What is Muscle Contraction?- Definition and Mechanism

When we are involved in certain kinds of activities, then muscle starts to tighten, contract, shorten, or lengthen, and this phenomenon is called muscle contraction.

Muscle contraction occurs along with muscle relaxation means muscles must return back to the normal state. Muscle contraction helps to maintain postures like sitting and standing, maintain the body temperature by producing heat, and maintain the stability of joints and connective tissues.

Our muscle consists of muscle fibers called myosin, and this muscle fiber either tightens up, loosens up, or stretches out depending on how we need to use our muscle. The contraction of heart muscle at a regular rate is also brought by the myosin.

The mechanism of muscle contraction is explained by the sliding filament theory and is one of the widely explained theories that explain how the muscle contracts. This theory was proposed in 1994 by a team of scientists named H.E Huxley, J. Hanson, A.F Huxley, and R. Niedergerke. According to this theory, myosin filaments utilize the ATP energy molecule and move along the actin filament with their cross bridges. This pulls the actin filament closer, and this movement leads to pulling the Z line close together, and the sarcomere is shortened.

  • Within the sarcomere, the arrangement of myosin and actin myofilament plays a vital role in muscle contraction.
  • When the myosin and actin myofilament pass each other by sliding, the muscle contracts and shortens the distance.
  • The sarcomere’s length contracts, leading to muscle contraction, and during the contraction of muscle, A-band (dark band) remains the same while I-Band (light band) and Z-zone get shortened.

Actin and Myosin myofilaments during contraction

Actin Myofilament

  • An actin myofilament is composed of actin molecules, tropomyosin, and troponin complex, where troponin is again subdivided into three components (troponin I, T, and C). 
  • Actin molecules (G-actins) are wrapped longitudinally in thin, twisted standard form by two helical strands called tropomyosin.
  • An ATP molecule is bound with each G-actin, and the actin molecule’s whole assembly is known as F-actin (Fibrous actin).
  • The muscle contraction mechanism is switched ON or OFF by tropomyosin.
  • A globular protein called troponin binds to calcium ions and tropomyosin.
Structure of Sarcomere

Myosin Myofilament

  • Two distinct regions make up myosin myofilaments: a long rod-shaped tail and two globular intertwisted heads called myosin rod and myosin head.
  • This globular head appears to be at a regular interval on the filament.
  • The myosin head binds with the actin myofilament and helps slide the filament, causing the muscles’ movement.

Mechanism of Muscle Contraction

Calcium ions play a vital role in muscle contraction. Calcium ions are released in the terminal axon when the nerve impulses from the brain and spinal cord are carried along motor neurons to the neuromuscular junction. When the calcium ion concentration increases, it stimulates the release of a neurotransmitter called Acetylcholine in the synaptic cleft. The released Acetylcholine then depolarizes and generates action potential across the muscle fiber for muscle contraction after binding to the receptor on the sarcolemma. Then the generated action potential propagates over the entire muscle fiber, and the movement is carried out along the adjacent fibers throughout the transverse tubules. Calcium ions are released from the sarcoplasmic reticulum due to the generation of the action potential in the transverse tubules, leading to muscle contraction.

1. Blocking of the myosin head:

  • A Cross bridge is formed by overlapping of actin and myosin filaments, and this cross bridge is active only when the myosin head binds to the actin filament.
  • Tropomyosin generally blocks the overlapping of the actin and myosin myofilament when the muscle is at rest.
  • During the blockage, the actin myofilament is said to be in the OFF position.

2. Release of the calcium ion:

  • The release of the calcium ion from the sarcoplasmic reticulum is triggered by the depolarization and action potential generation in the sarcolemma by nerve impulses.
  • The released calcium ion causes displacement of troponin complex and tropomyosin from its blocking site exposing the myosin-binding site after binding with the troponin complex on the actin myofilament.
  • The exposure of the myosin head leads to a cross-bridge with the actin myofilament, which is said to be at the ON position.

