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Updated: Oct 26, 2020

We started off this series on Strength Training with an overview of it. Do check it out here if you haven't yet. Now lets go back to basics and look into the muscles itself to get a better understanding of how they work.

Skeletal Muscle

The body has three types of muscle tissue, each with a different purpose. The muscles we voluntarily control are called ‘skeletal muscles’, and they work with the skeletal system to produce coordinated movements of our limbs. The other two are ‘smooth muscles’, which propel materials through our digestive, cardiovascular, urinary, and reproductive systems and the heart contains specialised cardiac muscle tissue to pump blood.

Skeletal muscles cover our skeleton, giving our body its shape and are attached to the foundations of our body, the bones, through tendons. Skeletal muscle, with its associated connective tissue, constitutes about 40% of the body’s weight. Pretty much all of our bodily movements are controlled by skeletal muscle contractions. Even for us to just sit still, our muscles are continuously making tiny adjustments to keep our body upright. Some of these skeletal muscles are directly attached to our skin and contraction of one of these muscles changes is what leads to our facial expressions.

Properties of a Muscle Tissue

All muscle cells including skeletal muscles share several properties:

  1. Excitability: is the ability to respond to a stimulus. This means that it can change due to some type of external stimuli, be it something electrical or in terms of hormones.

  2. Contractility: is the ability of muscle cells to forcefully shorten. For instance, in order to flex (decrease the angle of a joint) your elbow you need to contract (shorten) the biceps brachii and other elbow flexor muscles in the anterior arm. Notice that in order to extend your elbow, the posterior arm extensor muscles need to contract.

  3. Extensibility: is the ability of a muscle to be stretched. For instance, let's reconsider our elbow flexing motion we discussed earlier. In order to be able to flex the elbow, the elbow extensor muscles must extend in order to allow flexion to occur. Lack of extensibility is known as spasticity.

  4. Elasticity: is the ability to recoil or bounce back to the muscle's original length after being stretched. So you have extensibility which is the ability to stretch. Elasticity is the ability for it to go back to its original shape without damage. So elasticity plays as bigger role in terms as extensibility.

Key Tasks of Skeletal Muscles

  1. Movement of the body: Most skeletal muscles are attached to the bones and are responsible for the majority of body movements, including walking, running, chewing, and manipulating objects with the hands.

  2. Maintenance of posture: Skeletal muscles constantly maintain tone, which keeps us sitting or standing erect.

  3. Respiration: Skeletal muscles of the thorax carry out breathing movements.

  4. Production of body heat: When skeletal muscles contract, heat is given off as a by-product. This released heat is critical for maintaining our body temperature.

  5. Communication: Skeletal muscles are involved in all aspects of communication, including speaking.

Each skeletal muscle is a complete organ consisting of cells, called muscle fibers, which are associated with smaller amounts of connective tissue, blood vessels, and nerves. The connective tissue fibers that surround a muscle and its internal components extend beyond the centre of the muscle to become tendons, which connect muscles to bones or to the dermis of the skin.

Skeletal muscles work with tendons to pull on bones, the collagen in the muscle tissue (the mysia) intertwines with the collagen of a tendon (connects muscle to the bone. Ligament on the hand connects one bone to another). At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred though the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton.

The best-known feature of skeletal muscle is its ability to contract and cause movement. Muscles begin the actual process of contracting (shortening) when a protein called actin is pulled by a protein called myosin. More on this further down. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation.

Structure of the Muscle

To better understand the finer structural details of the muscle and the mechanism by which muscle contraction occurs, one must understand the molecular components of the muscle and of the structures associated with it. Every skeletal muscle is richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract.

Whole Skeletal Muscle Structure: Connective Tissue, Innervation, and Blood Supply.

A muscle is composed of muscle fascicles, each surrounded by perimysium. The fascicles are composed of bundles of individual muscle fibers (muscle cells), each surrounded by endomysium. This figure shows the relationship among muscle fibers, fascicles, and associated connective tissue layers: the epimysium, perimysium, and endomysium. Arteries, veins, and nerves course together through the connective tissue of muscles. They branch frequently as they approach individual muscle fibers. At the level of the perimysium, axons of neurons branch, and each branch extends to a muscle fiber.

Myofibril: A cylindrical organelle running the length of the muscle fibre, containing Actin and Myosin filaments. Each myofibril is a threadlike structure, approximately 1–3 μm in diameter, that extends the length of the muscle fiber. The myofilaments are composed of several different proteins, constituting about 50 percent of the total protein in muscle. Of the myofilament proteins, myosin and actin are known to play a direct part in the contractile event.

