Q: What's the difference between using machines and free weights? Do I need them both?
A: Yes, ideally, you should use both. "Most weight machines support your body to help isolate a muscle group and/or ensure you keep proper form," says Katie Krall, a certified trainer in Colorado Springs, Colo. "Free weights -- such as dumbbells and barbells -- force you to use additional muscles to help stabilize your body." Some "hybrid" machines, such as those by FreeMotion, utilize cables for resistance and eliminate much of the support, although they still guide your movement to a certain degree.
There's no hard-and-fast rule about when to use machines or dumbbells, but here are some guidelines: If you're a beginner, start with machines and add free-weight and cable moves as you get more familiar with the exercise. If you've been strength training consistently for at least three months, use machines for exercises that involve heavier weight -- like squats and chest presses -- or to help you learn proper form when you try a new exercise for the first time.