Natural fitness uses a series of free range of motion movements and body weight training.  You use your own body weight with or without resistance in order to build functional strength.  Strength you can actually use for everyday activities such as shoveling snow or cleaning the house.  Weight machines isolate your muscles and restrict your movement to the track of the machine.  Training isolated movements restricts your body from developing stability and support muscle around your larger muscle groups. Stability muscles prevent injury!  Your muscles were never meant to be isolated; they are designed to work together for overall functional strength.