Exercise makes us stronger. This we know. But how exactly does exercise make us stronger? It turns out to be a interesting story and not all of it involving dense cellular biology. I’d like to share with you a few of my favorite fun facts about what goes on in your muscles after you exercise. Keep them in mind the next time you’re in the midst of a tough workout, and you may find they give you a deeper understanding and appreciation of the experience.

Star Wars action figures smallFirst, I want to tell you about “dark muscle” and “white muscle” fibers. People have both types of these fibers in their muscles in varying proportions according to each individual.

“Dark muscle” fibers are dedicated to keeping us moving indefinitely. They’re called “slow twitch” fiber. You use this endurance-oriented-type fiber when you walk and run long distances. Our neck muscles have a lot of slow twitch fibers so that we can easily hold our head erect all day. Our deep calf muscles also have a lot of them so that we can run around from dawn to dusk if we want to. Look at “slow twitch” fibers through a microscope and they will appear dark. That’s because slow twitch fibers are filled with capillaries, giving them a rich blood flow that is a source of continuing energy (Muscle cells also produce their own energy.)

White and dark muscle fibers smallerBy doing lots of aerobic exercise, and you’ll infuse your slow twitch fibers with even more capillaries. Slow twitch fibers burn lots of calories, but no matter how long you work them, these fibers stay pretty much the same size, so they don’t contribute significantly to sculpting you. Finally, as their name implies, slow twitch fibers have a relatively slow reaction time.

Studies have shown that elite marathoners tend to be genetically endowed with a higher-than-average percentage of slow twitch fibers, while Olympic sprinters have more fibers that give them strength and quickness.

tiffany in thigh 2013 edit lighter text crop small1…Which brings us to the other major type of fiber in your muscles, “fast twitch” or “white muscle.” “Fast twitch” fibers are designed for power and speed. They have has less blood flow, which makes them lighter in color. Fast twitch fibers are what most reshapes your muscles. Do strength training, and these fibers get firmer. Keep training and these fibers will undergo “hypertrophy,” that is, they will get larger. Women’s muscles (fortunately) don’t increase size easily, so women can use heavy weights and still not see a lot of hypertrophy. Whatever sex you are, the manner in which you work out will determine the body shape you achieve. If you use heavy weights with few reps for several months, your “fast twitch” muscle fibers will increase in size. If your routine employs light weights with lots of reps, the result will be muscles that are firm and shapely but not significantly larger. The Bar Method’s practice of performing many reps with light weights produces just enough hypertrophy to sculpt muscles but not bulk them up.

One last note on dark and white fibers: Each of our muscles has a different proportion of “slow twitch” and “fast twitch” cells according to what that muscle does. For example, your hamstrings and arm muscles have a high percentage of fast twitch fibers for power and speed. Your glutes have a lot of both types (one reason they’re so large!)

hypertrophy text edited smallerHere’s how both types of fibers get stronger from exercise: First, working out causes “micro-tears” in your muscle fiber. The torn muscle fibers then heal themselves, building new fiber that is stronger than the old. The more intensely you exercise and the heavier the weights you use, the more micro-tears you’ll create, and the larger your healed “fast twitch” fibers will become (see “hypertrophy” above).

You know you’ve had a good workout when you get “delayed-onset soreness” (DOMS) 24 to 48 hours afterwards. This soreness is caused by your fibers’ healing process, not from lactic acid as has previously been believed.

Cindy ChuThere is a second type of strengthening that happens after exercise. It is less well known but has been found to give women most of the results they get from working out. Exercise physiologists called it “synchronous activation.” What this means is, in effect, improved mind/body coordination. Here’s how it works: An untrained muscle is weak in part because the message it gets from the brain and nerves is disorganized. A student literally does not have the neural connection between her brain and her muscle to perform the action she wants to. She may intend to fire a muscle to move a part of her body, but she can’t recruit enough of that muscle to make it behave. Mark A. W. Andrews, an associate professor of physiology, explains it this way: Exercise, especially when it focuses on form and precision, gives you “the ability to recruit more muscle cells – and thus more power strokes – in a simultaneous manner.” Andrews adds, “This neural adaptation generates significant strength gains with minimal hypertrophy and is responsible for much of the strength gains seen in women and adolescents who exercise.”

The Bar Method workout is especially focused on this fitness component. So the next time you’re doing a Bar Method workout and struggling with your form, take satisfaction in knowing that improving your form is increasing your strength and fitness as well.

best back exercise chartFinal fun fact: Most of your largest and most powerful muscles are on your back side. When you’re working out, pay attention to your back! That side of you houses most of the largest muscles in your body. They include the latissimus dorsi, the largest muscle in your torso; the triceps, the largest muscle in your arm; and the gluteus maximus, the largest muscle that moves your legs and the most massive muscle in your entire body.

So don’t assume your back has to be “out of sight out of mind.” During exercise try imaging your back in your mind’s eye and feel what’s happening back there. By doing so you’ll use more muscle and burn more calories, not to mention benefit your posture awareness as well.

5 replies
  1. Julie
    Julie says:

    I want to say thanks to Burr for taking the time out of her busy schedule to write these monthly blog posts! I look forward to them every month. I always learn something new. I don’t live near a studio so I appreciate the connection I have here. I use Burr’s DVDs, so thankful I found them. Just wondering if you might consider doing some of the exercises that use the barre. I have a fluidity bar and could do them using it. Thanks Again Burr for this awesome blog!

  2. Jessica Fisher
    Jessica Fisher says:

    This was such an informative AND interesting article. I am feeling strong and empowered in mind AND body now. Thank you for taking the time to write this.

  3. ann
    ann says:

    Another fun fact that could be mentioned here is that engaging your fast twitch muscles (vs your slow twitch)produces HGH. I have read that high intensity workouts are best for activating fast twitch muscles and therefore producing Human Growth Hormone. This matters to me because I have also red that HGH is a high powered fat burner that can continue targeting and burning fat for several hours after a high intensity workout (if you don’t shut it down with sugar – see Dr Mercola). If this is in fact true, it explains why a lot of long distance runners are not necessarily lean (in my experience)and why I would rather engage in fast twitch muscle workouts from a fat burning perspective.
    Great piece, very informative Thanks

  4. Jillian
    Jillian says:

    I didn’t know our muscles could have another part in them such as slow and fast twitch fibers. I thought muscles were just muscles. This is very informative for people who are not much into the fitness technicalities like me. I was laughing when I read that DOMS are manifestation of a good work out. During the first week when I started jogging and exercising, my body sores all over I almost couldn’t lift a leg the next day but after that there were no delayed onset sores. I just thought maybe I was doing extremely well that my body is adapted to exercising but after reading this post I just made a ridicule out of myself. I felt like I was an expert because I didn’t feel any soreness.


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