Free weights versus weight machines
Exercise balls allow a wider range of free weight exercises to be performed. They are also known as Swiss balls, stability balls, fitness balls, gym balls, sports balls, therapy balls or body balls. They are sometimes confused with medicine balls.
Free weights include dumbbells, barbells, medicine balls, sandbells, and kettlebells. Unlike weight machines, they do not constrain users to specific, fixed movements, and therefore require more effort from the individual's stabilizer muscles. It is often argued that free weight exercises are superior for precisely this reason. For example, they are recommended for golf players, since golf is a unilateral exercise that can break body balances, requiring exercises to keep the balance in muscles.
The weight stack from a cable machine.
Some free weight exercises can be performed while sitting or lying on an exercise ball.
There are a number of weight machines that are commonly found in neighborhood gyms. The Smith machine is a barbell that is constrained to vertical movement. The cable machine consists of two weight stacks separated by 2.5 metres, with cables running through adjustable pulleys (that can be fixed at any height so as to select different amounts of weight) to various types of handles. There are also exercise-specific weight machines such as the leg press. A multigym includes a variety of exercise-specific mechanisms in one apparatus.
One limitation of many free weight exercises and exercise machines is that the muscle is working maximally against gravity during only a small portion of the lift. Some exercise-specific machines feature an oval cam (first introduced by Nautilus) which varies the resistance, so that the resistance, and the muscle force required, remains constant throughout the full range of motion of the exercise.