Chapter 5: Training for Endurance 119 When to Replace Your Running Shoes Researchers recommend that running shoes be replaced after accumulating between 350-500 miles (Cook et al., 1985). While walking is not as hard on shoes as running, it is still unlikely they are able to provide adequate support and cushioning after 500 miles. Individuals who average 30 minutes per day of walking (or 3-4 hours per week) should consider replacing their shoes every six months. Individuals who average 60 minutes per day of walking (or 7-8 hours per week) should consider replacing their shoes every three months. In addition to mileage, body weight is another important factor to consider. The more an individual weighs, the faster the support and cushioning will wear out. Individuals weighing over 200 pounds may want to consider replacing their shoe even more often than the aforementioned recommendations. Summary • Cardiovascular fitness is the ability of the heart, lungs, and organs to consume, transport, and utilize oxygen. • There are three basic energy systems simultaneously at work during exercise: the phosphagen, glycolytic, and oxidative systems. They are used to replenish levels of adenosine triphosphate (ATP), the energy used to power muscle contractions. • VO2max is the maximum amount of oxygen that the body can take in and utilize in one-minute of high-intensity exercise. Knowing what your VO2max score is and how to effectively train to improve it is important as VO2max correlates well to certain distance run times (e.g., 1.5-mile). • Lactate threshold (LT) is the point in exercise where lactate production starts to exceed lactate removal in the blood. Research has shown LT to be a better predictor of one’s cardiovascular fitness than VO2max. • Exercise economy is the amount of energy required to maintain a particular running speed or generate a specific amount of power. Although not to the same extent as VO2max and LT, exercise economy does have a significant impact on endurance performance. • High-intensity interval training (HIIT) is a type of endurance training that utilizes repeated short to long bouts of high-intensity exercise combined with short recovery periods. Research has shown that the benefits of HIIT are almost identical to that of traditional endurance training.