Defining a Muscle
Muscle is a tissue found in the body, responsible for movement and force production. Made up of many smaller protein fibers, a muscle can contract and produce force. Muscle and connective tissue are essential for mobility, as it supports our skeleton, initiates movement, and absorbs shock.
- Defining a Muscle
- Muscle Function
- Frequently Asked Questions
- Mechanism of Contraction
- Muscle Fibers
- Ligaments and Tendons
- Muscle Function & Body Motion
- Muscular Facilitation, Control, and Coordination on Neural Level
- Autonomous Nervous System
- Key Points
If we look at muscles from a broad structural and functional point of view we can categorize them into three groups (1)
- Skeletal muscles, the ones around your bones, mainly function to exert force and provide movement that we can voluntarily control
- Smooth muscles, which are innervated by the autonomous nervous system, are separated into single-unit or multi-unit smooth muscles are the inner muscular tissues like organs and capillaries.
- The cardiac muscle or also known as the heart is an involuntary striated muscle that mainly functions to pump oxygenated blood throughout the body.
Different muscles have different functions, and structures and are innervated differently. There are voluntary muscles like our skeletal ones, which we can control. For example, we can straighten or flex our leg or arm, but when it comes to voluntarily making our organs function a certain way, we cannot do that because their innervation is automatic, by the autonomous nervous system. We can influence our heartbeat for example if we start running, it will become faster, but this is a secondary effect to our choice to start running in the first place, which is done with our skeletal muscles.
If you’re into anatomy, make sure you do not miss out on:
Functions of Skeletal Muscles
|To enable, allow, exert, facilitate, or start a movement.|
|To internally produce a force that equals our resistance in the external area.|
|To produce energy and release heat in the body.|
|To stabilize our movement, hold our posture.|
|To prevent skeletal injuries, improve mobility and provide support|
|To protect our internal organs|
|To store and release lipids and minerals|
Smooth Muscle Functions
There are many different functions of the smooth muscle, depends on where it is located and what its main purpose is. Here are some examples.
|Intestines||Push food throughout your body by contracting the walls|
|Bladder||Can change its shape and size to store or release urine through the urethra|
|Veins||To push deoxygenated blood back and u, the work against gravity.|
|Arteries||To transport, carry oxygenated blood throughout your body.|
|Stomach||Lining cells produce enzymes in order to break down food, so we can use it as energy|
So as we’ve seen, there are many different functions, but basically, smooth muscles are the inner ones that take care of everything going on in your body, so it maintains its homeostasis or balance.
Processes like breaking down food, pushing it through, allowing blood flow, pushing urine out, sweating, and many more, are controlled by the smooth muscles.
Frequently Asked Questions
How can I strengthen or regenerate my tendons?
Methods like consistent, proper weightlifting was shown to help, along with taking the needed nutrients, such as vitamin A, omega 3s, Manganese, and vitamin D. Note that tendons and ligaments don’t respond to exercise as muscles, but the strength gain may be due to increased need of regeneration, along with which thickening adaptation may happen to lift objects that weigh more.
What is the most commonly injured ligament?
ACL or Anterior cruciate ligament is the most commonly injured ligament in the human body.
How many tendons are in the human body?
There are approximately 4,000 ligaments in a human body.
Mechanism of Contraction
The main muscular function is force exertion, producing force through a contraction. This is why understanding how a muscle contracts is very important.
- The first step to any contraction is a message or order from our nervous system, voluntarily. The message travels from the nervous system to the muscle and triggers a chemical reaction.
- When every factor is on point, the message causes the chemical reaction that allows filaments like myosin and actin to slide on each other, which makes the muscle contract.
- When the signal stops, the muscle fibers rearrange and the contraction stops, the muscle relaxes.
On a more cellular level this looks like this:
- When a strong enough signal is sent from our nervous system, called an action potential, that signal transmits through the motor neuron (neuron firing). This comes to a neuromuscular junction, where the neuron reaches the muscle cell.
- Then acetylcholine, a neurotransmitter is released from the motor neuron. Acetylcholine then binds to muscle fiber receptors and starts to activate the mechanism of muscle contraction.
