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Muscle Contraction

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Muscle Contraction

Muscles are fascinating. Did you know there are many different types of muscles in the body? Did you also know that most muscles work in pairs? They carry out different types of contractions to generate movement.

Muscle classification

Muscle cells are classified into two categories based on their appearance: striated and non-striated (Figure 1).

Striated muscles are further broken down into two types, skeletal and cardiac muscles. One important feature that is common among striated muscles is that they contain myoglobin (a binding protein for oxygen and iron found in the cardiac and skeletal muscle tissues of vertebrates).

  1. Skeletal muscle: (also known as voluntary muscle). These muscles are:

    • The most common type of muscles in our body.

    • Under conscious control.

    • Attached to bones via tendons. They allow voluntary movement of limbs and the skeleton.

    • Examples:

      1. Bicep muscles

      2. Tricep muscles

      3. Quadricep muscles

  1. Cardiac muscle: (also known as myocardium).

    • This muscle is only found in the heart.

    • Its function is to contract and pump blood throughout the body.

    • Controlled involuntarily.

Non-striated muscles: (also known as smooth muscle). These muscles are different from skeletal muscles.

  • They do not contain any myoglobin.

  • They are controlled involuntarily.

  • They have various roles and functions in the body:

    1. Controlling the peristalsis process in the gut.

    2. Regulating the blood pressure by adjusting the resistance in blood vessels’ walls.

    3. Regulating the urine flow.

    4. Contractions of the uterus during pregnancy and labour.

Importance of myoglobin in muscle contraction

Myoglobin is a red protein that is structurally similar to a single subunit of haemoglobin.

While myoglobin and haemoglobin are both oxygen-storing molecules, myoglobin has a higher affinity for oxygen than haemoglobin (Figure 2). As a result, haemoglobin gives up oxygen to myoglobin, especially at low pH.

This behaviour is particularly important during an intense muscular activity where there will be a shortage of oxygen, and muscles will undergo anaerobic respiration.

A by-product produced during anaerobic respiration is lactic acid which lowers the pH in the muscles. Hence, during intense muscular activity, haemoglobin gives up oxygen more readily in the muscles to myoglobin. This oxygen is used in aerobic respiration to generate the ATP needed for muscle contraction.

Equilibrium dissociation curve

Affinity level of a molecule refers to how well it can interact and bind with another. This is reported by the equilibium dissociation constant ().

Figure 2 shows the ability of myoglobin and haemoglobin to bind oxygen. "" refers to partial pressure of oxygen, and " saturation" refers to how saturated myglobin and haemoglobin are with oxygen. As the partial pressure of oxygen gas increases, the oxygen saturation also increases until haemoglobin/myoglobin are saturated. Myoglobin has a higher affinity for oxygen and, therefore, it will become saturated with oxygen at lower pressures.

Types of muscle contraction

Skeletal muscle contractions are classified into two types based on the length of the muscle during contraction. These two types are isometric and isotonic.

Isometric muscle contraction

Isometric contractions generate force and tension while the muscle length stays relatively constant.

For example, muscles in the hand and forearm undergo isometric contraction when you make a tight grip. Another example would be during a biceps curl when you are holding a dumbbell in a static position instead of actively raising or lowering it (Figure 3).

Isotonic muscle contraction

As opposed to isometric contractions, the tension remains constant during isotonic contractions while the muscle length changes. Depending on the change in the muscle length, isotonic contractions can be either concentric or eccentric.

Concentric isotonic muscle contraction

Concentric contraction is a type of muscle activity that generates tension and force to move an object as the muscle shortens. Cross-bridge cycling between actin and myosin myofilaments and shortening of sarcomeres occur in concentric contraction.

This is the most common type of muscle contraction in our body.

For example, while lifting a dumbbell during a biceps curl, a concentric contraction causes the arm to bend at the elbow and lift the weight towards the shoulder (Figure 4).

Eccentric isotonic muscle contraction

During an eccentric contraction, the muscle elongates while still generating force. In other words, the resistance against the muscle is greater than the force generated, resulting in muscle elongation. Eccentric contraction is the strongest type of contraction which is mainly used for controlled weight movements.

Eccentric contractions can be either voluntary or involuntary. For instance, voluntary eccentric contraction allows the controlled lowering of a heavyweight object raised by a concentric contraction. An example of an involuntary eccentric contraction would be the involuntary lowering of a too-heavy object that slowly lowers under tension.

