Resistance exercises are part of resistance training, which is a type of strength training. Unlike cardiovascular training, which is about increasing respiration, this kind of training focuses on muscle size and strength. This kind of exercise also relies less on gravity. It can be done using the body parts in opposition to each other or another static force--or by using weights or machines. There are two main categories of resistance exercises: isotonic in which a body part is pushing against another body part, and isometric in which the body part is holding, or resisting a force.
Resistance
Resistance is simply putting a load on a muscle, making it move against a force. That force might be external, such as a weight, or it might be internal, like another muscle in your body. When a muscle tries to move, but encounters a problem (resistance), small tears occur in the muscle, which are then rebuilt and actually overbuilt to handle the next similar encounter. Muscle tissue gets bigger and more toned.
Isometrics
This kind of exercise requires virtually no movement of the muscle and can be done with no equipment. Contracting a muscle as you sit is a form of isometrics. Pushing the palms against each other, for instance, is one isometric exercise. The associated bones are not moving. Neither are they moving, say, if you are trying to open a stuck window. The muscles are working, but it is not causing movement. Thus, this kind of exercise does not require equipment, though a doorframe or the floor may be used. Because it does not require movement, it can be done in a relative small space. These types of exercise can be performed in a chair at work, in a car or in an airplane. Isometrics also build muscles used for many everyday functions, such as carrying and holding.
Isotonics
In isotonic exercise the force does not change. In contrast to isometrics, however, there is some movement in isotonics. The muscles get shorter during the concentric phase of the exercise and lengthen during the eccentric phase. Weights provide the resistance. In the classic bicep curl, lifting the weight causes the muscle to contract and lowering it extends the muscle. The arm bones move. The weight does not change, making the exercise isotonic.
Adding Weights and Machines
Progressive overload is the theory that the amount of weight resisted is increased each session. This is the most effective type of resistance training. It can most easily be done with free weights or exercise machines. Free weights are inexpensive and, if used properly, can be safe. Machines are more costly, and take up more space, but offer more variety of movement, more fluidity of movement and there's no risk of dropping the weights on your toe.
Advantages of Resistance Exercises
Researchers at Bispebjerg Hospital have found that resistance training has clearly increased muscle mass in the elderly, who may be limited in what kinds of exercise they can do due to previous muscle degeneration, inability to stand, etc. Many resistance exercises don't require equipment so are available to lower income people or those without access to equipment because they are busy or traveling. With the isometric exercises, the chances of overloading the muscle are small. These exercises build and tone muscles and supporting bones.
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