Reactive strength is a form of high-speed strength that has the stretch-shorten cycle as its basis. The cycle is an active lengthening of muscles followed by contraction of the same muscles. Suspended or limited, reactive strength occurs, for example, in neuromuscular diseases.
What is reactive force?
Reactive force is needed by humans to perform forms of movement such as jumps, sprints, or throws. All such movements are essentially reactive in nature. The nerve-muscle system overcomes various resistances with innervation processes and metabolic processes to contract muscles. This process is called concentric work. Different types of force characterize the nerve-muscle system. One of them is reactive force. In sports medicine, this refers to the force required for reactive movements. Accordingly, reactive force characterizes reactive movements. For example, sequences of movements for low jumps, sprints, or woofing movements are considered as such. One process of reactive force is, for example, the stretch-shortening cycle in the muscles. This cycle corresponds to the active lengthening of muscles followed by muscle contraction. Reactive movements always consist of a rapid succession of yielding and overcoming work that takes place against a resistance. In addition to reactive strength, sports medicine recognizes maximum strength and rapid strength as types of strength. Reactive strength is considered a special form of rapid strength.
Function and task
Reactive movements are a rapid succession of eccentric and concentric work modes of the musculature. In the eccentric phase, the tendo-muscular system stores a certain amount of kinetic energy within its parallel and serial elastic structures. In the concentric phase, the stored energy is released. This results in an increase in power and force. Reactive force depends on fundamentals from neuro-muscular factors and stretching ability of tendinous structures. The bases for the described increase in power is the stretch-shortening cycle to activate the muscle spindle. An eccentric stretching of the muscles in combination with the independent elasticity and innervation behavior is followed by a concentric phase. This concentric phase works with the pre-activation and the
stored tension energy and reflex innervation of the preceding phase. The muscle fiber cross-section determines performance. In addition, the composition and elasticity or innervation behavior of the muscles, ligaments and tendons determine the performance of the reactive force. The innervation and elasticity behavior is called reactive tension capacity. The reactive force corresponds to the eccentric-concentric rapid force for the shortest coupling of both phases of work. In simpler terms, reactive force is the human ability to generate momentum in the stretch-shortening cycle. In the stretch-shortening cycle, the body weight or other muscles create an eccentric and concentric contraction of a specific muscle. The muscle is plastic and elastic at the same time. For this reason, the contraction must occur immediately after the muscle stretch and focus on a phase in which the muscles have not yet adapted to the stretch. This stretch-shortening cycle benefits from the stored energy of previous movements and for this reason occurs particularly quickly. Some sources speak of energy storage in the muscles. Others see the connective tissue as the site of storage. The tendons and ligaments experience stretching to the limit during the stretch-shorten cycle. For this reason, stretchability is a critical factor in reactive strength. Stretchability varies from person to person, so reactive strength also varies from person to person.
Diseases and ailments
After typical sports injuries, reactive strength is limited. To regain it, rehabilitation including plyometrics typically takes place. This is a high-speed strength training program that promotes the stretch reflex of tendons and muscles. In addition, the patient regains control of the muscle spindle apparatus through plyometrics. The training is not limited to sports injuries, but is part of the standard training for high jumpers, sprinters, basketball players or goalkeepers.In all of the aforementioned sports, sprinting speed and jumping power are crucial skills. In addition to the exercises for jumping strength, there are plyometric training sessions for the upper body, which are used, for example, in boxing training or after sports injuries to the upper extremities. The plyometric training corresponds to a promotion of the stretch-shortening cycle. The own body weight creates a pre-tension within the musculature during the training sessions. Most often, deep jumps are performed on inclined planes to create as much preload as possible. Good reactive strength protects athletes from injury in the future. However, training takes place within rehabilitation after sports injuries only in combination with preceding muscle training, as plyometrics on weakened muscles can cause traumatic injuries. Individuals with decreased reactive strength are generally more susceptible to sports injuries and initially receive training sessions to promote stretchability. Low extensibility and such decreased reactive strength are usually related to a general lack of exercise. However, reactive strength can also be limited or even eliminated in the context of certain diseases. This is the case, for example, with neuromuscular diseases. The most significant diseases from this inhomogeneous group are the myopathies and the neuropathies. Myopathies are inherent diseases of the musculature that are not associated with a neuronal cause and manifest as muscle weakness. The neuropathies, on the other hand, are diseases of the peripheral nervous system that affect either single nerves or multiple nerves and also manifest as muscle weakness and even paralysis. In some cases, the neuropathy is caused by inflammatory damage to the nerves. Polyneuropathies in particular can result from previous injuries, viral infections, poisoning, vitamin deficiencies or autoimmune diseases. Guillain-Barré syndrome is an example of an autoimmune disease with neuropathies. Equally common, these neuromuscular disorders are observed after the use of chemotherapeutic agents, especially in association with platinum drugs.
