Physical exercise is important for maintaining physical fitness and can contribute to maintaining a healthy weight, regulating the digestive system, building and maintaining healthy bone density, muscle strength, and joint mobility, promoting physiological well-being, reducing surgical risks, and strengthening the immune system. Conversely, static exercise (such as weight-lifting) can cause the systolic pressure to rise significantly, albeit transiently, during the performance of the exercise. 'Dynamic' exercises such as steady running, tend to produce a lowering of the diastolic blood pressure during exercise, due to the improved blood flow. Types of exercise can also be classified as dynamic or static. Physical exercise can also include training that focuses on accuracy, agility, power, and speed. The goal is to improve the range of motion which can reduce the chance of injury. Activities such as stretching help to improve joint flexibility and keep muscles limber.
An aerobics exercise instructor motivates her class to keep up the pace in United States.