Athena Vale
Muscles lengthen when the force exerted on them exceeds the force they can generate, causing them to absorb mechanical energy. This is known as an eccentric contraction, where the muscle lengthens as it contracts, acting to decelerate the joint at the end of a movement or control the repositioning of a load. The energy absorbed by the muscle can be dissipated as heat, or stored as elastic recoil potential energy and recovered. While stretching can increase flexibility, it does not effectively lengthen muscles. Instead, strength training, particularly eccentric training, is more effective for muscle lengthening.
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What You'll Learn
Muscle contractions
Eccentric contractions occur when the tension generated while isometric is insufficient to overcome the external load on the muscle, and the muscle fibres lengthen as they contract. Rather than pulling a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or control the repositioning of a load. This can occur involuntarily, for example, when attempting to lift a weight that is too heavy, or voluntarily, for example, when resisting gravity during downhill walking.
Eccentric training has been shown to increase muscle strength and hopping frequency in humans, and spring stiffness in rats. Less muscle-building activities like yoga, pilates, and running do not build as much muscle as strength training, but they can still lead to longer muscles. Stretching is a nervous system strategy aimed at increasing flexibility, not a mechanical adaptation strategy.
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Muscle length and force
The force-length relationship is a static property of skeletal muscle. The active force generated by a maximally activated single fibre is maximal when filament overlap is optimised and decreases proportionally when overlap is diminished. The force-length relationship does not predict the consequences of dynamic contractions, as changes in sarcomere length during muscle contraction result in modulation of the active force.
Muscles that operate on the ascending limb of the force-length relationship typically function in stretch-shortening cycle contractions, while muscles that operate on the descending limb typically function in shortening contractions. The force-length relationship also has implications for sports performance. For instance, in squat jumps, people jump higher if they perform a countermovement downward before pushoff, as this allows for increased time for force development.
The importance and prevalence of lengthening contractions in normal locomotion have only recently received increasing attention. When the force exerted on the muscle exceeds the force developed by the muscle, the muscle absorbs mechanical energy and does "negative work". The absorbed energy can be dissipated as heat, in which case the muscle functions as a damper or shock absorber, or it can be stored as elastic recoil potential energy and subsequently recovered. For example, when hiking downhill, the energy that stretches the active muscle is lost as heat, while when running, most of this energy is stored as elastic recoil potential energy and recovered on the subsequent stride.
Chronic eccentric training can result in shifts in muscle properties, such as increased muscle fibre cross-sectional area, isometric strength, and hopping frequency. While stretching can increase flexibility, it does not increase muscle length. Strength training is more effective for lengthening muscles than stretching, as it involves eccentric contractions and movements that take the joints through their full range of motion.
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Muscle stretching
When a muscle is stretched, it can either be a static or dynamic stretch. Static stretching involves holding a stretch in a fixed position for a period of time, while dynamic stretching involves active movement that takes the joints through their full range of motion. The latter is often considered more effective for improving flexibility and joint health.
The length of a muscle is described as its flexibility, indicating the ability of the muscle to lengthen to the end of its range of motion across a joint. The amount of muscle lengthening depends on the muscle itself, the training status, and the time devoted to stretching. Muscles have an "ideal length", often their resting length, at which they operate with the greatest active tension. When stretched beyond this ideal length, the maximum active tension generated decreases due to the presence of elastic proteins within the muscle cell that oppose lengthening.
During an eccentric contraction, the muscle lengthens as it contracts, and this can occur involuntarily (e.g. when attempting to lift a weight that is too heavy) or voluntarily (e.g. when resisting gravity while walking downhill). In the latter case, the muscle acts as a damper or shock absorber, dissipating the absorbed energy as heat. However, during activities like running, the energy is stored as elastic recoil potential energy and recovered on the subsequent stride. Chronic eccentric training has been shown to result in increased muscle fibre cross-sectional area, isometric strength, and hopping frequency in humans.
