Muscle contraction is pretty cool. There are a ton of muscles in your body work in different ways to move. In fact, muscles work together in pairs! It's amazing to think about how these muscles contract and relax to create all the movements we make.

Muscle classification

Muscle cells come in two categories based on their appearance: striated and non-striated. Striated muscles are broken down into two types: skeletal and cardiac muscles. Striated muscles have myoglobin, which is a protein that binds oxygen and iron found in cardiac and skeletal muscle tissues. Skeletal muscles are the most common type in our body and are under our conscious control. They are attached to bones with tendons, helping us move our limbs and skeleton. Examples include the bicep, tricep, and quadricep muscles. Cardiac muscles are only found in the heart, aiding in pumping blood throughout the body. They are controlled involuntarily. Non-striated muscles, also known as smooth muscles, don't have myoglobin and are controlled involuntarily. They have many different roles in the body, like controlling peristalsis in the gut, regulating blood pressure by adjusting resistance in blood vessels, regulating urine flow, and contracting the uterus during pregnancy and labor.

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Importance of myoglobin in muscle contraction

Myoglobin and haemoglobin are molecules that store oxygen, butoglobin has a higher affinity for oxygen than haemoglobin. This means that when there is low pH, haemoglobin gives up oxygen to myoglobin. This is important during intense muscular activity when there is a shortage of oxygen and muscles undergo anaerobic respiration, producing lactic acid which lowers the pH in the muscles. Haemoglobin gives up oxygen more easily to myoglobin during intense muscular activity, and this oxygen is used in aerobic respiration to generate the ATP needed for muscle contraction. The level of a molecule refers to how well it can bind with another molecule, reported by the equilibrium dissociation constant (). Figure 2 shows the ability of myoglobin and haemoglobin to bind oxygen, where "" is the partial pressure of oxygen, and "saturation" shows how much oxygen myoglobin and haemoglobin are saturated with. Myoglobin has a higher affinity for oxygen, becoming saturated at lower pressures.

Types of muscle contraction

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

There are two types of muscle contractions: isometric and isotonic. In isotonic contractions, the tension remains constant while the muscle length changes. Depending on the change in muscle length, isotonic contractions can be either concentric or eccentric. Concentric contraction 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, making it the most common type in our body. For example, lifting a dumbbell during a biceps curl uses concentric contraction to bend the arm at the elbow and lift the weight towards the shoulder. Eccentric contraction, on the other hand, elongates the muscle 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 and is mainly used for controlled weight movements, either voluntary or involuntary. 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

Myofibers, or muscle cells, contain contractile proteins such as actin and myosin filaments, which are collectively known as myofilaments. In skeletal muscles, these myofilaments are arranged into groups called sarcomeres, which give the myofibers a striated appearance. When a muscle is stimulated by nerves, calcium ions are released into the muscle fiber's cytoplasm, causing the thin actin and thick myosin filaments to slide past each other in a process called the sliding filament theory. This process is driven by cross-bridges that extend from myosin filaments and interact with the actin filaments. Muscle contraction is a high energy-demanding activity, with energy being supplied via ATP hydrolysis at myosin heads. As a result of these fibers sliding over one another, the sarcomeres and muscle fibers shorten, leading to muscle contraction.

How do skeletal muscles bring about movement?

In order for effective movement to occur, muscles need to produce tension on a structure that does not change shape, such as bone. This is why movement of limbs requires both muscles and a firm skeleton. The human body has over 600 skeletal muscles, which cross over each other in multiple directions. These muscles are typically attached to bones via strong connective tissue called tendons. Despite their high flexibility, tendons do not stretch when the muscle contracts and pulls on them, allowing them to transmit all of the generated force onto the bone. Some muscles have long tendons, while others directly attach to bones. However, not all tendons are attached to bones, as some connect muscles to the tendons of other muscles, such as the lumical muscles in the hand, which are connected to the FDP tendons.

Antagonistic action of muscles

Muscles are limited to producing tension by pulling or contracting, and are unable to push or compress. As a result, muscles work in pairs to generate movements in different directions. When two different muscles pull at in opposite directions, they are acting antagonistically to each other. An example of this can be seen in the quadriceps and hamstring muscles of the thigh when we flex and extend our leg at the knee joint. To extend the knee, the quadriceps muscles contract and the hamstrings relax. To bend the knee, the hamstring muscles contract and the quadriceps relax. It is important to note that this antagonistic action results in movement due to the incompressible bones. One of the primary functions of muscles is to maintain posture, which is achieved when pairs of antagonistic muscles contract isometrically at joints to keep the joint angle constant.

Synergistic action of muscles

When lifting heavy, a more complicated contraction process is typically required with more muscles. For example, the biceps brachii muscles are the prime flexors of the elbow. In addition to the biceps brachii, the brachialis and brachioradialis muscles also flex the elbow when they contract. These muscles are said to act synergistically, meaning that they assist each other during contraction. Muscles are generally divided into two categories: striated and non-striated muscles. Striated muscles include cardiac and skeletal muscles, which both contain myoglobin and are composed of many contractile units called sarcomeres that give them their striated. Non-stri smooth muscles, which do not contain any myoglobin or sarcomeres. Myoglobin is an oxygen-binding protein found in striated muscles that has a higher affinity for oxygen than hemoglobin, allowing it to readily unload oxygen from the blood and store it in the striated muscles for when it is 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, with their actions either being antagonistic or synergistic. Antagonistic action involves two muscles that generate opposite movements by pulling on a joint in opposite directions, while synergistic actions involve one or more muscles that work together to generate movement by pulling on a joint in the same direction.

Muscle Contraction

How do muscles contract?

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. 

What happens when a muscle contracts?

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.

What causes muscle contraction?

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.

What is skeletal 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.

What is an example of an isometric contraction?

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

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