Skeletal muscle is incredibly resilient. It can stretch and shorten at very fast speeds without sustaining damage. However, in addition to its ability to elongate and compress, muscle also has two other important characteristics: irritability and elasticity. All four of these properties affect how muscle performs under different loads and at different speeds, and this blog post will look at them briefly.
The technical term for a muscle's ability to elongate or lengthen is called extensibility. When you perform a stretch, you elicit this property of the muscle. It's essential to bear in mind that no muscle can lengthen on its own. Some external force is always needed, which can be provided by the active contraction of an agonist (shortening) muscle, a training partner moving the joint passively through its range of motion, and so forth. While sarcomeres can lengthen during a stretch, extensibility is determined primarily by the muscle's connective tissue elements.
As the name suggests, contractility is a muscle's ability to generate active tension by harnessing chemical energy and (usually) shortening in length. However, a muscle can tense while remaining the same length (isometric contraction) or increasing in length (eccentric contraction). Flexibility may be limited not necessarily by lack of extensibility in the antagonists but by lack of contractility in the agonists. For example, a dancer may possess all the hamstring extensibility necessary to raise their leg 180 degrees into the air, but they might lack sufficient strength in the hip flexors to lift the limb. A well-rounded flexibility training programme will address both of these factors according to the demands of a person's chosen sport or activities.
This term does not refer to the muscle getting annoyed. Instead, it means how well the muscle responds to a particular stimulus. Another word that is often used in place of irritability is excitability, which perhaps better describes this characteristic. Skeletal muscle is second only to nerve tissue in terms of sensitivity and responsiveness. Therefore, a muscle's excitability allows it to be recruited for a given task very quickly. The nervous system can exert considerable control over the number of muscle fibres it stimulates during movement, which is fundamental to good athletic performance.
The last characteristic is something I have discussed in previous posts, but a muscle can return to its previous shape (i.e., its resting length) after a stretching force has been removed. We call this ability elasticity, and much like extensibility, it is determined mainly by the connective tissue in the muscle-tendon unit rather than by the myofibrils that we tend to think of as the "muscle." Together with extensibility, elasticity protects the structural integrity of the muscle and allows the body to maintain homeostasis throughout the kinematic chain. Elasticity also plays a role in the amount of force output the muscle can generate during stretch-shortening cycles, as occurs in reactive jumping actions.