You use more effort to move more body mass.
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Body weight is one of the many factors that affect your running times. Even a slight change in weight, higher or lower, fat or muscle, will alter the pace of your runs. Weight is not the only variable for speed, so do not fret if your weight is not where you want it to be. Your physical health and aerobic capacity may make up the difference.
A Massive Task
The more you weigh, the more energy your body requires to move that weight. This process also requires more time. In contrast then, the less you weigh, the easier it is to move which results in a faster run. According to the book, "Body Composition and Physical Performance: Applications for the Military," a 10 percent increase in body weight results in a 9 percent decrease in acceleration speed. If your weight increases by 20 percent, you can anticipate a 17 percent decrease in acceleration speed.
One reason that weight affects running speeds is in relation to gravity. Each time you raise your leg off the ground to step forward and run, you pull against gravity. This increases in difficulty with the amount of weight. Your body requires more energy to raise a heavier leg than it does a lighter leg. To compensate for extra weight, your running stride may shorten to support a heavier mid-section and raise your body off the ground. When this occurs, your run time is lengthened.
Lighter body weights do not always mean faster running times. If you are light weight, but do not have the aerobic endurance, good running technique or the ability to bring in enough oxygen, your time will differ. However, even small amounts of weight loss may affect your running time over long distances. For example, Runner's World suggests that a 10-pound weight loss improves your mile time by 20 seconds. Over a 5K course, you gain approximately one minute.
Up for Debate
Excess body weight does not always mean a slower run time, but excess body fat does show adverse affects on running speeds. If you have reached a speed plateau and are not within a healthy weight range, consider altering your workout to lose weight and improve your endurance. For example, instead of running at a steady pace, add speed intervals by jogging for one minute and sprinting for 20 seconds. Combine your running workouts with a healthy diet that is not too high in calories to keep your weight in a healthy range.