The American College of Sports Medicine (ACE) defines five components of physical fitness: muscular strength, aerobic capacity, flexibility, and bone density. The organization does not discount the importance of nutrition when it comes to physical fitness, however. According to Jeffrey E. Oken, deputy chief of staff at the Edward Hines Jr. VA Hospital, the most important factor for maintaining health is mental health. Although many people think that diet and exercise are unrelated, they are not. Physical fitness, or health, is essential for the maintenance and development of the body's systems. It allows people to carry out their daily activities efficiently. A fit person can perform well in schoolwork, fulfill home responsibilities, and participate in leisure activities. They are also better equipped to handle emergency situations. As a result, physical fitness is essential to a healthy lifestyle. The World Health Organisation outlines four main categories of fitness. The first category is cardiorespiratory endurance, which is vital in combating heart disease, and the second category, muscular endurance, is critical for endurance sports. A healthy definition of fitness is based on how well a person performs in each of the components. Total fitness cannot be determined based on how many reps a person can bench press, but also takes into consideration endurance during long runs and distance cycling. Additionally, cardiorespiratory fitness is assessed with the VO2 max test. It is important to increase heart rate and increase oxygen consumption when a person engages in aerobic activity.