Cross-bridge cycles between actin and myosin filaments still occur in eccentric contraction, but the sarcomere and muscle length are elongated.

Mechanism of muscle contraction

Muscle cells (myofibers) contain contractile proteins such as actin and myosin filaments, collectively called myofilaments.

In skeletal muscles, these myofilaments are arranged into groups called sarcomeres which cause the myofibers to have a striated appearance (Figure 6).


Following nervous stimulation and release of calcium ions into the muscle fibre’s cytoplasm, the thin actin and thick myosin filaments slide past each other in a process called the sliding filament theory. Briefly, this process is driven by cross-bridges that extend from myosin filaments and recurrently interact with the actin filaments (Figure 7).

Muscle contraction is high energy-demanding activity. This energy is supplied via ATP hydrolysis at myosin heads. As a result of these fibres sliding over one another, the sarcomeres and muscle fibres shorten, leading to muscle contraction.

How do skeletal muscles bring about movement?

Muscles only produce tension which does not lead to effective movement unless it is being acted upon a structure that does not change shape, i.e., bone. Therefore, the movement of limbs requires both muscles and a firm skeleton.

Skeletal muscles are the most common type of muscles in the human body, with over 600 of them crossing over each other in multiple directions.

Muscles are usually attached to bones via lengths of very strong connective tissue called tendons. One of the many important properties of tendons is that despite their high flexibility, they do not stretch when the muscle contracts and pulls on them. Hence, they transmit all the generated force onto the bone. Some muscles have very long tendons, and others directly attach to bones.

Not all tendons are attached to bones, though. Some tendons connect muscles to the tendons of other muscles, such as the lumbrical muscles in the hand, which are connected to the FDP tendons.

Antagonistic action of muscles

Muscles are only capable of producing tension by pulling or contracting. Hence, they are unable to push or compress. Because of this limitation, muscles have to work in pairs to generate movements in different directions.

When two different muscles pull at a joint in opposite directions, they are acting antagonistically to each other. An example of antagonistic muscle action can be seen in the quadriceps and hamstring muscles of the thigh when we flex and extend our leg at the knee joint (Figure 8).

  • To extend the knee: the quadriceps muscles contract and the hamstrings relax.

  • To bend the knee: the hamstring muscles contract and the quadriceps relax.

Again, it is important to point out that this antagonistic action results in movement due to the incompressible bones.

One of the main functions of muscles is to maintain posture. This is achieved when pairs of antagonistic muscles contract isometrically at joints to keep the joint angle constant.

Synergistic action of muscles

In most cases, lifting heavy objects requires a more complicated contraction process with more muscles involved. For example, the biceps brachii muscles are the prime flexors of the elbow. In addition to biceps brachii, brachialis and brachioradialis muscles also flex the elbow when they contract (Figure 9). Therefore, these muscles are said to act synergistically, meaning that they assist each other during contraction.

Muscle Contraction - Key takeaways

  • Muscles are generally divided into two categories: striated and non-striated muscles. Striated muscles include cardiac and skeletal muscles. They both contain myoglobin and are composed of many contractile units called sarcomeres that give them their striated appearance. Non-striated muscles include smooth muscles. They do not contain any myoglobin or sarcomeres.
  • Myoglobin is an oxygen-binding protein that is found in striated muscles. It has a higher affinity for oxygen than haemoglobin. Therefore, it is able to readily unload oxygen from the blood haemoglobin and store it in the striated muscles for when they are needed.
  • There are two main types of muscle contraction: isometric and isotonic. Isotonic contraction is further divided into two categories: concentric and eccentric.
  • Muscles often work in pairs. Their actions are either antagonistic or synergistic. Antagonistic action involves two muscles that generate opposite movements by pulling on a joint in opposite directions. To generate movement, one needs to be relaxed for the other to contract. Synergistic actions involve one or more muscles that work together to generate movement by pulling on a joint in the same direction.

Frequently Asked Questions about Muscle Contraction

Muscle contraction is stimulated when an action potential from a motor neuron reaches the muscle. The action potential triggers an increase in the calcium ion concentration in the sarcoplasm. Calcium ions play a key role in cross-bridge formation between actin and myosin filaments. The energy released from ATP hydrolysis is utilised for the sliding of actin and myosin filaments over each other in a process called the sliding filament theory. As a result, the sarcomeres and muscle fibres shorten, causing muscle contraction.

