Strength Through Understanding: The Muscular System

It is not by muscle, speed, or physical dexterity that great things are achieved, but by reflection, force of character, and judgment.”
– Marcus Tullius Cicero

Yoga places a special emphasis on the muscular system. In order to come into a yoga pose, known as an asana, we engage our muscles to move our bodies. The physiology of our bodies and the relationship between our muscles allows us to apply specialized techniques that increase the effectiveness of our stretching. As we deepen our understanding of how muscles function we can increase the efficiency of our asana practice.

There are around 640 muscles in the human body. Many people classify groups of muscles differently, creating disparities around this number. Almost all of these muscles form a bilateral pair that create opposite movements, like the biceps and triceps. On average, muscles account for 42% of body mass in males and 36% of body mass in femalesi

Types of Muscles

There are three types of muscles in the human body: smooth muscle, cardiac muscle, and skeletal muscle.

Smooth muscles are involuntary, such as the muscles lining our esophagus and intestines. These muscles contract without our awareness in a motion called peristalsis. The next time that you swallow, peristalsis will move your saliva and food down your throat without you having to think about any muscular engagement. It works similar to squeezing toothpaste out of a tube.

Cardiac muscle is specialized muscle found in the heart. Cardiac muscle contains small discs that can carry an electrical signal. This electrical signal causes an involuntary contraction of the muscles. When medics try to revive a patient whose heart has stopped, they actually send an electrical current into the person’s heart in an attempt to restart the beating of the heart. Like smooth muscle, we do not have to think about contracting our hearts for our muscle to engage. This contraction happens automatically and speeds up or slows down in response to the information sent to the brain from the nervous and endocrine systems.

Skeletal muscle is very different from smooth and cardiac muscle in that it can be controlled voluntarily. In reading this article, you have almost certainly engaged muscles to scroll down the page, to change your gaze as you read from left to right. In yoga we are concerned with skeletal muscles because we can purposefully engage these muscles. We aim to strengthen and stretch our skeletal muscles so we can interact with our environment more effectively and comfortably.

Some of the major skeletal muscles.
Some of the major skeletal muscles.

Fast Twitch vs. Slow Twitch

Skeletal muscle is divided into slow twitch and fast twitch muscle. Think about a cooked turkey. Some of the meat is white while other meat is dark. The white meat represents fast twitch muscles, low in the darker colored mitochondria. Dark meat represents slow twitch muscles, high in mitochondria. These differences are an example of our intelligent design for accomplishing different tasks.

Slow twitch muscle fibers contract for long periods of time with little force. To achieve this, slow twitch muscle is rich in capillaries and mitochondria. This allows them to contain a larger amount of oxygen and stored fats and carbohydrates to be used for aerobic activity. While jogging or walking we engage many slow twitch muscles.

Fast twitch muscle fibers contract for shorter periods of time with a large amount of force. Fast twitch fibers are less dense in capillaries and mitochondria and can only sustain contraction for a short period of time as a result. They are capable of generating a large amount of force and have a greater potential for an increase in mass. While lifting weights we engage many fast twitch muscles that generate a large amount of force quickly but also tire and feel sore quickly.

Types of Contractions

The contraction of a muscle fiber is an all-or-nothing deal. The base unit of a muscle, called a sarcomere, can only either be fully contracted or fully relaxed. In order to generate more strength, more sarcomeres are recruited to contract. As more sarcomeres contract, the muscle fibers and overall muscle generates more force for greater strength. If you are holding up a bag of groceries and someone were to put a rock in the same bag, your individual muscle fibers wouldn’t be working any harder to accommodate the extra weight. Rather your nervous system would signal for more fibers to contract, generating more force to accommodate the increased weight.

There are three types of contraction that are classified the following ways:

Isotonic Concentric Contraction: the muscle becomes shorter while generating force

  • Imagine lowering down in a pushup. Your biceps become shorter and start to bulge out as they are generating force. Both contraction and a shortening of the muscle are occurring.

Isotonic Eccentric Contraction: the muscle becomes longer while generating force

  • Imagine pushing yourself back up in a pushup. Your biceps exert force as they stretch back towards their resting length. Both contraction and a lengthening of the muscle occur.

Isometric Contraction: the muscle is contracted but no movement is taking place

  • Imagine holding yourself halfway down in a pushup, you are performing isometric contraction. Your chest and biceps are engaged but they are not changing length.

We use a variety of combinations of these contractions in our asana practices. Oftentimes we use isotonic contractions to bring ourselves into a posture, and then use isometric contractions to maintain our position while finding stillness. Understanding the difference between these three types of contractions can aid us greatly in applying specific techniques to deepen the relaxation of certain muscles.