- The proteins inside muscle fiber, myosin, and actin can organize themselves when acetylcholine reaches muscle fiber membrane receptors. This, therefore, causes channel opening where sodium ions enter the cytoplasm of the fiber. This sends a trigger to release calcium ions, which diffuse into the fiber, binds to troponin, and shift actin filaments which enable actin and myosin connection (cross-bridge) so they slide on each other, which produces contraction when the sarcomere is shortened.
- The contraction uses ATP as energy, which first binds to myosin where it is hydrolyzed into ADP and inorganic phosphate by ATPase (enzyme). This enables the myosin head angle to change, so the sliding motion or actin-binding is possible, and the muscle contracts.
Once the action potential or nerve signal is gone, the muscle relaxes back into its original end, and this is how the contraction ends.
The three types of muscle fibers found in the human body are:
Type IA are Slow-Twitch muscles that are usually higher in endurance athletes. These muscle fibers are red in color which comes from the higher hemoglobin content. These fibers are rich in mitochondria which enables them to function longer, use up oxygen to create ATP, and endure more. These fibers do not fatigue easily, have slower firing speeds, and can produce less force. They are rich in vessels, and myoglobin and have low energy utilization
Type IIA are in between, Fast-Oxidative Fibers which means have characteristics of both fast and slow-twitch muscle fibers. They are large in muscle fiber size, can produce high force quickly, and have a medium level of mitochondria, capillaries, and myoglobin. Usually found in most athletes up to a certain percentage, but are more on the quicker, whiter, explosive side.
Types IIB are the Fast-Twitch Muscle Fibers mostly present in powerlifters, explosive sprinters, Throwers, and Jumpers. These fibers can produce the highest force in the quickest time, are whitish in color, and have large muscle fiber sizes. Very high force production and contraction speed, but the oxidative abilities are low. Low in myoglobin and mitochondria, can get tired and fatigued very fast. They function best under higher amounts of CRP and have higher ATPase levels.
No human has only one type of fiber, but the ratios are different. All three muscle fiber types are present, as they are very important in everyday activities, of them.
Marathoners and Ironmans will have around 60-80% of type I slow-twitch fibers while a Sprinter or a Powerlifter will have around 60% of white fast-twitch fibers.
When babies are born, muscle fibers are mostly white and fast-twitch which allows us to understand the uncontrolled and fast movements a baby makes. However, throughout life, this changes rapidly, so much so that children have usually more red, slow-twitch fibers than adults. With training, one can change the ratio of muscle fibers one possesses. If two identical twins are trained in sports like marathon and powerlifting, the marathoner will have a higher content of slow-twitch while the powerlifter of fast-twitch fibers.
Types of Skeletal Muscles
- Convergent muscle is usually triangular shaped where more fascicles attach to a broad tendon at the end, converging into a narrower one at the other. An example would be pectoralis major.
- Parallel muscles have parallel running fascicles and depending on the diameter ratio (center to periphery) they can be fusiform or non-fusiform, flat-shaped, or as spindles. An example would be the Sartorius muscle.
- Fusiform a type of parallel muscle, with parallel fibers running narrow. An example would be the biceps brachii which can flex the arm.
- Unipennate muscles are one-tendoned or one-headed, as the name itself implies. It has a tendon running through the muscle’s length. Unipennate is called the muscle where the fascicles run from only one end (head). These muscles have less range of motion and a higher ability to transmit and withstand tension. An example would be Extensor digitorum
- Bipennate & Multipennate muscles are in the same pennate family, have similar characteristics, and a tendon running through the center of the muscle. An example of a bipennate is rectus femoris, while a multipennate is deltoid muscle, which has more (multi) tendons to which fascicles are attached.
- Circular muscles or also known as sphincters are the muscles on your mouth and anus, where fascicles are organized in a circular shape, concentrically, having a hole in the middle.
What is the main function of ligaments?
To provide support, prevent an excessive range of motions, and attach bone to bone.
How many ligaments are in the human body?
There are approximately 900 ligaments in a human body.
How to prevent ligament injury?
Don’t overstretch your ligaments to access hypermobile positions and work your muscle stabilizers of a specific joint.
Ligaments and Tendons
A ligament is a type of connective tissue that connects bone to bone, mainly providing support and limiting excessive mobility and movement. It is adaptable, flexible, and provides passive support.
