Muscle Contraction

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 (${\mathrm{K}}_{\mathrm{d}}$).

Figure 2 shows the ability of myoglobin and haemoglobin to bind oxygen. "${\mathrm{P}}_{{\mathrm{O}}_2}$" refers to partial pressure of oxygen, and "${\mathrm{O}}_2$ 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.

Fig. 2 - Haemoglobin versus myoglobin oxygen dissociation curve

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.

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.