Muscle Biomechanics: How Does The Muscle Work?

The muscles are very important for the functioning of the human body and so its survival.

Muscles have different important functions in the human body such as:

Movement: Muscles allow the movement of bones and related joints they are attached to.
Body Posture: the muscles support and sustain the entire human body’s skeletal system by supporting the weight of our body and by preventing us from collapsing.
Vital Functions: Some muscles constitute special organs such as the heart and the walls of the intestine that are essential to human life.
Heat production: Muscles during contraction produce heat that helps to keep our body warm at the right temperature.

Muscles are classified into three types:

Skeletal Muscles: These are a type of muscles that are attached to bones and allows us to move our limbs and body. They are also known as striated muscles because it has a striped appearance. They are voluntary muscles, which means that they are under conscious control.
Smooth Muscle: They are involuntary muscles, which means they are not under conscious control because they are found in the walls of internal organs, such as the stomach, intestines, and blood vessels that function without our voluntary control.
Cardiac Muscle: As the name specifies it is a special type of muscle that has a similar structure to striated muscles but it is an involuntary muscle as the heart is always contracting without the voluntary control of our brain.

Anatomy of the Muscle

Muscles are composed of specialized cells called muscle fibers. These muscle fibers are long, cylindrical cells.

The muscle fiber is composed of many myofibrils that are made up of bundles of protein filaments that form the contractile units called sarcomeres.

The sarcomeres represent the functional unit of the muscle fiber and they are disposed in series alongside the muscle fiber.

The sarcomere is the unit that contracts and relaxes when the muscle contracts and relaxes.

At the two ends of a muscle, there are two tendons that connect the muscle to the bones allowing its movement.

A tendon is a band of dense, fibrous tissue. It is made up of collagen fibers, which are arranged in parallel bundles. The collagen fibers are very strong and provide the tendon with its tensile strength.

Anatomy of the Sarcomere

The Sarcomere represents the contractile unit of the muscle fiber. The sarcomere is disposed of alongside the muscle fiber in series.

Sarcomeres are delimited by two Z lines, which are made up of specialized proteins called alpha-actinin that connect other two adjacent sarcomeres.

The sarcomeres are responsible for muscle contraction and are composed of two types of protein filaments:

Thin Filaments: They are made up primarily of the protein Actin, along with smaller amounts of other proteins such as tropomyosin and troponin.
Thick Filaments: they are made up of the protein Myosin.

The Z lines anchor the thin filaments and define the boundaries of the sarcomere.

Within a sarcomere, the thick and thin filaments overlap partially, forming distinct regions called the A band, I band, H Band, and M Line.

The A band contains both the thick and thin filaments.

The I band contains only thin filaments.

The H zone is a region where only thick filaments are present.

The M line is the central region of the sarcomere where the thick filaments are anchored.

The arrangement of actin and myosin filaments within the sarcomere is highly organized.

Each myosin filament is surrounded by a hexagonal array of actin filaments.

Controll system of the muscle fiber

At the core of this control system is the motor unit, which consists of a motor neuron and the muscle fibers it innervates.

The motor neuron innervates the muscle fiber through a specialized synapse, the neuromuscular junction.

When a motor neuron receives a signal from the CNS, it releases a neurotransmitter called acetylcholine at the neuromuscular junction.

Acetylcholine binds to receptors on the muscle fiber, triggering a series of events that terminated with calcium ions being released from the sarcoplasmic reticulum into the surrounding cytoplasm of the muscle fiber, which leads to muscle contraction.

The degree of muscle contraction is controlled by the recruitment of motor units. Motor units vary in size, with smaller motor units controlling fine movements and larger motor units responsible for more forceful contractions.

Motor neurons are specialized nerve cells that transmit signals from the central nervous system (CNS) to the muscles.

By selectively activating different motor units, the nervous system can control the strength and precision of muscle contractions.

The control system of muscles also includes sensory feedback.

Sensory receptors in muscles and tendons (such as Golgi Tendon Organ ) provide information about muscle length, tension, and position to the CNS.

This feedback allows for adjustments in muscle activity to maintain the body’s posture, balance, and coordination.

Biomechanics of the muscle fiber

The biomechanics of muscle contraction is the effect of the interaction between thin and thick filaments and so between the proteins Myosin and actin.

During muscle contraction, the myosin heads interact with the actin filaments, causing the thin filaments to slide inward toward the M line, shortening the sarcomere and resulting in muscle contraction.

Structure of the contractile proteins Mysion and Actin

Myosin

Myosin is a protein that consists of two heavy chains and four light chains, with the heavy chains forming the tail and the light chains forming the head. The myosin represents the thick filament. The myosin heads interact with the actin filaments during muscle contraction.

Image 2 A. Thick filament formation: The filament is formed by end-to-end association of myosin molecules.

Image 2 B. Thick Filament Segment: Crowns of three cross-bridges project at 14.3nm intervals and 120° apart from each other.

Actin

Actin is a globular protein that can assemble into a double-stranded helical filament. Actin is the primary component of the thin filaments.

The actin filaments in muscle fibers have additional associated proteins that regulate their interactions with myosin and contribute to the overall function of the muscle.

One such protein is tropomyosin, which lies along the length of the actin filament and helps regulate the exposure of the myosin-binding sites on the actin.

Another protein, troponin, binds to tropomyosin and undergoes conformational changes in response to calcium ions, allowing the myosin heads to bind to actin and initiate muscle contraction.

Muscle Contraction Process

The muscle contraction is represented by the so-called cross-bridge cycles ( Image A ) that refer to the series of molecular events that occur during the interaction between actin and myosin filaments within the sarcomere.