Tendon is a type of connective tissue that connects bone to muscle. It is a white, tough, dense, and fibrous band that can withstand tension and transmit force
|Ligament Function||Tendon Function|
|Holding the bones together||To attach bones to muscles or other structure|
|Limiting excessive mobility, twists, and movements||To attach muscles to bones or other structure|
|Providing passive support and injury prevention||To store elastic energy|
|Preventing bone separation||To transmit mechanical forces|
|Stabilizing joints||To provide passive support|
|To support and connect the musculoskeletal system|
The ligament has a hierarchical structure.
Starting from the whole ligament, this splits into smaller parts called fascicles. These fascicles are made up of more basic fibrils and fibroblast (cells that produce collagen and build ECM). Then each fibril is made up of Sub-fibril, smaller units, which consist of many microfibrils.
The ligament is composed of fibrous collagenous fibers and fibrocytes with gel-like ground substances.
The main component of tendons is collagen fibers. The outer layer of a tendon is the fascia, while the fascicles are bound through the endothelium. The tendon consists of smaller fibers that have fascicles that contain collagen fiber made up of smaller fibers known as collagen fibril, inside of which is tropocollagen.
The specialized ECM creating cells found in tendons are tenocytes, found in the collagen fibers of a tendon. Collagen type I is dominant with around 60-80%, while collagen type III is usually under 10% present. Tendons also have elastin, proteoglycans, and minerals.
Types of Ligaments
There are many different types of ligaments, categorized by their location, function, or color:
There are white Ligaments that are more fibrous and non-elastic, rigid and made for passive tension and ROM limitation, and yellow Ligaments, which are the more flexible ones. By location we can find cruciate ligaments, crossing each other, a great example would be the ACL and PCL in the knee and capsular ligaments located in the joint, and capsular ligaments that hold the biggest ball-socket joints together like hips and shoulders. There are also Articular (Natural, human) and artificial ligaments (artificially made).
Ligaments are flexible and adaptable, which means they can be changed in their elasticity. Ligaments have collagen fibrils which are smaller in volume in comparison to tendons, plus ligaments have a higher percentage of proteoglycan matrix than tendons. Ligaments are viscoelastic. This means that they change shape if stretched, or under tension but they return to their original length. Slightly limited self-regeneration abilities, due to lower blood nourishment.
Viscoelastic tissue – which means it can change its shape under tension, and return back. While starched and under tension at the same time, tendons can save up elastic energy which can be released later on for more efficient movement. This phenomenon is famous in the Achilles tendon in runners and jumpers, where the use of this elastic energy has a high impact on the performance of the jump.
Muscle Function & Body Motion
The muscles are the primary organs that keep the body in motion, and keep the body moving.
Both physical movement that we can see and internal movement of the organs, or walls are moving constantly in order for our body to b in a state of homeostasis.
Internal movement – Smooth & Cardiac muscles
When we are breathing the diaphragm muscles are working, plus the obliques and other thoracic muscles to keep our inhale-exhale muscles in tune.
- When we are having a meal, the chewing that we consciously do, along with all the digestive system support that allows the food to travel, break down, and transport to the colon, has involved muscular movement on many levels.
- Just to keep being alive our heart must function properly to pump out oxygenated blood throughout our bodies so we can survive. In the meantime, the veins have to work against gravity and move that deoxygenated blood back up.
- All the glands that produce hormones affect our moving motions, especially of the muscles of our internal organs that are innervated by the autonomic nervous system, which means we are not conscious of but they are still moving.
Physical Movement – Skeletal Muscle Function
Skeletal muscles are the ones responsible for body movements. They are attached by the bones with a tendon on which they pull when contracted. This tendon then pulls the bones that facilitate movement in our physical body, which was done voluntarily. Aside from just moving, the whole body is held up by and supported by muscles, which give us our frame.
They can also generate higher forces, especially the bigger muscles which can help us jump, or push and pull heavy objects from or off the ground. Bigger muscle groups like shoulders, hip extensors, and leg flexors are the ones that produce the highest forces and have bigger motor units.
