The issue: "Functional fitness" exercises that mimic the moves we do every day strengthen the muscles used most often and help us avoid injury. Weight machines, on the other hand, usually focus on one muscle group at a time, and they don't replicate the way we move in the course of our daily lives, says Vonda Wright, MD, a Pittsburgh-based orthopedic surgeon who specializes in injury prevention and mobility. (When's the last time you laid down on your back and pushed a heavy object up with the soles of your feet, like you would on a leg-press machine?) They're not a the most efficient way to build muscle—for example, you'd use three different machines (leg press, back extension, overhead press) to work the same areas you would with one total-body dumbbell move (squat and overhead press), and you could still get injured. What's more, these machines are usually designed to fit men's bodies, which tend to have a longer limbs and an extended reach.
The fix: For maximum efficiency, try total-body exercises that involve squatting, bending, lunging and reaching. Wright suggests cardio-strength moves like walking lunges with a weight ball, squat jumps and wood-choppers. Or alternate your machine workouts with boot camp classes that focus on strength-building moves.