feet well supported," says Cedric X. Bryant, Phd, chief exercise physiologist and vice president of educational services for the American Council on Exercise. For example, with the leg extension, make sure your thighs are in the center and the backs of your knees just about touch the front of the seat. With any upright machine, put your head, back and hips in contact with the back pads.

Fitness Flaw #4: The First Thing You Do Is Stretch
For years people thought pre-workout stretching was essential to prevent injury. But cold muscles are like uncooked noodles…they need to be heated up before they can move freely.

The Fix:
To warm up before a workout, skip stretching and do 5 to 10 minutes of cardio or a lighter, easier version of whatever movements you do in the workout itself. Afterwards, do a regular stretching routine that hits the major areas—back, hamstrings, calves, shoulders, hips, neck and feet.
As a reminder, always consult your doctor for medical advice and treatment before starting any program.