Introduction
The muscular system play an essential role in the body as it produces movement and provides protection and support of various organs. Different types of activities require muscles to work in different ways while many of these activities require muscles to contract. Muscle cells contain an abundant amount of actin and myosin filaments which are specialised for contraction [1]. Muscle filaments can be broken down into three main types, namely smooth muscles, skeletal muscles and cardiac muscles. Contraction of cardiac muscles and smooth muscles is an involuntary reaction while the contraction of skeletal muscles is voluntary. Muscle contraction may be classified as isotonic or isometric depending on the arrangement of produced tension [2].
What is muscle contraction?
Skeletal muscles are known as contractile organs which consist of many motor units. Each unit consists of muscle fibres connected to a single motor neuron [1]. If there is a force which acts in opposition to the muscles, such as a weight for example, the muscle fibres will stretch thereby resulting in an increase in the tension. The contraction may not be enough to produce movement, but they do keep the muscle at a resting level of tension or tone [3]. Muscle tone is the resting tension in skeletal muscles and it helps to stabilize the position of the bones and the joints.
Isotonic contraction
The phrase ‘isotonic contraction’ is directly defined as ‘same tension’ whilst the word ‘isotonic’ is derived from two Greek words: ‘iso’ meaning ‘the same’ and ‘tonikos’ means ‘tension’ with respect to muscles [1]. As the name suggests, an isotonic contraction is one in which the muscles will maintain the same tension as it contracts or shortens. During isotonic contractions, a tension or force will develop up to a certain level. After this level, the tension remains constant while the length of the muscle will subsequently change. These motor units within the skeletal muscles are actually activated thereby allowing for the necessary tension in the muscle to develop [4]. Isotonic contractions are commonly used when moving limbs. Common examples of such activities include walking, running or even lifting objects.
Mechanism of isotonic contraction
Two main types of proteins found within muscles are responsible for isotonic contractions. These are the actin and myosin proteins. During isotonic contractions, thick strands of myosin and thin strands of actin move over each other. This sliding movement leads to a reduction in size within each individual muscle cell and in the overall entire muscle [4].
Types of isotonic contractions
Depending on the amount of force working against an individual’s body, one of two kinds of isotonic contractions will take place. These are concentric contractions and eccentric contractions [5]. Concentric contractions occur when muscles shorten while its tension is greater than the force opposing it [2]. Eccentric contractions on the other hand occur when muscles extend in length. The force in eccentric contractions is usually greater than the muscle tension which causes the elongation. The lengthening of muscles during eccentric contractions place a high level of stress on the working muscles and thus the possibility of muscle injury is much higher as when compared to concentric contractions [3].
Examples of isotonic contractions
Examples of a concentric contraction occur when an individual curls their arm. During curling, the muscles will shorten as the arm flexes at the elbow [4]. Extension of the elbow, walking down stairs or sitting on a chair would be a perfect example of an eccentric contraction which aids in controlling the rate of movement. As the arm is extended, the same muscle will lengthen and maintain the tension.
Isometric contraction
Isometric is directly defined as ‘same length’ whereby ‘iso’ means same and ‘metric’ means ‘length’ when referring to muscles [5]. During isometric contractions, the muscle itself does not change in length while the tension never exceeds the load that must be carried. This means that while the muscle itself does not shorten, the tension will never exceed the opposing force.
Mechanism of isometric contractions
One of the key facts about isometric contractions is that the muscles do not change in length during the contraction. Instead, they will remain their normal length. For example, consider a person holding a weight at a fixed position in front of their body [3]. Without any resistance, the weight will pull the persons arm to the floor however when they apply some form of resistance, the resulting stress will lead to an isometric contraction in the biceps of the upper arms. The amount of force produced during the isometric contraction will increase the length of the muscle being affected.
Examples of isometric contractions
Common examples of activities where muscles use isometric contraction include holding a weight in a certain place above the ground or pushing an object that was initially stationary [2]. As already mentioned, the length of the entire muscle will not change during an isometric contraction however, the respective muscle fibres will shorten which in turn leads to strengthening of the muscles.
Difference between isotonic and isometric contraction
While isotonic and isometric contractions form essential parts of the muscular contraction system, there are however major differences between them. In an isotonic contraction, the muscles maintain the same tension as it shortens while in an isometric contraction, the muscle remains the same length as the tension changes [5]. Isotonic contractions are known to have shorter contraction and relaxation times while isometric contractions have longer contraction and relaxation times. Changes in temperature affect each kind of contraction differently. While an increase in temperature increases the time taken for shortening of muscles during an isotonic contraction, it however decreases the time taken for an isometric contraction [3]. Isotonic contractions release a great deal of heat during muscle contraction making this less energy efficient while isometric contractions release less heat, making this a more energy efficient form of contraction. In addition, isotonic contractions occur in the middle of a contraction while isometric contractions occur at the beginning and end.
Conclusion
Daily activity involves a combination of both isotonic and isometric contractions. Establishing the difference between these two types of contractions is important as it can help individuals understand what happens when their muscles come under some form of physical stress. In addition, this understanding will aid redefining their work out routines and helping them to better take care of their bodies.
Summary of differences between isotonic and isometric contractions
| Isotonic contractions | Isometric contractions |
| The muscle length varies | The muscle length remains the same |
| The tension is constant | The tension varies |
| Shorter latent period, shorter contraction period, and a longer relaxation period. | Longer latent period, longer contraction period, and a shorter relaxation period |
| An increase in temperature increases the shortening period | An increase in temperature decreases isometric tension |
| Isotonic contractions are less energy efficient as more heat is released | Isometric contractions are more energy efficient as less heat is released |
| There is external work being done as shortening occurs | There is no external work being done as no shortening occurs |
| Isotonic contractions occur in the middle of a muscle contraction | Isometric contractions occur at the beginning and end of all muscle contractions |
| During muscle contractions, the isotonic of contraction decreases when the load increases | During muscle contractions, the isometric phase of contraction increases when the load increases |