As a Pilates instructor, I love educating clients on how to move. From the Hundred to Swan to Mermaid, it's a joy seeing clients develop strength, mobility and awareness. Even though you may have noticed the changes Pilates has made in your body, what exactly is going on below your skin that allows you to bend, lift, curl, twist and reach?

Allow me to introduce skeletal muscle. Yes, your brain, bones, and myofascia definitely play a role but it is your muscle that performs the action. Even though most of us simply call it "muscle," it is technically skeletal muscle since it is different at the cellular level from your heart muscle and the muscle in your digestive system. Since it is the type of muscle that moves your skeleton we call it skeletal muscle.

Now, time to dive deeper. Skeletal muscle is similar to a large rope composed of several smaller ropes. All of the ropes bound together would be the muscle itself but if you were to isolate out one of the smaller ropes you would have a muscle fascicle. If you went down to a single string of the rope you would have a muscle fiber (Photo 1. Courtesy of http://www.medicalook.com/human_anatomy/organs/Skeletal_muscle_fiber.html).

Each skeletal muscle fiber is made up of small contractile protein units called sarcomeres. The sarcomeres of a skeletal muscle are lined up one after another. When the muscle receives a signal to contract from the skeletal muscle nerves (alpha motor neurons) each of the sarcomeres contract to allow the muscle  to shorten. If you've ever seen one of us at the studio explain skeletal muscle contractions using the wide-tooth hair picks, the two picks together would be a sarcomere.  Depending on the contraction strength needed your body recruits a few, some, or all of the muscle fibers for a contraction. For a more nerdy look at this, check out the supplemental YouTube video.

While the structure is certainly interesting in design (yup, I am a nerd) have you ever been sore? Or have you wondered how you can get stronger from Pilates? And why is it that if you break your arm and you wear a cast for a few months, the arm becomes significantly smaller?

Skeletal muscle strength is determined by the size of your skeletal muscle ropes. While you cannot increase or decrease the number of your "ropes," you can increase and decrease the circumferences. When you do strength training exercise (i.e., Pilates) the first noticeable increase in strength is due to neuromuscular signaling. This is basically an increase in the efficiency of your brain telling your body how exactly to move. The second noticeable increase in strength comes from an increase in the size of your muscle. But how does skeletal muscle increase in size?

Within each skeletal muscle fiber there are resident stem cells called satellite cells. Satellite cells are an adult stem cell. As an "adult" stem cell they have limited number of choices in what type of cell they become unlike pluripotent stem cells found within developing embryos that can become any type of cell.  At rest, satellite cells are dormant and live just outside of the skeletal muscle fiber membrane (Photo 2. Copyright of Hawke and Garry. 2001. J Appl Physio. Aug;91(2):534-51). Essentially, satellite cells are like having Bob the Builder live next door & you can call him anytime you need. When your skeletal muscle is "injured" (i.e., some surgeries, contusion, etc...) the satellite cells are activated via the immune system and molecules released from the damaged skeletal muscle. This signaling is like you making a phone call to Bob the Builder to tell him that you've decided to renovate your living room and need his help to install new hardwood floors. Once activated, the satellite cells migrate into the muscle fiber and to the site of injury.

At the site of damage, satellite cells work together with immune cells such as macrophages. Macrophages are little Pac-Man types of cells that chomp up unneeded damaged tissue & debris. Then the satellite cells fuse to the site of injury and through a process known as differentiation they become part of the newly repaired muscle fiber (Photo 2).  This is also how skeletal muscle becomes multinucleated.

This brings us to soreness, when you work out and do new movements or increase resistance the soreness that you feel is slight microtrauma. Essentially when you work out and perform eccentric movements (the movement when your muscle is lengthening yet still striving to maintain control of spring tension) your muscle proteins struggle to maintain control. As a result, slight micro-tears can occur. It is important to note that these "tears" are very slight. They occur at the molecular level and are actually an important step to getting stronger. When micro-tears occur, satellite cells are activated to repair the muscle. This process also helps increase the size and strength of your skeletal muscles.

As you age, the percentage of satellite cells within your skeletal muscle will decrease. However, current research is now showing that if you stay active as you age, you can either prevent this loss or delay this loss of satellite cells. However, if you choose a sedentary lifestyle as you age you will lose the percentage of satellite cells that you once had in your 20's and lose the robust capacity of your skeletal muscle to be able to continually repair itself. Thus, again, I cannot stress enough how wonderful Pilates & exercise is as you age. It truly is that type of "fountain of youth" that many wish they could find.