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Muscle tension is the force exerted by a muscle on an object, and it is generated by the contraction of the muscle. Muscle contraction can be described in terms of two variables: length and tension. The force generated by a contracting muscle is dependent on the length of the muscle and its shortening velocity. Muscle tension can be produced without changes in the length of the muscle, such as when holding a dumbbell in the same position. Muscle tension is also generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.
| Characteristics | Values |
|---|---|
| Muscle contraction variables | Length and tension |
| Muscle tension | The force exerted by the muscle on an object |
| Load | The force exerted by an object on the muscle |
| Isotonic contraction | Constant tension with a change in muscle length |
| Isotonic contraction types | Concentric (shortening) and eccentric (lengthening) |
| Isometric contraction | Change in tension without a change in muscle length |
| Muscle fibre shape | Variable with biochemical, mechanical, and metabolic phenotypes |
| Muscle cell organelles | Change in structure and function with exercise levels |
| Muscle relaxation | Return of muscle fibres to a low-tension state |
| Muscle types | Skeletal, cardiac, and smooth |
| Muscle tone | Small degree of tension even when muscles are at rest |
| Muscle tone regulation | Controlled by neuronal impulses and muscle spindle |
| Muscle force | Dependent on length and shortening velocity |
| Muscle elasticity | Due to the presence of titin |
| Muscle activation | Predictive of the load it will encounter |
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What You'll Learn
Muscle contraction types: isometric and isotonic
Muscle contractions are fundamental to our ability to perform different movements. There are three main types of muscle contractions: isotonic, isometric, and isokinetic.
Isotonic Contractions
Isotonic contractions are characterized by constant muscle tension and a change in muscle length. This type of contraction occurs when the contraction force matches the total load on a muscle. Isotonic contractions are further divided into two types: concentric and eccentric. A concentric contraction causes the muscles to shorten, thereby generating force. An eccentric contraction causes the muscles to elongate in response to a greater opposing force. Isotonic contractions are seen during activities such as walking, running, or squatting.
Isometric Contractions
Isometric contractions are characterized by a change in muscle tension without a change in muscle length. These contractions occur when pushing against an immovable object or trying to lift a weight that is too heavy. Isometric contractions are frequently used to maintain posture and maintain bone and joint stability. They are also required for some functional activities.
Isokinetic Contractions
Isokinetic contractions are similar to isotonic contractions in that the muscle changes length during contraction. However, the key difference is that isokinetic contractions produce movements of a constant speed. An example of this type of contraction is the breaststroke in swimming, where the water provides a constant, even resistance to the movement.
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Muscle fibre structure
These muscle fibres are composed of myofibrils, which are made up of contractile, regulatory, and structural proteins. Myofibrils are the protein structures that give skeletal muscle its striated appearance under a microscope. Each myofibril contains sarcomeres, which are the smallest functional unit of a skeletal muscle fibre. Sarcomeres are composed of thick and thin filaments of myosin and actin, respectively, which are types of protein. The thick and thin filaments overlap, and this zone of overlap is where the muscle generates tension and contracts.
The shape of muscle fibres is variable and depends on biochemical, mechanical, and metabolic phenotypes. The muscle cell organelles can change in structure and function with exercise levels; for example, endurance training induces mitochondrial biogenesis, increasing their size and number. The T-tubule system is part of the muscle fibre's cell membrane and is important for conducting nerve impulses to the cell's interior.
Skeletal muscle fibres are surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. The sarcolemma is surrounded by three layers of connective tissue called the mysia, which enclose, provide structure, and compartmentalise the muscle fibres. The outermost layer is the epimysium, which separates the muscle from other tissues and organs, allowing independent movement. The middle layer is the perimysium, which surrounds bundles of muscle fibres called fascicles. The innermost layer is the endomysium, which surrounds each individual muscle fibre and contains capillaries and nerve tissue to supply the muscle fibres.
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Muscle tone
There are two main types of skeletal muscle contractions: isotonic and isometric. Isotonic contractions are characterized by constant muscle tension with a change in muscle length, such as when walking, running, or squatting. During isotonic contractions, the tension in the muscle stays constant, and the load is moved as the length of the muscle changes. Isometric contractions are characterized by a change in muscle tension without a change in muscle length, such as when pushing against an immovable object or trying to lift a weight that is too heavy. During isometric contractions, the muscle length does not change because the load exceeds the tension the muscle can generate.
