Exercise | Isometric| Isotonic | Contraction

                       

                        Isometric | isotonic contraction

This article discusses different types of muscle contractions during exercise. Learn about the types of muscle proteins, the structure of muscles, and more here.

Keywords:  isotonic | isometric | muscle proteins | A band| I band | Z line |M line | size | sarcomere|

Table of contents

1.	Introduction
2.	Types of muscle protein
3.	Structure
4.	Isotonic and isometric
5.	work done by muscles

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 Introduction:

Exercise is any physical activity to improve or maintain physical fitness. Physical exercise is beneficial for an individual.

Muscle protein is more than any protein in the human body. Types of Muscle protein :

1. Myofibrillar or myofibrils, thick and thin filaments, are made up of myosin and actin, respectively. Other proteins are also present in small amounts.About 35% of skelteol protein is myosin.

2. Regulatory protein:-troponin, tropomyosin,beta-actin, gamma actin, and M-protein.

3. Stromal proteins: Collagen, Elastin, and reticulum.

4. Other proteins: for example, Z disc and other proteins.

Structure of skeletal muscles:

Muscle fibers show striations due to alternate dark and light cross bands due to refractive indices in muscle fiber. Muscle fiber and a muscle cell is the same thing. The muscle fiber is covered by areolar connective tissue, which covers the plasma membrane -sarcolemma, like other body cells.

Sarcoplasm is the cytoplasm of muscle fiber,sarcosomes are mitochondria, and sarcoplasmic reticulum is the endoplasmic reticulum of muscle fibers.

A band:

The highly refractile area is due to anisotropic myosin protein and produces the dark band in the muscle fiber called the 'A band.' Its length is 1.5 microns out of 2.5 microns of the sarcomere. Its length remains constant during contraction or stretching of the sarcomere.

H band: is a slightly less refractile area of (0.5 microns)in the center of the A band. Hensen discovered this band.

M line: is present in the center of the H band.

Pseudo-H-zone is the M line, along with narrow, less refractile areas on either side of the M line.

I band less refractile area due to isotropic actin protein alternating with A- band. Its length is 1 micron.

Its length changes during contraction and stretching.

Z -line is a narrow dark line in the I band center.

Sarcomere extends between two Z lines.

       Structure of a sarcomere, A,I, and H bands, Z and M lines 

                                        AI junction

                 This Photo is by Sandhya Prasad, Licensed under CCBY.

[Tendon is a rope-like connective tissue fiber that connects muscle to the bone.]

When muscle fibers contract, it pulls the endomysium, perimysium, and epimysium. They are elastic tissue.

                          

                       

% of contractile component and Parallel and Series elastic components.

                          This Photo is by Sandhya Prasad, Licensed under CCBY.

Muscle consists of contractile components representing the elasticity of muscles -

1. Three-fifths of muscle contractile components are due to the contractile protein of muscles as thick and thin filaments. Thick and thin filaments lie parallel to the muscles' elastic component. The thick and thin filaments act as a passive viscous part and offer little resistance to stretch.

2. Two-fifths of the total muscle protein represents the elasticity of the muscle fiber

(a)The Parallel elastic component (PEC) is attached to the connective tissue sheath parallel to the muscle fibers.

(b)The series elastic component(SEC) represents the Tendon's elasticity and is attached in series with the muscles.

Muscle protein= 3/5th myofibrils (contractile component) + 2/5th

 elastic component.

the elastic component consists of

(1) Parallel elastic component (PEC)

(2) Series elastic component (SEC)

Resistance to stretch is mainly due to the 'series elastic component,' and the 'parallel elastic component' provides little resistance to stretch.

Muscle does not obey the Hooks law as it is not a completely elastic component.

Tension develops when the resting muscle is stretched, which is not proportional to the stretch applied.

Lengths of muscles:

The optimal length of a muscle is the length of the muscle at which it develops maximum active tension.

The resting length of a muscle is the length of the muscle at which it is in a relaxed position+—usually resting length = optimal length.

The equilibrium length of a muscle is the length of the relaxed muscle when it is removed from its bony attachments.

The initial length of a muscle is its length before it contracts.

Types of muscle contractions:

Two types of contractions occur in skeletal muscles:

Isometric contraction: While contraction, muscle length does not changes but tension increases.

Isotonic contraction is when muscle length changes, but

tension remains equal. However, in isotonic contraction first part is an isometric contraction. That means before the isotonic contraction starts. It begins with an isometric contraction.

In isometric contraction (iso=equal,metric=length), as its name suggests, in an isometric contraction, muscle length is constant. However, in asymmetric contraction, muscle does not move, although it contracts.

Work done in isometric contraction is negligible.

Work done is calculated by:

{work done= force applied x distance}

In isometric contraction, there is no change in length,i.e., no displacement occurs. Therefore no work is performed.

Due to the contraction of thick and thin filaments of a muscle, shortening in muscle length occurs, which gets compensated by stretching a series of elastic components. Therefore, there is no change in the overall size of the muscle, but tension increases.

However, the distance is minimal, so the muscle does little(negligible) external work during isometric contraction.

Examples of isometric contraction:

Muscles in maintaining posture against gravity,

Contraction of the arm muscles when trying to push a wall.

Relation between length and tension of a muscle:

Passive or resting tension: Tension develops when the muscle is stretched without stimulation by applying weights to its lower end. The tension generated is called passive or resting tension due to the stretching of parallel and series elastic components.

Passive tension is due to PEC+ SEC.

Active tension is due to the contractile component(CC)

Active tension:

Stimulation transforms contractile components into an active structure, which converts chemical energy into mechanical work. As a result, PEC and SEC remain unchanged during contraction.

Total tension developed after stimulation

=active tension due to CC+ passive tension due to PEC and SEC.

Active tension= Total tension -Passive tension.

The contraction component can no longer contract when muscle length increases twice the optimal size. Therefore, active tension becomes zero, and total tension is due to PEC and SEC and is equal to the passive tension.

When muscle length is thrice optimal, the muscle will rupture when more tension develops.

When muscle length decreases less than optimal length and then is stimulated, passive tense will not develop because there is no stretch on the muscle. Instead, the total tension that creates after stimulation is due to the active tension.

The active tension is maximum when the muscle is at optimal length, that is, the length at which the muscle is present in that natural condition at rest in the body.

Tension development in isometric contraction is proportional to the cross-linkages between the actin and myosin filaments.

The average sarcomere length is 2.5 microns. At this length, cross-bridges between the head of myosin and actin molecules are maximum.

When a muscle is stretched, the overlap between actin and myosin is reduced, and the number of cross-linkages. This decreases the development of active forces.

When the muscle is shorter than the resting length-2.5 microns-the thin filaments overlap, reducing the number of cross-linkages.

Isotonic contraction (iso=same,tonic= tension).:

In isotonic contraction, muscle length shortens, but tension remains unchanged.

The length of the muscle changes; therefore, external work is done.

For example, movement of legs, walking, jogging, running, or weight lifting.

What is afterload preparation?

When a load is given to a muscle, the load will work only when the whole muscle begins to shorten.

On stimulation, muscle contracts. In addition, due to stimulation, the contractile component contracts and pulls the 'series elastic component,' increasing muscle tension.

When the force (tension) developed by the muscle exceeds the effect of load, the muscle begins to contract, and the tension in the muscle remains constant throughout the remainder of the shortening.

The load affects the isotonic contraction. Therefore, it varies according to the load.

Summary

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