3. Active cross-bridge formation:

  • The active cross-bridge is formed when the myosin head is attached to the neighboring actin myofilament like a hook. Now the cross-bridge between actin and myosin myofilament acts as an enzyme (Myosin ATPase).
  • The energy is released by the hydrolysis of stored ATP into ADP, and inorganic phosphate by the same enzyme called Myosin ATPase. The released energy is used for the movement of the myosin head to the actin myofilament. The myosin head tilts itself and pulls the actin filament alongside so that the myosin and the actin myofilaments slide each other.
  • Within the sarcomere, the opposite end of the actin myofilament moves towards each other leading to muscle contraction.
  • Once the filaments slide along each other, the formed cross-bridge detaches, and the myosin and actin filaments come to the original position. The active cross-bridge forms and reforms 50-100 times within a fraction of a second by the use of ATP quickly. As there is a requirement for energy in a rapid fashion, muscle fibers consist of the mitochondria.
  • During muscle contraction, the sarcomere can contract up to 30-60 % of its original length.
cross-bridge formation
cross-bridge formation

Tests for checking Muscle Health

It is necessary to check the health of the muscle, and there are different tests for muscle health checkups. Some of them include:

  • Complete blood count (CBC): 

It is a kind of blood test that evaluates overall health and checks for infections.

  • Electromyography (EMG):

It is one health test that measures the relationship between the nerve and muscle or how they work.

  • MRI:

It is one of the imaging techniques that shows pictures of damaged tissues.

  • Muscle Biopsy is used to test the sample of the muscle tissue for diseases.

How can we keep our muscles healthy?

So the big question arises about keeping the muscles healthy. We can do that by focusing on staying healthy overall:

  • Plenty of exercises:

Staying active is one of the main factors for keeping the muscles strong including the heart. Performing cardiovascular activity and weight-bearing exercise daily is the side for remaining healthy along with the muscles being healthy. It is necessary to take the proper advice of the trainer about the exercise that is right for oneself.  Also, warming up is a good idea before doing any heavy exercise because warm-up helps to protect the muscles from injury.

  • The right food and smart choices:

To keep muscles strong and healthy, a balanced diet and healthy food is necessary. Different kinds of food should be avoided that contain a huge amount of sodium and trans fat (mostly fried foods), which can be one of the potential causes of heart diseases. If an individual has a habit of smoking, then it is necessary to ask for suggestions to quiet the smoking habits.

  • Maintaining a healthy weight:

Gaining an extra pound of weight is not good at all for muscles as it can cause injuries. It also increases the problems related to pressure like high blood pressure, which is not good for health. To have a better lifestyle and healthy muscle, it becomes necessary to maintain the weight of the body. We can ask the trained person about the correct weight according to the person’s height and age.

  • Rest during any injury:

After a strain, an individual should give time to repair it, and it becomes an important task to take rest. One should also rest when they feel soreness after doing rigorous exercise. Allowing your muscle to repair and giving time for rest can help to recover from injuries.

  • Regular screenings:

One of the important factors for keeping the muscles healthy is by keeping records of everything, including types of food, exercise time, and health checkups. When there is a higher risk of muscle problems related to the diseases, it is necessary to get health tips and track muscle injury records. When the problems are detected early, it becomes easy to provide the right treatment at the right time.

  • Healthy bones: 

Most of the muscle weight is carried and supported by bones throughout the body. So healthy bones go side by side with healthy muscles. If the bone density is less, bones get weak, because of which, we cannot perform any kind of exercise to retain the muscles. So to get the bones strong, a diet rich in calcium and potassium is necessary.


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About Author

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Mohan Gupta

Mohan Gupta is a recent undergraduate in Biotechnology and recently working on Probiotics to identify the gene responsible for inhibition of Antimicrobial resistance bacteria. He is a Biotechnologist, Biohacker, Researcher, Business lead at Media Lab Nepal, and ex-cofounder at Baja Nepal.

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