While stretching can be beneficial, it is important to note that it does not drastically change muscle length. Strength training, particularly with eccentric contractions, is more effective for lengthening muscles and improving flexibility. Activities like yoga and pilates are often touted for their ability to create "long, lean muscles", but they do not actually lengthen the muscle, only changing the fibre type from short fast-twitch fibres to slow twitch or long muscle fibres.
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Muscle lengthening and locomotion
During locomotion, muscles can either shorten or lengthen. Classic studies in muscle physiology have traditionally focused on the work done by shortening muscles, considering it essential for movement. However, recent research has shed light on the importance of lengthening contractions in normal locomotion. When a muscle lengthens, it absorbs mechanical energy, doing what is known as "negative work." The absorbed energy can be dissipated as heat, acting as a damper or shock absorber, especially during activities like hiking downhill. Alternatively, the energy can be stored temporarily as elastic recoil potential energy and later recovered, as observed in running mammals.
The length of a muscle can be influenced by various factors, including the specific muscle being considered, its training status, and the time devoted to training. While stretching can increase flexibility, it does not directly lead to muscle lengthening. Instead, it involves decreasing the brain's threat response, allowing the muscle to stretch further. Strength training, particularly eccentric contractions that take joints through their full range of motion, is more effective for lengthening muscles than stretching alone.
Longitudinal muscle growth can be induced through specific interventions or mechanical loading strategies, such as stretching or lengthening contractions. Animal studies have shown that prolonged lengthening immobilization results in increased serial sarcomere numbers, suggesting that similar adaptations may occur in humans. Additionally, eccentric training can lead to significant increases in muscle fiber cross-sectional area, isometric strength, and hopping frequency.
In summary, muscle lengthening plays a crucial role in locomotion, and its understanding has evolved beyond the traditional focus on muscle shortening. Lengthening contractions enable muscles to absorb and utilize energy during movement, contributing to overall locomotor performance. The length of a muscle can be influenced by training and is associated with improved muscle function and reduced strain injuries.
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Muscle lengthening and strength training
Muscles can lengthen through eccentric contractions, where the tension generated while isometric is insufficient to overcome the external load, and the muscle fibers lengthen as they contract. This can occur involuntarily, such as when attempting to lift a weight that is too heavy, or voluntarily, such as when resisting gravity during downhill walking. During an eccentric contraction, the muscle behaves like a shock absorber-spring complex, absorbing and dissipating mechanical energy as heat, or storing it as elastic recoil potential energy to be recovered later.
The length of a muscle also affects its strength. Muscles operate with the greatest active tension when they are close to their ideal length, often their resting length. When stretched or shortened beyond this, the maximum active tension generated decreases. This relationship between muscle length and tension is described as the length-tension relationship. Additionally, the speed at which a muscle changes its length, regulated by external forces, is related to the amount of force it generates, known as the force-velocity relationship.
Strength training involving eccentric contractions can be effective for increasing muscle strength and length. Chronic eccentric training has been shown to increase muscle fiber cross-sectional area, isometric strength, and hopping frequency in humans. However, it is important to note that exercise-induced muscle damage is also greater during lengthening contractions. While activities like yoga, Pilates, and running are often associated with lean and slender muscles, they may not build as much muscle as strength training. Instead, they may create the appearance of slender muscles by building less muscle mass and reducing fat.
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Frequently asked questions
An eccentric contraction is when a muscle lengthens as it contracts. This can occur when the tension generated is insufficient to overcome the external load on the muscle. The muscle acts to decelerate the joint at the end of a movement or control the repositioning of a load.
Muscles can be lengthened through stretching and strength training. Stretching is a nervous system strategy aimed at increasing flexibility, not a mechanical adaptation inducing strategy. Strength training is more effective for lengthening muscles than stretching, as it involves eccentric contractions and movements that take the joints through their full ranges of motion.
Isometric contractions are when muscle tension changes without any corresponding changes in muscle length. Isotonic contractions are when muscle tension remains the same throughout the contraction. In an isotonic contraction, the muscle can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction.
