Many of us just go about our day using our muscles, and we don't need to think twice about how they work, they just do! But if we want to know how to treat problems when they arise, it really helps to understand their basic function. Today's post will be for those who have not studied muscle physiology before, and it may come in handy for future posts about how to work on your own trigger points at home, how to determine the direction you would stretch a muscle, or otherwise just be interesting to learn.
Contraction of individual fibers/groups
A muscle's action is to pull its ends together. On a large scale, the muscle attaches to bones and across a movable joint, pulling one side of the joint closer to the other... and on a small scale each tiny muscle fiber contracts to shorten the small band of muscle. It contracts in one direction, that is, the direction of its fibers. The protein filaments that make up the muscle fiber start at a maximum length and contract by pulling(1) the outside bands closer to the center. There are many of these microscopic fibers in each muscle, and nerves feed into them in small groups called "motor units". The interesting thing about these small groups is that they follow an "all-or-none" principle, meaning that if the nerve sends the signal to contract, then they all contract, or none of them do. But since the groups are relatively small, you can only use a few motor units to lift something light, and then recruit more and more muscle fibers when you lift something heavier.
Energy needed for contraction
For the muscles to contract, they must use up energy. I will devote a separate post to cellular respiration, the means of creating the energy we use. Basically, energy is stored in the muscle ready to use in two forms: creatine, and a form of glucose (a sugar aka carbohydrate, it is stored as glycogen in the muscle) that your muscles will use as a quick burst of power, but will only last a short time. This energy is quick but inefficient, and produces lactic acid which can be used for more fuel. When you have used up all the sugar storage, your body can then start to convert fatty acids into sugars to restore the sugar levels ready for more energy. In longer endurance activities, more fats are burned than sugars, and the energy production is much more efficient than quick/powerful movements. Proteins can be used for fuel too, but that usually means your body is breaking down muscle to use for energy to keep you alive, which is usually a bad thing. Similar to how sugars can be stored in the muscle for energy, oxygen can be stored as well for the more efficient energy production process.
To conduct the signal for contraction, Calcium, some neurotransmitters such as acetylcholine, and hormone regulation (such as epinephrine, insulin levels) are all important parts of the process as well.
The main summary here is: sugars tend to feed your quick/powerful muscle fiber types, fats tend to feed your endurance muscles. Part of the explanation of fatigue is running out of these nutrients in the muscle, and despite eating the nutrients there is only so much your blood can carry to every muscle in your body to restore the energy supply.
Muscle contraction is an electric process, using chemical transfer from nerves to start the electric signal to contract. In the mid 1780's the discovery was made that frog legs would jump when a wire sent electricity through them.
Types of contraction
Muscle contraction during movement comes in two flavors. We call them "concentric" for when you shorten the muscle, and "eccentric" for when the muscle is elongated but still firing. You can think of the concentric portion as your biceps lifting a weight, and the eccentric portion as your biceps slowing the fall of the weight as the elbow straightens.
The third type of muscle contraction is isometric, where you are just holding the weight and keep it from moving.
Changes to muscles
When you use a muscle, you make it stronger. Even more interesting is that a trained muscle group can adapt to store more of the fuel needed for energy production. Other changes include an adaptation of the nervous system, muscle fiber types, enzyme content of the muscle, and hormone adaptations that allow the muscles to function more efficiently. This will be explored in more depth in the future.
Contraction by reflex
One of my favorite features on the subject of how muscles work (and one that will get several of its own posts in the future) is that the skeletal muscles can be voluntarily contracted, contracted by external electric pulse, or contracted by reflex. I'll tell you why that's exciting to know, but to do that, a little about how it happens:
In your muscles and tendons are special cells that receive information from the surroundings. They cause an automatic response when they detect changes in muscle length or tension. So when someone drops a heavy book in your hands, part of the response isn't something you even think about, your arms automatically lift up in response to the sudden weight in your hands (sometimes more than necessary). The knee-tap test (very useful video) your doctor does, when he/she taps a tendon on your knee and makes you kick, is an example of this reflex. The other reflex forces your muscle to relax to protect itself against damage from contracting too much. It inhibits contraction without you even having to think about it.
The really cool thing about reflexes is they happen automatically, you don't have to think about them! The other cool thing is that your massage therapist can take advantage of this information to help your muscles relax. This works because when you contract your biceps on the front of your arm, your triceps on the back of your arm must relax or else your arm doesn't move anywhere. The inhibitory signal is sent through your nervous system to relax the muscles opposite of those that are contracting. This is an automatic process, so it can sometimes help with muscles that are too tight, and in managing cramps. The next time you get a charlie horse, you can try identifying the group that is cramping, then contract the opposite group in order to attempt to force the signal to relax the cramping muscle.
There is a certain amount of contraction that goes on throughout the body at all times, this is called the resting tone of the muscle. The partial contraction helps with posture, it keeps you from flopping around like a rag-doll. When you stretch your muscles, you are not actually elongating the muscle itself (which would cause damage or tearing of the muscle) but you are removing the inhibitory nerve signal to maintain tone. So stretching is more of a nervous system function than an actual change to the muscle structure (or at least it should be, injury would result from elongation of the muscle).
This post is full of links that will take you much deeper into each subject, and I will be expanding on each subject more in the future! Please let me know if you found this post helpful and tuned with the right amount of introductory information for how muscles work.
What's going on with me, research articles, interesting little blurbs. This blog is an attempt to consolidate research into an easily digestible format.
Alex Moon has been a Licensed Massage Therapist since 2012, did his undergraduate studies at Utah State, and is currently working on his Doctorate in Physical Therapy.