Exercise bikes are used for exercise, to increase general fitness, for weight loss, and for training for cycle events. The exercise bike has long been used for physical therapy because of the low-impact, safe, and effective cardiovascular exercise it provides. The low-impact movement involved in operating an exercise bike does not put much stress on joints and does not involve sporadic motions that some other fitness equipment may require.[citation needed] However, as with typical biking. Stationary bikes are also used for physical testing. Traditionally this is done by imposing a certain level of resistance mechanically and/or measuring this gives a good overview. Modern ergometers and even many consumer exercise bikes are fitted with electronic sensors and displays. Some models feature handlebars that are connected to the pedals so that the upper body can be exercised along with the lower body. Most exercise bikes provide a mechanism for applying resistance to the pedals which increases the intensity of the exercise. Resistance mechanisms include magnets, fans, and friction mechanisms. Some models allow the user to pedal backwards to exercise antagonist muscles which are not exercised in forward pedaling. Exercise bicycles are typically manufactured using a crankshaft and bottom bracket, which turns a flywheel by means of a belt or chain.