On the other side, there are smaller motor units, meaning the muscle is innervated by more motor axons (neurons) which have higher control over detailed movements. This includes movements we do with our mouth or tongue when speaking or even the drawing movements for which we need high preciseness.
Muscular Facilitation, Control, and Coordination on Neural Level
Your muscles need stimulation to work. We call that excitation in physiologic terms.
The central and peripheral nervous system is made up of our brain, spinal cord, and peripheral (efferent and afferent) nerves that control and facilitates movement.
For a muscle to function properly, all of the needed actions in the chain (previously explained) must work. This includes
- strong enough nerve impulses
- enough acetylcholine secretion
- proper muscular anatomy
- enough calcium and creatine
- enough water and oxygen
- sufficient mineral content
How Movement is Initiated
Your nervous system controls the movements of your body, both voluntary and involuntary.
The physical muscle movement is coordinated and facilitated but also starts in your brain. The motor cortex is part of the brain (cerebral cortex) involved in motion, execution, coordination, and facilitation of voluntary movement.
Movement is first created in the motor cortex, then a nerve signal is transmitted through the spinal cord onto your efferent neurons and then muscles. When all parts of the chain are properly functioning, the muscle contracts (activates, flexes). This produces movements like lifting your hand, flexing your abdominals, or extending the knee joint. The left brain controls the right side and the right brain controls the left side of the body.
The cerebellum mainly functions to coordinate movement, which is done by comparing the information given from the motor cortex and how the movement is actually done. Sensory muscular input is required, so all the intramuscular sensors, as muscle spindles or proprioceptors work to bring quality information to the cerebellum. This feeling is translated through the afferent neurons to the cerebellum, where the comparison is made.
Autonomous Nervous System
Autonomic nervous system or ANS
The Autonomic nervous system or ANS works by itself and controls smooth muscle function, to ensure proper body homeostasis. Anything from fluid balance, breathing, and heart beating which holds homeostasis as the main priority is controlled by ANS. There are two subdivisions:
Parasympathetic (PSNS) functions to slow down the body, known as the rest and digest state. Characteristics would be digestion, feeding in the body, higher blood intake from internal organs, slower heartbeat, and breathing.
The sympathetic nervous system (SNS) functions to speed up and activate the body and its processes. Usually activates when we are active, and prepares the body for stress and action. Sweating is increased, as well as heartbeat, and breathing frequency. More blood is taken to the skeletal muscles.
Muscles can relax and contract. Contraction is the activation mode, while relaxation is like turning off. Not always when a muscle relaxes your bones will get back, which means that the antagonistic muscles have to work after your agonistic ones deactivate.
This means if you want to flex your arm, the bicep will pull on your bones, but when you relax it, the triceps needs to activate in order for you to extend the arm unless the proper gravitational position is present. Muscles work by pulling the bones, not pushing.
The muscle is the main tissue in the body responsible for the movement and production of force. Other muscular functions are support, energy production, lipid and mineral storage, internal organ protection, and full body stabilization.
Muscle categories: muscles can be divided into three broad categories: Cardiac (Heart), Smooth (Internal organs), and Skeletal Muscles (physical movement). The skeletal muscles can be further categorized based on structure as uni, bi or multipennate, flat or parallel, circular, fusiform, and convergent.
Around each muscle, there are tendons, ligaments, bones, and other connective tissue that helps with movement. Ligaments connect bone to bone, while tendons connect bone to muscle. Both provide stabilization and support to our frame, preventing injury and limiting ROM.
Muscle Contraction Mechanism: to flex, activate or contract a muscle we need a strong enough signal. This requires a chain of actions from electrical nerve signal transmission from the brain through the spinal cord, peripheral efferent neurons to muscle, acetylcholine secretion, calcium ions, and actin & myosin cross-bridge connection.
There are three main types of muscle fibers, IB slow oxidative ones, red in color with more hemoglobin, IIB white, fast and explosive ones that get fatigued quickly but can produce more force in a shorter time and in-between ones IIA, known as Intermediate fibers, that function between fast glycolytic and slow oxidative.
Muscle activation is controlled by the nervous system. Muscles can produce force and move our bodies by pulling the bones. There are many sensors involved in this process that give feedback to the cerebellum, known as proprioceptors.