For a detailed look at the process and proteins involved in causing muscle contractions click here.

Origin and Insertion

Each muscle has an origin and insertion based on where it attaches to the skeletal system. The origin is the attachment that is more proximal, or closer, to the midline (or main mass) of the body. The insertion is the attachment that is more distal, or farther away, from the midline of the body.

Take the largest calf muscle, the gastrocnemius. Its origin is the heads of the medial and lateral condyles (the round parts at the bottom) of the femur. The calf inserts into the posterior (front) part of the heel (calcaneus). Since the femur is closer to the torso than the heel, it is classified as the origin for the calf and the heel is classified as the insertion.

The origin and insertion of the calf muscle.
The origin and insertion of the calf muscle.

In our yoga postures it makes a large difference whether the origin or insertion is fixed, as we will see in later posts.

Agonists, Antagonists, and Synergists: The Physiology of Stretching

Skeletal muscles can be subdivided into three groups depending on the type of motion that we are performing. These are the agonists, antagonists, and synergists. Please note that an agonist in one asana may be an antagonist or synergist in a different asana and vice versa.

Agonist: the contracting muscle which is causing movement

Antagonist: the relaxing muscle which is being stretched (if contracted the antagonist would create the opposite movement to the agonist)

Synergist: a contracting muscle that aids the agonist in movement

Here are some common agonist/antagonist muscle pairs:

  • Biceps & Triceps
  • Quadriceps &Ham strings
  • Abdominals & Back Muscles
  • Adductors & Abductors

Reciprocal Inhibition: An Important Concept

These muscles operate in a relationship known as reciprocal inhibition. As the brain signals for the agonist to contract, it simultaneously signals for the antagonist to relax. Many times agonists and antagonists are on opposite sides of a joint, like the biceps and triceps. When one of these muscles is contracted, its counterpart relaxes. This creates a yin-yang in the muscular system as every force has an equal and opposite reaction. Without this reciprocal relationship, we would place too much stress on our joints and connective tissues and cause serious harm to our bodies.

Let’s explore this relationship in action in paschimottonasana (seated forward fold).  Before reading on, see if you can come into the posture and discern some of the agonist, antagonists, and synergists based on what you feel engaged and what you feel stretching.  Remember to hinge forward at your hips, maintain a microbend in your knees, and only come forward as far as you can with a straight spine.  


Agonists: Quadriceps, Abdominals

Antagonists: Hamstrings, Back muscles (latissimus dorsi and others), Calves, Triceps

Synergists: Iliopsoas, Biceps, TFL (among others)

In a seated forward fold we aim to flex at the hips, drawing the upper body towards the legs (the chest towards the thighs). The abdominals and quadriceps contract to draw the torso forward. The muscles on opposite sides of the respective joints, the back muscles and hamstrings, relax in response.

As we contract other synergists like the hip flexors that help in flexion of the hip and torso we move a bit deeper. When we engage the biceps to pull our torso further forward (either by pulling a strap or the fingers around the toes), the triceps relax in response and we move deeper. The deeper our understanding of which muscles are agonists, antagonists, and synergists in a pose, the more we can isolate the contractions to agonists and synergists to relax stretched muscles more effectively.

Interesting Muscle Facts

  • Smiling requires 17 muscles while frowning requires 43 muscles.

  • The tongue is often considered the strongest muscle proportionate to size in the body, but is actually composed of 8 muscles.

  • The masseter, responsible for chewing, can generate more force than any other muscle.

  • The largest muscle in the body is the gluteus maximus, regardless of how big a booty we have.

  • Around 200 muscles are involved in taking a single step.

  • The smallest muscle in the body is the stapedius. Along with other small muscles it helps to hold the ear together.

  • We do not grow more muscle fibers, we are born with as many as we will ever have. They can become thicker through being worked.

Concluding Thoughts

Muscles play a vital role in everyday life, giving us the capability for movement, providing a gateway for endless possibilities. Sometimes we become focused on the superficial muscles and having bulging biceps or a ‘six pack’. In yoga our goal isn’t to develop a lean, ripped body. Sometimes that happens to be a byproduct of the way we are moving our bodies. Just like there are numerous muscles deeper within the body that we can’t see, there are many aspects of yoga beyond the physical. But we can use our physical bodies to develop a greater awareness of our overall selves.

In the next two posts we will examine the stretch reflex and some techniques for allowing our muscles to relax so that we can move deeper in our postures and deeper into our understanding of our selves.

iMarieb, EN; Hoehn, Katja (2010). Human Anatomy & Physiology (8th ed.). San Francisco: Benjamin Cummings. p.312.

3 thoughts on “Strength Through Understanding: The Muscular System

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    1. Hey Bryant,

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