It is a cyclical process that involves the binding of myosin heads to actin filaments, the hydrolysis of ATP, and the detachment of myosin heads.

We can identify different stages of the cycle:

Binding:

The first act of this stage occurs when a muscle is stimulated by a nerve impulse and calcium ions are released from the sarcoplasmic reticulum into the muscle cell cytoplasm. Calcium binds to troponin, causing a conformational change that moves tropomyosin away from the actin-myosin binding sites. The second act is the binding of myosin heads with the actin, thanks to the hydrolysis of ATP in ADP+Pi, and the exposure of actin-binding sites, forming cross-bridges.

The cross-bridge structure between the actin and myosin head has a conformation of 90°.

This interaction releases the inorganic phosphate (Pi) molecule from the myosin head.

At this stage, there is still no contraction of the muscle fiber or sliding between thin and thick filaments.

Power Stroke:

During this stage, the myosin head undergoes a conformational change. This conformational change occurs after the release of ADP+Pi from the myosin head and consists of the change of the myosin head from 90° to 45° with respect to the actin filament ( Image B ).

This movement results in the myosin head pivoting and pulling the actin filament toward the center of the sarcomere.

Detachment:

During this stage, the ATP binds to the myosin head, causing it to detach from actin.

Recovery:

During this stage, ATP is hydrolyzed by the myosin head, providing the energy for the myosin head to return to its original position.

The myosin head can repeat the cycle by binding to another actin molecule further along the filament. The cycle continues as long as calcium is present, ATP is available, and there is sufficient stimulation.

Tension-length relationship of the muscle fiber

The tension-length relationship of the muscle fiber and so of its sarcomere refers to the relationship between the length of the sarcomere and the amount of tension or force it can generate during contraction.

This relationship is influenced by the overlap of actin and myosin filaments within the sarcomere.

When a muscle is at its optimal length, where there is an optimal overlap between actin and myosin filaments, it can generate the maximum amount of tension. This length is often referred to as the “resting length” or “physiological length” of the muscle.

At this length, there is optimal cross-bridge formation between actin and myosin, allowing for efficient force generation.

If the sarcomere is stretched beyond its optimal length, the overlap between actin and myosin filaments decreases. This reduced overlap decreases the number of cross-bridges formed, leading to a decrease in tension generation.

This phenomenon is known as the lengthening or passive tension.

Conversely, if the sarcomere is shortened beyond its optimal length, the actin and myosin filaments begin to overlap excessively. This excessive overlap leads the myosin to collide with line Z and the overlap of the actin filaments with itself, reducing the force-generating capacity of the muscle.

This phenomenon is referred to as the shortening or active tension.

In summary, the tension-length relationship of the sarcomere shows that there is an optimal length ( L0 ) at which a muscle can generate the maximum tension or force. Deviating from this optimal length, either by stretching or shortening the sarcomere, leads to a decrease in tension generation ( Image Below ).

This relationship is crucial for muscle function and helps explain the relationship between muscle length and force production in the body.

As the length and the density of tick and thin filaments are similar to the skeletal muscles of the vertebrates, they generate at L0 the same maximum force.

The maximum force generated by the muscle cross-sectional area in vertebrates is approximately 30N/cm² ( around 3 kg/cm² ).

Factors that can influence the generating force of a muscle.

For example, the biceps brachii muscle in the arm is designed to flex the elbow joint. When the elbow is flexed at approximately 80 to 100 degrees, the biceps brachii muscle is at its optimal length and can produce the most force.

In the image above, the last condition of the sarcomere at the bottom of it represents a condition of the muscle fiber when it is not able to contract anymore as there is no further contact between the myosin heads and the actin filament. For example, this is a condition that could happen when we have a muscle strain or a muscle tear, where the muscle fibers, due to excessive stretching, reach the breaking point and are not able to contract anymore.

Main Factors that can influence the generating force of a muscle

There are different factors that can affect the capacity of the sarcomere and so of the muscle fiber to generate the maximum force.

Here there is a list of the most important factors that influence muscle power:

Motor Unit Recruitment: The strength of neural input from motor neurons to the muscle fibers determines the degree of muscle activation and subsequent power generation. Higher neural drive, in terms of increased frequency and recruitment of motor units, can lead to greater force and power output.
Number of Cross-Bridges: As explained above, the power generated by a sarcomere is dependent on the number of cross-bridges formed between the thin and thick filaments.
Calcium Ions Availability: Higher calcium concentrations can increase the number of active cross-bridges and therefore enhance power generation.
ATP Availability: ATP provides the energy required for the detachment of myosin heads from actin, allowing them to reset and form new cross-bridges. Insufficient ATP levels can limit the cycling of cross-bridges and decrease power output.
Muscle Fiber Type: There are two main types of muscle fibers: slow-twitch (Type I) fibers and fast-twitch (Type II) fibers. Slow-twitch fibers are more fatigue-resistant and have lower power output but are capable of sustaining contractions for extended periods. Fast-twitch fibers, on the other hand, generate higher power output but fatigue more quickly.
Muscle temperature: Muscles work more efficiently and can generate more force when they are warm. Warmer muscle can improve many activities of the muscle fiber such as enzymatic activity, ATP turnover, Calcium Ion sensitivity, viscoelastic properties, and conduction velocity of action potentials. It’s worth noting that excessively high muscle temperatures can have negative effects on power generation! In fact, very high temperatures can denature proteins, impair enzymatic activity, and cause muscle fatigue. Low muscle temperatures can lead to decreased metabolic activity and reduced contractile function. Therefore, maintaining an optimal muscle temperature within a certain range is crucial for maximizing power output and overall muscle performance.

Mr Salus Sporting Lab