Sarcomere: A functional unit of the Myofibril, sarcomere, is a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins. These are divided into I (light), A (dark) and H bands. The sarcomere is the basic structural and functional unit of skeletal muscle because it is the smallest portion of skeletal muscle capable of contracting. Structures called Z disks separate one sarcomere from the next. A Z disk is a filamentous network of protein that forms a stationary anchor for the attachment of actin myofilaments. Each sarcomere extends from one Z disk to the next Z disk. A dark line, called the M line, helps hold the myosin myofilaments in place, similar to the way the Z disk holds actin myofilaments in place. The arrangement of the actin (thin, therefore light) and myosin (thick, therefore dark) myofilaments gives the myofibril a banded, or striated appearance.

Parts of a Muscle: (a) Part of a muscle attached by a tendon to a bone. (b) Enlargement of one muscle fiber. The muscle fiber contains several myofibrils and specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) called the sarcoplasmic reticulum (SR). (c) A myofibril extended out the end of the muscle fiber. The banding patterns of the sarcomeres are shown in the myofibril. (d) A single sarcomere of a myofibril is composed of actin myofilaments and myosin myofilaments. The Z disk anchors the actin myofilaments, and the myosin myofilaments are held in place by titin molecules and the M line.

Actin: A group of thin, globular proteins that are the most abundant proteins in sarcomeres, which constitute about 25 percent of the protein of myofilaments and help in providing shape, structure, and mobility to the body. Actin also plays an essential role in cell division, cell motility, and cell signalling. Actin interacts with myosin to support muscle contraction.

Myosin: a family of motor proteins that, together with actin proteins, form the basis for the contraction of muscle fibers. It is a thick, contractile protein filament, with golf head like protrusions known as Myosin Heads. Each myosin molecule have two heads. One head is for binding of ATP and the other one acts as a binding site for actin. Myosin initiates muscle contraction by generating a force by binding to the ATP molecule. It has two important roles: a structural one, as the building block for the thick filaments, and a functional one, as the catalyst of the breakdown of ATP during contraction and in its interaction with actin as part of the force generator of muscle.

Structure of Actin and Myosin: (a) The sarcomere consists of actin (thin) myofilaments, attached to the Z disks, and myosin (thick) myofilaments, suspended between the actin myofilaments. (b) Actin myofilaments are composed of F actin (chains of purple spheres), tropomyosin (blue strands), and troponin (red spheres and rod). Myosin myofilaments are made up of many golf-club-shaped myosin molecules, with all the heads pointing in one direction at one end and the opposite direction at the other end. (c) G actin molecules (purple spheres), with their active sites (yellow), tropomyosin, and troponin, make up actin myofilaments. Myosin molecules (green) are golf-club-shaped structures composed of two molecules of heavy myosin wound together to form the rod portion and double globular heads. Four small, light myosin molecules are located on the heads of each of

the myosin molecules.

Troponin: Is a regulatory protein and is part of the contractile mechanism of Actin. It plays an important role in the binding of calcium, which initiates a series of events eventually leading to ATP hydrolysis and muscle contraction. Muscle contraction is regulated by the intracellular calcium concentration.

Tropomyosin: Tropomyosin is an elongated rod shaped protein about 40 nm long, that winds along the groove of the actin. It acts to inhibit the myosin cross-bridges from binding to the myosin binding site on actin thereby blocking actin’s active binding site from binding to myosin. This regulates and coordinates muscle contraction depending on the need and stimulus. If not for this mechanism in place, our muscles would always be in a relaxed state. This is disabled when the muscle gets the signal to contract, which pushes calcium in and binds to troponin, moving tropomyosin out of the way.

Titin: In addition to actin and myosin, there are other, less visible proteins within sarcomeres. These proteins help hold actin and myosin in place, and one of them accounts for a muscle’s ability to stretch (extensibility) and to recoil (elasticity). Titin is one of the largest known proteins, consisting of a single chain of nearly 27,000 amino acids. It attaches to Z disks and extends along myosin myofilaments to the M line. The myosin myofilaments are attached to the titin molecules, which help hold them in position. Part of the titin molecule in the I band functions as a spring, allowing the sarcomere to stretch and recoil.

Muscle Spindle

The muscle spindle is a proprioceptor (specialised sensory receptors), a sense organ that receives information from a muscle, that senses STRETCH and the SPEED of the stretch. When you stretch and feel the message that you are at the ENDPOINT of your stretch the spindle is sending a reflex (stretch or myotactic reflex) arc signal to your spinal column telling you not to stretch any further. This sense organ protects you from over stretching or stretching too fast and hurting yourself. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.