During muscle contraction, the actin and myosin filaments slide past each other. Therefore, the sarcomeres and muscle fibres shorten in length. Skeletal muscles are attached to bones either directly or via tendons or by aponeuroses. The force created by the sliding of myofilaments during muscle contraction is transmitted to bones. Due to the rigid nature of bones, this force results in a change of angle at the joints and brings about movement.

Action potential received from a motor neuron triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin C and cause movement of tropomyosin away from actin-binding sites. Hence, allowing myosin and actin cross-bridge formation. The repeating cycle of actin and myosin cross-bridge formations, driven by ATP hydrolysis, results in the shortening of the sarcomeres’ length and causing muscle contraction.

When stimulated by a motor neuron, a skeletal muscle fibre contracts as the thin actin filaments are pulled and then slide past the thick myosin filaments within the myofiber's sarcomeres. This process generates tension and force, which are transferred to the skeletal system either directly or via tendons.

The plank, holding the dumbbell during a biceps curl, sitting stationary.

Final Muscle Contraction Quiz

Question

Name the three types of muscle in the body and give one example of each.

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Answer

1. Cardiac muscle found exclusively in the heart (myocardium)

2. Smooth muscle, found in the walls of blood vessels and the gut. 

3. Skeletal muscle, Biceps brachii

Show question

Question

What is the role of myoglobin in striated muscles?

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Answer

Myoglobin has a higher affinity for oxygen than haemoglobin. Therefore, it assists in unloading oxygen from haemoglobin and delivering it to muscles during intense activity.

Show question

Question

Define the terms isometric and isotonic contraction.


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Answer

Isometric contraction: Generation of tension in the muscle while its length remains constant. 

Isotonic contraction: Tension remains constant, but muscle length changes. 

Show question

Question

Name the three types of muscle contraction.

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Answer

Isometric, concentric isotonic, and eccentric isotonic contractions

Show question

Question

Why do sarcomeres shorten in length during muscle contraction? 

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Answer

Due to actin and myosin filaments sliding over one another.

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Question

What is the source of energy for muscle contraction? 

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Answer

ATP hydrolysis by the myosin head.

Show question

Question

What is the source of energy for muscle contraction? 

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Answer

ATP hydrolysis by the myosin head.

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Question

What does the term ‘antagonistic pair of muscles’ mean?


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Answer

Muscle can only pull and cannot push. Therefore, they need to work in pairs to move bones in different directions at the joints. Antagonistic pairs pull in different directions. When the agonist muscle contracts, its antagonist is relaxed.

Show question

Question

How do antagonistic muscles maintain posture?


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Answer

When they undergo isometric contraction simultaneously at a joint to keep the joint angle constant.

Show question

Question

Define the term ‘Concentric isotonic contraction’ and give an example.


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Answer

Concentric isotonic contraction is a type of muscle contraction during which the length of the muscle shortens and, as a result, brings the origin and insertion of the muscle closer. An example would be the contraction of the biceps muscle while lifting a dumbbell.

Show question

Question

In __________ contraction, the force generated than the resisting force, and the muscle shortens as it contracts.


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Answer

Concentric

Show question

Question

In __________ contraction, the force generated is smaller than the opposing force on the muscle. Hence, the sarcomeres lengthen as they contract. This type of contraction is used for decelerating a body part or gently lowering a load.


Show answer

Answer

Eccentric

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Question

In __________ contraction, the force generated equals the resisting force, and the muscle length remains constant. An example would be holding an object without any movement. 


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Answer

Isometric

Show question

Question

What is the definition of muscle?

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Answer

It refers to a number of muscle fibres bundled together in layers of connective tissue.

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Question

What is a myofiber?

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Answer

An individual muscle cell.

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Question

A muscle fibre contains multiple _______?


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Answer

Nuclei.

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Question

Can an injured muscle fibre be replaced by dividing existing skeletal muscle fibre? 


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Answer

No. Muscle fibres cannot divide.

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Question

How do muscle fibres get bigger? 


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Answer

By hypertrophy.

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Question

_____ filament is composed of 2 intertwined helical chains. Each of which contains a binding site for _____.


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Answer

Actin, myosin.

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Question

There are two I bands per sarcomere, one at each end. One end of the I bands is free. To what is the other end bound?

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Answer

Z line.

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Question

The portion of the thin filaments that does NOT overlap the thick filaments is called _____.


