Ignoring the effect of gravity in creating resistance during all movements, free weights (dumbbells) keep the resistance on the muscle constant throughout the joint’s range of motion (ROM), while weight machines use variable resistance, with the resistance changing throughout the ROM. Machines have geometrically shaped cams that change the torque required of the muscles by changing the lever arm of the resistance force (external weight) or the applied muscular force. Thus, machines place more stress on the muscles at the angles at which muscles can produce greater force. Since there are points in a joint’s ROM where the muscle is stronger and points where it is weaker, and the amount of weight your clients can lift is limited by their weakest point, free weights serve only as a strong enough training stimulus for the weak joint positions. With machines, the load changes to provide optimal resistance throughout the entire ROM.
On the other hand, movements using free weights occur in a three-dimensional plane, while most weight machines allow movement only in a single plane. With machines, the movement is guided, so only the major muscles required to perform the movement are used. With free weights, the added task of balancing the weights in the three-dimensional plane recruits other functional muscles that machines do not recruit.
Clients new to weightlifting should probably begin with machines to train the major muscles, and then use free weights to train more specific movements.