Fitness equipment is a large group of tools that are used in exercise and are referred to also as exercise equipment. These can include simple items such as balls, track shoes and skipping ropes as well as more complex pieces of equipment such as treadmills, weights, bicycles and many others. Today a wide range of fitness equipment is available for sale with new designs constantly being produced due to the high demand for equipment that is effective in building muscles and strength as well as for losing weight. Some of the types of fitness equipment available for use can be classified according the type of exercise for which they are built. The first class to be discussed is the strength training equipment. These include free weights and machines referred to as the resistance machines. The most common type of resistance machine is one that uses gravity as a resistance source. This design is used mainly in weight machines. Gravity in combination with a simple design of pulleys, wheels and inclines, uses a percentage of the weight of the user as the resistance transferred. Other types of resistance machines can use other forces other than gravity to provide resistance. These include friction, spring-loaded, fan-loaded or fluid-loaded machines. Another type of equipment belonging to the strength training equipment is the bands and tubing, which are lightweight options for training at home and in the office. The other class is the aerobic fitness equipment which is mainly the endless-path machines. These include devices such as the stationary bicycles, the treadmills, the elliptical trainers, aerobic riders, the stair climbers, the ski and rowing machines. The stationary bicycle focuses the exercise on the legs by the action of pedaling. The treadmills are an all body exercise as you may walk or run at various speeds. There are also some models that can simulate slopes for a more intense workout. The stair climbers replicate the action of climbing a stairway. This device now also has modifications that replicate the pulling up action off the hands for a more through workout. The skiing and rowing machines work both the arms and legs simultaneously with the rowing machine also working the back muscles. The elliptical trainers are a cross between the stair climbers and the ski machines. The aerobic riders also exercise the arms and legs at the same time by the use of a simultaneous pull and push motion. There is also a miscellaneous group of fitness equipment. These include items such as heart rate monitors and exercise balls as well as many others. The heart rate monitors are used as a motivational tool in exercise as they monitor the heart rate and provide feedback to the individual exercising. The use of exercise balls has become very important in training such as flexibility training and for balance as well. When using any type of fitness equipment it is always important to seek professional advice as the improper use of this equipment can cause severe damage.