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Answer

I band.

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Question

What do we call a narrow dark band in the centre of the H zone that corresponds to proteins linking together the central region of adjacent myosin filaments?


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Answer

M line.

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Question

What is the section between two Z lines called?


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Answer

A sarcomere.

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Question

Name a series of tubular sleeve-like segments surrounding each individual myofibril. Their main role is to store calcium ions.


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Answer

Sarcoplasmic reticulum.

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Question

Name three common causes of muscle spasms.


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Answer

  1. Overuse and muscle fatigue.
  2. Dehydration.
  3. Electrolyte abnormalities.

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Question

The sliding filament theory involves the act of five different molecules + calcium ions. Name them.

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Answer

1.Myosin 

2.Actin

3.Tropomyosin

4.Troponin, 

5.ATP

 + 6.Calcium ions

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Question

What are the six steps of the cross-bridge formation cycle?

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Answer

1. The influx of calcium ions 🡺 triggers the unblocking of the actin-binding sites.

2. Myosin head binds to actin. 

3. The power stroke of the myosin head causes the sliding of the thin actin filaments.

4. The binding of ATP to myosin head resulting in the cross-bridge detachment.  

5. The hydrolysis of ATP, which re-energizes the myosin head and makes it ready for the next cycle. 

6. The transport of calcium ions back to the SR.

Show question

Question

An ___brings about the release of calcium ions from the _______.


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Answer

  • Action potential
  • SR

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Question

Calcium ions flood into the sarcoplasm and _____, causing a ________of the troponin-tropomyosin complex.

This conformation change ____the binding sites on ____.

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Answer

  • Bind to the troponin c
  • Conformational change
  • Unblocks
  •  Actin filaments

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Question

The binding of myosin to actin brings about a _____ of the cross bride, resulting in the release of ___and___.

At the same time, the cross-bridge ___, pulling the ____inward toward the _____. This movement is called the "_____."

The _____has been transformed into the ____of contraction.

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Answer

  • conformational change
  • ADP 
  • phosphate ion
  • flexes
  • actin filament
  • M line
  • Power Stroke
  • chemical energy of ATP hydrolysis
  • mechanical energy

Show question

Question

The release of the myosin cross-bridge from actin triggers the ____of the ATP molecule into ___ and ___.


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Answer

  • hydrolysis
  • ADP  
  • phosphate ion

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Question

Calcium is _____ from the sarcoplasm into the ____by ____.

As the calcium is removed, the troponin-tropomyosin complex again ____the binding sites on ____.

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Answer

  • Actively transported
  • SR
  • Calcium ion pumps (SERCA)
  • Blocks
  • Actin

Show question

Question

What is a myofiber?


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Answer

An individual muscle cell.

Show question

Question

A muscle fibre contains multiple what?


Show answer

Answer

Nuclei

Show question

Question

The portion of the thin filaments that does NOT overlap with the thick filaments.


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Answer

I band

Show question

Question

Slow-twitch muscle fibres mainly use _______ respiration?

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Answer

Aerobic

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Question

Fast-twitch muscle fibres mainly use _______ respiration?

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Answer

Anaerobic

Show question

Question

What are the characteristics of slow-twitch muscle fibres contraction?


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Answer

Slow, less powerful, used over a longer period.

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Question

What are the characteristics of fast-twitch muscle fibres contraction?


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Answer

Rapid, more powerful, used over a shorter period.

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Question

What activities are the slow-twitch fibres best suited for?


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Answer

Endurance type activities

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Question

Where would you be likely to find slow-twitch fibres? Why?


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Answer

Neck and back muscles. They need to contract for long periods to maintain the body in an upright posture.

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Question

Where would you be likely to find fast-twitch fibres? Why?


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Answer

Bicep muscle. They are used for short bursts of intense activity like lifting a heavy dumbbell.

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Question

Which muscle fibre contains large stores of glycogen? why?


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Answer

Fast-twitch fibre. Glycogen provides a source of glucose for rapid glycolysis to generate ATP.

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Question

Which muscle fibre contains large amounts of myoglobin? why?


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Answer

Slow-twitch fibres. Myoglobin helps in the delivery of oxygen from the blood to the tissues

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Question

Which muscle fibre type contains more mitochondria?


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Answer

Slow-twitch fibres

Show question

Question

What does affinity mean?

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Answer

The ability of two molecules to interact and bind.

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