Muscle Fibers

The cells found in skeletal muscle are highly specialized with a unique structure. Skeletal muscle fibers are long, cylindrical cells, hence the term 'fiber'; each with several nuclei located near the plasma membrane. A single fiber can extend the entire length of a muscle. In most muscles, the fibers range from approximately 1mm to about 4cm in length and from 10μm to 100μm in diameter. Most muscles contain a mixture of small- and large-diameter fibers.

Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently. The endomysium, a thin connective tissue layer of collagen and reticular fibers contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue.

Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP. Using these criteria, there are two major types of skeletal muscle fibers: slow-twitch and fast-twitch. Not all skeletal muscles have identical functional capabilities. They differ in several respects, including the composition of their muscle fibers, which may contain slightly different forms of myosin. The myosin of slow-twitch muscle fibers causes the fibers to contract more slowly due to its oxidative reliance to generate ATP and therefore is more resistant to fatigue, whereas the myosin of fast-twitch muscle fibers causes the fibers to contract quickly owing to its glycolytic pathway (Anaerobic) and to fatigue quickly.

Characteristics of Skeletal Muscle Fiber Types

The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.

Slow Twitch/SO/Type 1 Muscle Fibers - Endurance

Slow twitch muscle fibers (also called Type I Muscle Fibers) contract slower, are very durable, and have a high resistance to fatigue. They play a role in maintaining posture and are predominantly responsible for lower-intensity activities such as walking, jogging, housework etc. Why is that so? One of the reasons is the fact that such fibers have more myoglobin and mitochondria, a large oxidative capacity, and a slower release of ATP for energy. In addition, slow twitch muscle fibers have less ability to store glycogen.

These fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. Hence, amount of force these muscles can produce is lower than that of fast twitch muscle fibers.

These fibers also possess myoglobin, an O2-carrying molecule similar to O2-carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O2 within the fibers themselves (and gives these fibers their red color). All of these features allow Slow Twitch - Type1 fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.

The fact that this type of fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilising bones and joints, and making small movements that happen often but do not require large amounts of energy.

High concentrations of myoglobin in muscle cells allow organisms to hold their breath for a longer period of time. Diving mammals such as whales and seals have muscles with particularly high abundance of myoglobin. Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria. So, increasing myoglobin levels can enhance workouts and thereby strength/muscle gains.

Fast Twitch Muscle Fibers

There are two types of fast twitch muscle fibers: type IIA and IIX. Fast muscle fibers are often referred to as white muscle fibers. They Have a larger diameter than slow twitch as they have more myosin filaments in them (thick filament), which leads to greater force generation.

Type IIA/FOG – Moderate Intensity

Type IIA are also called oxidative-glycolytic since these muscle fibers use oxidative (aerobic) and glycolytic (anaerobic) mechanisms for energy. Type IIA fibers are activated predominantly during movements and activities that are quick, repetitive and of somewhat lower intensity. IIA fibers are activated after the type I. This type of muscle fiber has a relatively large amount of mitochondria and can quickly recover after activities. However, they do not possess significant myoglobin, giving them a lighter color than the red and less O2 carrying capacity to fuel prolonged workouts sessions like Type 1 muscle fibers.

These fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension.


Type IIX muscle fibers are fibers that are rapidly contracted and are activated during high-intensity activity, such as High Intensity Interval Training. A small number of mitochondria, lesser oxidative and a greater glycolytic ability are characteristics that describe the type IIX fibers. It is important to understand that this fiber type gets tired very quickly, their recovery is slower, and occurs mainly after physical activity. However, in contrast to type I fibers, type IIX fibers have the possibility to generate a lot of force, and are almost exclusively responsible for overcoming maximum load.

Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color.

Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle. ATP provides the energy for muscle contraction. The three mechanisms for ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic metabolism. Creatine phosphate provides about the first 15 seconds of ATP at the beginning of muscle contraction. Anaerobic glycolysis produces small amounts of ATP in the absence of oxygen for a short period. Aerobic metabolism utilizes oxygen to produce much more ATP, allowing a muscle to work for longer periods.

Muscle fatigue, which has many contributing factors, occurs when muscle can no longer contract due to exhaustion of ATP!

Now that basic anatomy is out of the way, in the next blog we will look into the neural adaptations, which is the very first step that our body needs to go through to facilitate strength gains. That's coming up.

P.S: Shout out to Seeley's and Rice University Anatomy and Physiology authors and publishers for the incredible content, some of which (including pictures) I have used in this blog!!


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