Increased muscle with increased mileage?


Over the past year-and-a-half or so I have increased the amount of miles I run for a workout from 1 to 6. I know that muscle weighs more than fat, and since I'm quite skinny to begin with, any weight gain is likely due to an increase in my body's muscle mass. However, I've also gone up in pants size, and I know I haven't been eating more than usual (I'm a healthy eater, and I watch what I eat). Could this be because my quadriceps and the muscles around my upper thigh and glute area have gotten bulkier? What are typical physiological changes that women runners tend to go through as they increase their mileage?


Dear Reader,

Yes, your pants size has probably increased as a result of INCREASED MUSCLE MASS in your legs from your longer runs. Upping your running mileage creates physiologic changes that support endurance. As you increase mileage, your legs build up more muscle mass to tolerate the increased work they need to do. When you feel sore after a run, it indicates that a small amount of injury occurred to certain muscle groups. Our bodies respond by repairing the area of injury and building more muscle mass. This increases endurance and overall strength. Muscle mass is enhanced in several ways via these physiologic adaptations:


Increased capillaries
Capillaries, the body's smallest blood vessels, bring oxygen and fuels in, and waste products, such as carbon dioxide, out of the cells. Longer runs increase the number of capillaries per muscle fiber, improving this delivery and removal system.


More myobglobin
Myoglobin carries oxygen from the cell membrane of the muscle cell to the mitochondria for energy production. Increasing running mileage results in higher myoglobin content in muscle fibers, with more oxygen delivered to those fibers.


Mighty mitochondria
Mitochondria, the aerobic energy centers in your cells, increase in number and size in your muscle fibers as you increase your runs. Enzymes in the mitochondria speed up aerobic energy production, increasing enzyme activity and helping maintain a faster pace while running.


Glycogen storage
Glycogen is the storage form of carbohydrate. As your mileage increases, more and more stored glycogen is used. Glycogen depletion stimulates muscles to store more glycogen.

As you can see, these physiologic changes, as a result of an increase in running mileage, support your leg muscles in becoming bigger and stronger. You don't have to worry about your leg muscles growing exponentially with each added mile in your run because the hormone estrogen prevents muscles from getting too big or too bulky. Rather, your leg muscles will become lean mean running machines! In addition to these physiologic changes, your body also uses fat as an energy source at a higher rate than carbohydrates and glycogen. The type of muscle fibers in the legs is also transformed. Slow-twitch fibers in muscle help in endurance because they have a higher proportion of the adaptations listed above. Longer runs give your fast-twitch muscle fibers, usually reserved for sprinting or power actions, more of the characteristics of slow twitch fibers. So, feel good about your new pants size. Your legs will take you where you want to go!

Last updated May 13, 2015
Originally published Aug 12, 2005

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