Intensity and duration are two important elements of fitness programming that answer the questions "how hard?" and "how long?" To a large extent, your training goals will determine the answers to those questions. But health considerations and your schedule may also come into play.
Endurance versus Intensity
Deciding whether to work harder or longer depends on your goals and your current fitness level. If you are sedentary and overweight or obese, long-duration moderate-intensity exercise may help you lose weight. A study of 201 overweight women published in the "Clinical Journal of Sports Medicine" found that longer duration exercise at a moderate intensity had a more profound effect on weight loss than training for less time at a higher intensity. However, a 10-year longitudinal study of more than 10,000 male and female subjects published in "BMJ Open" found that training at higher intensities was more effective than increased volume for reducing the risk of metabolic disease.
Endurance training is characterized by rhythmic large muscle activity performed for an extended duration of time. According to the IDEA Health and Fitness Association, the standard approach to improving cardiovascular fitness is to add greater volume to your training regimen by increasing the amount of time spent jogging, biking, walking or doing other rhythmic steady-state activity. The American College of Sports Medicine recommends a minimum of 150 minutes weekly of moderate-intensity aerobic activity for healthy adults, and 250 minutes or more for adults who want to lose weight.
Increasing your exercise intensity may help you realize health benefits beyond those offered by traditional endurance training. In an analysis of the benefits of high-intensity interval training, the IDEA Health and Fitness Association listed increases in mitochondria, the powerhouses of muscle contraction; increased fatty acid oxidation; elevated post-exercise oxygen consumption, meaning a prolonged caloric burn after the cessation of exercise; an increase in the number of oxidative enzymes that facilitate muscle contraction; and improved cardiovascular fitness similar to or exceeding improvements from endurance training.
Factors to Consider
In addition to your fitness goals, other factors to consider when deciding on an exercise program include how much time you are willing or able to spend, your current fitness level and health status, and your access to exercise equipment or space to exercise. If you are busy and time-challenged, exercising for less time at a higher intensity may be a good option. If you are sedentary with a low fitness level, starting out with moderate-intensity endurance training can help you lose weight and increase your fitness level so you can graduate to more intense training.