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Muscle contractions are the tightening, shortening, or lengthening of muscles during activity. They can be described in terms of two variables: length and tension. Muscle length can change while muscle tension remains the same, resulting in an isotonic contraction, or muscle tension can change while muscle length stays the same, resulting in an isometric contraction. During an isotonic contraction, the muscle can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction. This means that muscles can actively lengthen during activity, and this phenomenon has been observed in various movements and activities.
| Characteristics | Values |
|---|---|
| Definition | Muscle contraction is the activation of tension-generating sites within muscle cells. |
| Muscle Lengthening and Activity | A muscle can lengthen and be active (an eccentric contraction), can lengthen and be inactive (a relaxed muscle), or can lengthen and gradually change from active to inactive or vice versa. |
| Muscle Contraction Types | Muscle contraction can be isometric (constant length) or isotonic (shortening against a constant load). |
| Muscle Length and Tension | Muscles operate with the greatest active tension when close to an ideal length (often their resting length). |
| Muscle Lengthening and Locomotion | Relatively little is known about the mechanics and energetics of activated muscle during forced lengthening. |
| Muscle Lengthening and Energy | When an active muscle is lengthened during an eccentric contraction, it behaves like a shock absorber-spring complex, and energy is lost as heat. |
| Muscle Lengthening and Growth | Eccentric exercises improve the length-force characteristics by improving both the maximum force and length range of active force exertion. |
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What You'll Learn
Muscle contractions are isometric or isotonic
Muscle contractions can be described in terms of two variables: length and tension. In natural movements, muscle contractions are multifaceted as they can produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is likely to remain the same in skeletal muscles that contract during locomotion.
Muscle contractions can be classified as either isometric or isotonic. An isometric contraction is when the muscle tension changes but the muscle length remains the same. In other words, an isometric contraction generates force without changing the length of the muscle. For example, when holding a heavy weight steady, neither raising nor lowering it, the muscle is under tension but neither shortens nor lengthens. Isometric contractions are performed without joint motion and are frequently used to maintain posture.
On the other hand, an isotonic contraction occurs when the muscle length changes while the tension remains the same. This means that the muscle changes length while maintaining constant tension. Isotonic contractions are performed with joint motion and can be further classified into two types: concentric and eccentric. A concentric contraction occurs when the muscle shortens, resulting in joint motion. For example, when lifting a heavy weight, the biceps contract and shorten, causing the arm to bend at the elbow and lift the weight towards the shoulder. An eccentric contraction occurs when the muscle lengthens while still generating force. For instance, when lowering a heavy weight, the bicep muscle remains contracted but lengthens as the weight is lowered.
It is important to note that the terms "isometric" and "isotonic" refer to the length of the muscle during contraction, rather than the actual physical size of the muscle. Additionally, most physical activities involve a combination of both forms of muscle contraction, although one form usually predominates.
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Muscle length changes during locomotor activity
Muscle contractions are the activation of tension-generating sites within muscle cells. Muscle contractions do not always result in muscle shortening because muscle tension can be produced without changes in muscle length. For example, when holding something heavy in the same position, muscle tension is produced without any change in muscle length.
In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is likely to remain constant when the muscle is active during locomotor activity.
Muscles operate with the greatest active tension when they are close to their ideal length, which is often their resting length. When stretched or shortened beyond this, the maximum active tension generated decreases. This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal.
The force–velocity relationship relates the speed at which a muscle changes its length to the amount of force that it generates. Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. The reverse is true when the muscle is stretched – force increases above the isometric maximum, until finally reaching an absolute maximum.
There are three types of muscle contractions: isometric, isotonic, and eccentric. Isometric contractions occur when muscle tension changes without any corresponding changes in muscle length. Isotonic contractions occur when muscle length changes while muscle tension remains the same. In an isotonic contraction, the muscle can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction. Eccentric contractions happen when a muscle is actively lengthened during normal activity, such as walking.
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Muscle length changes through active or passive mechanical loading
Muscle length changes are a result of active or passive mechanical loading. Muscle contraction is the tightening, shortening, or lengthening of muscles when an activity is performed. This can occur when holding or picking up an object, stretching, or exercising with weights. Muscle contraction is followed by muscle relaxation, which is a return of the muscle fibres to their low tension-generating state.
