In my last post, I talked about what happens from a cardiovascular standpoint when you exercise, describing how the blood is pumped though the body. As I discussed, your heart rate increases with physical activity to supply more oxygenated blood to your muscles. The fitter you are, the more efficiently your heart can do this, allowing you to work out longer and harder. As a nice little bonus, this increased efficiency will also reduce your resting heart rate and lower your blood pressure.

But what happens within the muscles themselves as you exercise? Depending on what you are doing, different muscles are required to move your body. These muscles contract to move your skeletal system around. This muscle contraction is made possible by ATP (Adenosine Triphosphate) molecules. Your muscles use billions of ATP every second. This ATP molecule is found in all living cells. The energy in ATP that your muscles use to contract is obtained from breaking down the foods that we eat. Several energy sources can be used to power the production of ATP (Creatine Phosphate, Fat, Carbohydrate and Protein). Depending on what type of exercise you are doing (aerobic or anaerobic), a different source may be used to produce ATP. These energy systems do not work independently of one another. From very short, very intense exercise, to very easy, prolonged workouts, all energy systems make a contribution.

Our muscles are made up of three different types of fibers (slow twitch, fast twitch A and fast twitch B). The ratio of these fibers varies greatly from person to person. The slow twitch muscles are more efficient at using oxygen to generate more fuel for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run or bicycle for hours. Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. This is why bodybuilders look different than marathon runners. They are both strong, but their ratio of muscle fibers is very different.

Slow Twitch vs. Fast Twitch

So how do you get stronger or faster muscles? The enlargement of a muscle due to an increase in the size of muscle cells – particularly the muscle’s fibers – is known as hypertrophy. Muscle hypertrophy is something that happens as a physical response to damaged muscle. When you stress your muscles, you break muscle fibers during the process. This damage is the cause of the soreness that you feel the day or two following a workout. As the body heals these muscle fibers, it triggers a process which causes the new fibers to be larger than the ones they are replacing. This increase in size makes the fibers stronger so that future activity will have less ability to do damage to them. Each fiber becomes larger or thicker. The long-term increase in muscle size is referred to as chronic hypertrophy. Short-term or transient hypertrophy refers to the pump you get while you are exercising or lifting (thus the need to flex in the mirror or take a selfie at the gym).

While the research isn’t conclusive, the general consensus is that consistent anaerobic strength training will produce hypertrophy over the long term, in addition to its effects on muscular strength and endurance (yes, anaerobic training can improve endurance). Muscular hypertrophy can be increased through strength training (weights) and other short-duration, high-intensity anaerobic exercises (sprints, hill repeats, etc.). Lower-intensity, longer-duration exercise increases stamina and oxygen supply, allowing the body to burn energy for longer periods of time. Slow-twitch muscle fibers have a relatively poor ability to get bigger, but as a result of repeated aerobic workouts over time, they do increase in size very slightly. The bigger gains are with their work capacity.

So there you go. This is what happens to your muscles while you exercise or lift weights. It’s important to remember that without regular stress, your muscles will get smaller and weaker. This can occur in as little as 72 hours. The fitter the muscles are, the slower the loss. That’s why it’s important to try and stay active. Even if it’s just doing some body-weight exercises if you can’t complete your normal exercise or lifting routine.

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