Intracellular calcium concentration increases when calcium enters the cell and is released from the SR. Calcium binds to calmodulin, which activates myosin light chain kinase (MLCK). MLCK phosphorylates myosin head light chains and increases myosin ATPase activity. Active myosin cross-bridges slide along actin and create muscle tension. Smooth muscle relaxation occurs as free calcium in the cytosol decreases when calcium is pumped out of the cell or back into the SR.
The nervous system plays a crucial role in controlling muscle tension. Muscle tone may reflect a state of preparedness for movement, with the neuromotor apparatus responding to commands from upper levels of movement construction. This definition positions muscle tone as an active contributor to movement and postural tasks.
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Muscle tension and length
Muscle tension is the force exerted by a muscle on an object, while a load is the force exerted by an object on the muscle. Muscle tension is generated by the contraction of the muscle, which causes the muscle fibres to shorten and create a force. The force generated by a contracting muscle is what we refer to as muscle tension.
The physiological concept of muscle contraction is based on two variables: length and tension. Muscle shortening and muscle contraction are not the same thing. Tension within a muscle can be produced without any changes in its length, such as when holding a dumbbell in the same position or holding a sleeping child in your arms.
When a muscle contracts, the myosin heads attach to actin to form cross-bridges. The thin filaments then slide over the thick filaments as the heads pull the actin, resulting in sarcomere shortening and creating the tension of the muscle contraction. Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80% to 120%, approximately 1.6 to 2.6 micrometres. This is known as the ideal length of a sarcomere.
There are two main types of skeletal muscle contractions: isotonic contractions and isometric contractions. In isotonic contractions, the tension in the muscle stays constant, but the length of the muscle changes as it moves a load. This can be further divided into concentric contractions, where the muscle shortens to move a load, and eccentric contractions, where the muscle lengthens under tension. An example of an eccentric contraction is the controlled lowering of a heavy weight.
In isometric contractions, the muscle length does not change because the load exceeds the tension the muscle can generate. Isometric contractions involve sarcomere shortening and increasing muscle tension, but they do not move a load because the force produced cannot overcome the resistance provided by the load. For example, when trying to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the position of the hand weight.
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Muscle tension and load
Muscle tension is the force exerted by a muscle on an object, while a load is the force exerted by an object on the muscle. The force generated by a contracting muscle is called muscle tension. Muscle tension can be produced without changes in the length of the muscle, such as when holding a dumbbell in the same position or holding a sleeping child in your arms.
To move an object, or load, the muscle fibres of a skeletal muscle must shorten. The shortening of the muscle fibres or sarcomeres results in the generation of muscle tension. The amount of tension generated depends on the magnitude of overlap between the actin and myosin myofilaments. Sarcomeres produce maximum tension when thick and thin filaments overlap by about 80-120%. If a sarcomere is stretched too far, there will be insufficient overlap of the myofilaments and less force will be produced.
When the muscle contracts against a load that does not move, it results in two main types of skeletal muscle contractions: isotonic and isometric contractions. In isotonic contractions, the tension in the muscle stays constant, and the load is moved as the length of the muscle changes. There are two types of isotonic contractions: concentric and eccentric. A concentric contraction involves the muscle shortening to move a load. For example, when performing a biceps curl, the biceps brachii muscle contracts, resulting in an increase in muscle tension and a decrease in the angle of the elbow joint.
In eccentric contractions, the muscle lengthens while still generating force, and the resistance is greater than the force generated. An example of an eccentric contraction is the controlled lowering of a heavy weight. During isometric contractions, the muscle tension changes without any change in muscle length. An example of an isometric contraction is trying to lift a weight that is too heavy, resulting in increasing muscle tension without any change in the position of the weight.
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Frequently asked questions
Muscle tension is the force exerted by a muscle on an object.
There are two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.
Isotonic contractions are contractions where muscle tension remains constant, but the length of the muscle changes.
Isometric contractions are contractions where muscle tension changes but the length of the muscle remains the same.
Muscle tone, or tonus, refers to the small degree of tension that is present even when muscles are at rest.
