Calisthenics are an important part of Karate training. Prior to the start of physical Karate practice preparatory calisthenics (jumbi undo) are done to loosen and stretch the muscles, joints, tendons, and ligaments. These exercises also increase the intake of oxygen and stimulate blood circulation. The heart and lungs are strengthened and do not become fatigued as easily during strenuous training. As the body is thoroughly warmed, mobility is gained and reflexes become more rapid. By performing preparatory calisthenics, the risk of injury (such as pulls, strains, sprains, and dislocations) during Karate practice is greatly reduced. Exercises should be done in a pattern starting with the lower extremities and working upward to promote circulation and to avoid putting too much stress on the heart too soon. This type of warming up also relaxes the mind and relieves tension so better concentration can be achieved in training. Preparatory calisthenics should never be too heavy or too light. Ten to twenty minutes is generally spent before hard formal training. For light training sessions, less time is required. After practice, supplementary calisthenics (seiro undo) should be performed to relax the body and gradually cool down. The pulse and respiration rate is slowed to normal and lactic acid is burned as more oxygen is carried to the muscles and circulation is improved. This process helps prevent the joints from stiffening and the muscles from cramping. Approximately five minutes should be spent on these exercises after training.