Deconditioned athletes may feel the burn at 50 to 60 percent of their maximum heart rate. A highly trained athlete can raise the threshold to 80 or 90 percent of maximum heart rate. However, your ultimate ability to delay muscle burning is determined by factors other than training. Your age, gender, and genetics all have an impact. For example, older individuals tend to have less tolerant muscles that burn more slowly after exercise. In general, younger individuals can tolerate high levels of exertion for longer periods of time before becoming exhausted. In contrast, adults over 40 years old can endure exercise for only about 30 minutes without rest. The reason why aging bodies require more recovery time is because they no longer produce new cells as quickly as they destroy them. As such, the body is left with less energy reserves to fuel exercise at later stages in life. There are several ways you can increase the duration of your workouts. You can increase the intensity level of your exercise program, which will help your body become used to dealing with greater amounts of stress on a daily basis. You can also adjust how you eat before a workout to provide your body with the necessary nutrients it needs to function at its best. For example, eating foods containing protein and carbohydrates shortly before exercising helps your body build up resistance to fatigue during exercise. Last, but not least, you should try to relax and have fun while working out.