Logan Steele
Flexion and extension are two basic ways the human body moves at its joints. Extension involves increasing the angle between two bones, i.e., straightening a bend. While muscles do not change in length, they can be extended and retracted. When a muscle is lengthened, it behaves like a shock absorber-spring complex. During an extension, the muscle can store energy as elastic recoil potential energy, which can be converted into mechanical energy. This energy can be recovered in the subsequent stride. The distance a muscle can be extended is determined by the sensitivity of the stretch reflex, which is the body's automatic defence against overstretching.
| Characteristics | Values |
|---|---|
| Do muscles elongate when extended? | Muscles do not elongate when extended. |
| Muscle extensibility | Muscle extensibility can be increased by slowly applying a static stretch, which stimulates neuromuscular reflexes that induce muscle relaxation. |
| Muscle length | Muscle length does not change when stretched. Stretching simply extends the muscle further before it locks. |
| Muscle extension | Muscle extension is the straightening of a bend, increasing the angle between two bones. |
| Muscle contraction | When a muscle is lengthened during an eccentric contraction, it behaves like a shock absorber-spring complex. |
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What You'll Learn
Muscles do not change length, they extend and retract
It is a common misconception that muscles can be stretched and elongated. In reality, muscles do not change length—they extend and retract.
Muscles are not very elastic, and their length remains the same despite what the word "stretch" implies. For instance, consider a 12-foot extension ladder. It can appear to be anywhere between six and 12 feet long, depending on how it is adjusted, but its length is always 12 feet. Similarly, the human hamstring or any other muscle in the body does not change length when stretched. Stretching simply extends the muscle further and further before it locks.
The ability of a muscle to extend is called "muscle extensibility." This is determined by the sensitivity of the stretch reflex, which is the body's automatic defense against dangerous and damaging overstretching. When a muscle is stretched, it activates a stretch reflex, causing the muscle to contract and resist further stretching. This is a protective mechanism to prevent injury.
Research has shown that stretching does not make muscles permanently longer. Instead, it re-educates the nervous system to tolerate a greater degree of muscle extension without firing pain signals. The nervous system learns to relax the muscle, allowing for a greater range of motion without causing any structural changes to the muscle itself.
It is important to note that deviations from optimal extensibility can contribute to muscle imbalances, faulty posture, and dysfunctional movement. Therefore, it is crucial to work with the nervous system to achieve lasting gains in flexibility and range of motion without compromising muscle health.
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Muscle extensibility is determined by the stretch reflex
Muscle extensibility is not determined by an increase in muscle length but by an increase in the tolerance of the muscle to stretching. The muscle length does not change, but the muscle extends and retracts.
The stretch reflex, also known as the myotatic reflex or muscle stretch reflex, is a muscle contraction in response to stretching a muscle. It is a useful example of examining the properties and functional consequences of ionotropic PSPs. The muscle spindles in the muscle detect a stretch and convey this information to the central nervous system. The stretch reflex can be a monosynaptic reflex that provides automatic regulation of skeletal muscle length. The monosynaptic reflex is when the signal entering the spinal cord arises from a change in muscle length or velocity.
The stretch reflex can also include a polysynaptic component, as in the tonic stretch reflex. When a muscle lengthens, the muscle spindle is stretched, and its nerve activity increases. This increase in activity causes the muscle fibers to contract and thus resist the stretching. Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle.
The stretch reflex is a simple behaviour mediated by the central nervous system. The tap of a neurologist's hammer to a ligament elicits a reflex extension of the leg. The brief stretch of the ligament is transmitted to the extensor muscle and is detected by specific receptors in the extensor muscle. The action potentials initiated in the stretch receptors are then propagated to the spinal cord by afferent fibres.
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Stretching re-educates the nervous system to tolerate extension
Stretching does not make muscles longer. Instead, it re-educates the nervous system to tolerate a greater degree of muscle extension without firing pain signals. Muscles are not short or long and cannot significantly change length without injury. When a muscle or muscle group demonstrates insufficient extensibility, the muscle appears to be short, and the contraction results in restricted motion.
The nervous system is a complex network of nerve cells that create a highway with a variety of routes, impacting all the structures it encounters. Our body is able to move and function through two-way messages carried along these nerves. Information is constantly travelling along nerve cells from the brain to muscles to enable them to contract. Additionally, information from the body goes back to the brain so that the brain can make sense of what is happening in the body and adjust outgoing messages as needed.
Neurodynamics refers to the communication between different parts of the nervous system and to the nervous system's relationship with the musculoskeletal system. It has been shown that nerves move independently from other tissues. A healthy nerve cell should be able to glide and slide freely back and forth in the myelin sheath. When compressed or compromised, the myelin can become sticky and limit the ability of the nerve cells to glide back and forth.
Physical therapists can assess nervous system issues and find the most appropriate strategies to address them. They may choose movements and manual techniques that improve the neurodynamics of the body.
To increase muscle extensibility, it has been proposed that slowly applied static stretch stimulates neuromuscular reflexes that induce relaxation of muscles undergoing static stretch. Some authors have suggested that neuromuscular reflexes adapt to repeated stretch over time, which enhances the stretched muscles' ability to relax and results in increased muscle extensibility.
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Muscles extend during eccentric contractions
It is a common misconception that muscles can be stretched and become longer. In reality, muscles extend and retract, but their length remains the same. The length of a muscle cannot change without injury.
When a muscle is lengthened during an eccentric contraction, it behaves like a shock absorber-spring complex. An eccentric contraction occurs when a force is applied to a muscle that exceeds the force produced by the muscle, resulting in the lengthening of the muscle-tendon system while it is contracting. During this process, the muscle absorbs energy from an external load, which is why eccentric action is referred to as "negative work".
Eccentric contractions are characterized by the production of muscle force associated with the lengthening of the muscle-tendon system. This can cause micro-lesions followed by a regeneration process. Eccentric exercises are used in rehabilitation for their positive effect on collagen synthesis and resistance training to increase muscle strength and mass in athletes.
Eccentric contractions are integral to most movements during daily or sports activities. For example, when running downhill, the energy that stretches the active muscle is lost as heat. In contrast, when running, most of the energy required to stretch the muscle is stored as elastic recoil potential energy, which can be recovered on the subsequent stride.
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Muscle length is dependent on the tensile force applied
The relationship between muscle length and tension can be plotted on a passive length/tension curve, with multiple length measurements according to the amount of passive tension required to reach each length. In human studies, these curves are often presented as torque/angle curves, as human muscle length measurements are usually joint angle measurements, and the tensile force is applied in a rotational manner.
The length of a muscle can be increased by stretching, but this does not make muscles permanently longer. Instead, stretching teaches the nervous system to tolerate a greater degree of muscle extension without firing pain signals. The muscle appears longer, but this is due to the muscle demonstrating excessive extensibility, which results in limber movement.
The length of a muscle can also be affected by the force applied to it. When the force applied to a muscle exceeds the force produced by the muscle, it will lengthen, absorbing mechanical energy. This is known as an eccentric contraction and acts as a braking force to protect joints from damage.
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Frequently asked questions
No, muscles do not elongate when extended. Stretching does not make muscles permanently longer. Instead, stretching teaches the nervous system to tolerate a greater degree of muscle extension without firing pain signals.
Flexion and extension refer to the basic ways the body moves at its joints. Flexion makes the joint angle smaller, bringing two body parts closer together. Extension, on the other hand, makes the joint angle larger, moving two body parts farther away from each other.
Examples of exercises that involve extension include the triceps kickback and the act of driving out of a squat. The triceps brachii and anconeus are muscles that extend the elbow.