Muscle contractions can be described in terms of two variables: length and tension. Muscle contractions can be isometric, where muscle tension changes but the muscle length remains the same, or isotonic, where muscle tension remains the same throughout the contraction. If the muscle shortens, the contraction is concentric, and if the muscle lengthens, the contraction is eccentric.
The length-tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with the greatest active tension when close to their ideal length, which is often their resting length. When stretched or shortened beyond this, the maximum active tension generated decreases. This decrease is minimal for small deviations but drops off rapidly as the length deviates further from the ideal.
The force-velocity relationship describes the speed at which a muscle changes its length relative to the amount of force it generates. The force declines as the shortening velocity increases and vice versa when the muscle is stretched. This property of active muscle tissue plays a role in the active damping of joints with opposing muscles.
Mechanical loading of muscles can originate from surrounding muscle tissue via myofascial force transmission. Increased load due to the resection of synergistic muscles has been shown to increase muscle fibre cross-sectional area (CSA), leading to hypertrophy and increased passive force, which affects the length-force relationship of the muscle.
Passive stretching is a type of muscle contraction that helps gently lengthen the muscles. This can be achieved by stretching them to their physical limit, activating them without using force.
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Muscle length affects the force generated
The length-tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with the greatest active tension when they are close to their ideal length, which is often their resting length. When stretched or shortened beyond this, the maximum active tension generated decreases. This decrease is minimal for small deviations but drops off rapidly as the length deviates further from the ideal.
The force-length relationship of sarcomeres can be explained by the sliding filament and cross-bridge theories. The sliding filament theory assumes that length changes in sarcomeres, fibres, and muscles are accomplished by the relative sliding of the essentially inextensible myofilaments, actin, and myosin, within a sarcomere. The cross-bridge theory suggests that the relative sliding of actin and myosin is caused by independent force generators, i.e., the cross-bridges.
The force generated by a muscle is also influenced by the frequency at which the muscle is stimulated. As the stimulus frequency is increased, the force is increased until a maximum is reached, at which point it begins to decrease. In addition, an increase in the level of circulating epinephrine and norepinephrine from the sympathetic nervous system also increases the force of contraction.
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Muscle length affects the speed of movement
Muscle length does affect the speed of movement. The force–velocity relationship describes the speed at which a muscle changes its length relative to the amount of force it generates. The force generated by a muscle is dependent on the degree of overlap between thin and thick myofilaments. The greater the number of cross-bridges attached to the actin filaments, the larger the contraction force.
The force–velocity relationship demonstrates that as the velocity of shortening increases, the force generated decreases hyperbolically until it reaches zero at some maximum velocity. Conversely, when a muscle is stretched, the force increases above the isometric maximum until it reaches an absolute maximum. This is demonstrated in the length-tension relationship, which describes how the active tension generated by a muscle decreases when it is stretched or shortened beyond its ideal length, which is often its resting length.
The length-tension relationship also applies to the heart muscle, where the active force developed increases as muscle length increases, reaches a maximum, and then decreases. The force developed by the heart muscle is also dependent on the frequency of stimulation. As the stimulus frequency is increased, the force generated increases until a maximum is reached, after which it decreases.
The force–velocity relationship is observed in activities such as walking and running. As walking speed increases, the force generation ability of certain muscles, such as the vasti, increases, while the force generation ability of other muscles, such as the plantarflexors, decreases. The transition from walking to running also affects force generation by changing fiber lengths and velocities. For example, the soleus muscle behaves more eccentrically in running than in walking, with earlier peak fiber length and a transition from isometric to shortening fibers. Similarly, the force-generating ability of muscles crossing the hip, such as the gluteus maximus, gluteus medius, and semitendinosus, decreases with increasing running speed. These muscles would require higher levels of activation to meet the demand for higher forces at higher running speeds.
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Frequently asked questions
Yes, muscles can lengthen and be active. This is called an eccentric contraction, where the muscle is active as it lengthens.
Muscles lengthen because an outside force (such as gravity or another muscle) acts more strongly than the muscle being lengthened. This can happen actively or passively.
Walking, hiking downhill, and lowering something heavy are all examples of eccentric contractions.
Strategies to improve muscle length include stretching, electrical stimulation, loaded inter-set stretching, and lengthening contractions.
