Muscle contraction involves the sliding of thick filaments of myosin beyond thin filaments of actin. The interaction of myosin and actin begins when a high-energy ATP molecule located in the head of the myosin filament is hydrolyzed into an inorganic phosphate (Pi) molecule and ADP. The myosin head then attaches to an actin filament, which forms a transverse bridge. The ADP and Pi are then released, and the myosin head undergoes a conformational change that causes the actin filament to move relative to the myosin filament. ATP then binds to the myosin head and causes myosin to dissociate from actin filament. These steps are repeated very quickly, causing the myosin head to “go” along the actin filament, resulting in muscle contraction. Only when ATP is present can the myosin head detach from the actin filament to continue the process. When ATP is not present, the muscle becomes stiff and cannot relax, as is the case with rigor mortis. In bright animals such as earthworms and bloodsuckers, circular and longitudinal muscle cells form the body wall of these animals and are responsible for their movement.  In an earthworm moving in soil, for example, contractions of the circular and longitudinal muscles occur reciprocally, while the coelomaal fluid serves as a hydroskeleton maintaining the turgor of the earthworm.  When the circular muscles of the anterior segments contract, the anterior part of the animal`s body begins to shrink radially, pushing the incompressible coelomaal fluid forward and increasing the length of the animal. As a result, the front end of the animal advances. When the front end of the earthworm is anchored and the circular muscles of the anterior segments are relaxed, a wave of longitudinal muscle contractions runs backwards, pulling the rest of the animal`s dragging body forward.
  These alternating waves of circular and longitudinal contractions are called peristalsis, which underlies the creeping movement of earthworms. Eccentric muscle contraction is called negative work. Your muscle reacts eccentrically to help you lower something heavy. An example of these two contractions is lifting a dumbbell during exercise. In the areas of fitness and movement training, the words lengthen and stretch are used in a variety of ways. It is important to understand that a muscle can lengthen and be active (an eccentric contraction), prolong and be inactive (a relaxed muscle), or lengthen and gradually go from active to inactive or vice versa. ACh is broken down into acetyl and choline by the enzyme acetylcholinesterase (AChE). AChE is located in the synaptic cleft and breaks down ACh so that it does not remain bound to ACh receptors, which would lead to prolonged unwanted muscle contraction (Figure 7). Muscle contraction begins when the nervous system produces a signal. The signal, a pulse called the action potential, travels through a type of nerve cell called a motor neuron. The neuromuscular connection is the name of where the motor neuron reaches a muscle cell.
Skeletal muscle tissue is made up of cells called muscle fibers. When the signal from the nervous system reaches the neuromuscular connection, a chemical message is released by the motor neuron. The chemical message, a neurotransmitter called acetylcholine, binds to receptors outside muscle fibers. This triggers a chemical reaction in the muscle. The strength of skeletal muscle contractions can be roughly divided into contractions, summation and tetanus. A contraction is a unique cycle of contraction and relaxation generated by an action potential in the muscle fiber itself.  The time between a stimulus to the motor nerve and the subsequent contraction of the innervated muscle is called the latency period, which typically lasts about 10 ms and is caused by the time it takes to distribute the nerve action potential, the chemical transmission time at the neuromuscular junction, and then the subsequent steps of the excitation-contraction coupling.  In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.
 This happens when the force generated by the muscle exceeds the load that prevents it from contracting. Myofibrils are made up of smaller structures called myofilaments. There are two main types of filaments: thick filaments and thin filaments; Each has different compositions and locations. Thick filaments occur only in the A-band of a myofibril. Thin filaments attach to a protein in the Z disk called alpha-actinin and occur along the entire length of band I and partially in the A band. The area where the thick and thin filaments overlap has a dense appearance because there is little space between the filaments. Thin filaments do not extend into the A-bands, leaving a central area of the A-band containing only thick filaments. This central area of the A-band appears slightly brighter than the rest of the A-band and is called the H-band (Figure 4). The center of the H zone has a vertical line called the M line, where accessory proteins hold the thick filaments together. The Z disk and the M line hold the myofilaments in place to maintain the structural arrangement and stratification of the myofibrillus. Myofibrils are connected by intermediate filaments or desmin that adhere to the Z disk. The main component of thin filaments is actin protein.
Two other components of the thin filament are tropomyosin and troponin. Actin has binding sites for binding to myosin. Tropomyosin strands block binding sites and prevent actin-myosin interactions when muscles are at rest. Troponin consists of three spherical subunits. A subunit binds to tropomyosin, a subunit binds to actin, and a subunit binds to Ca2+ ions. Advanced insects such as wasps, flies, bees and beetles have asynchronous muscles that form the flight muscles of these animals.  These flight muscles are often called fibrillary muscles because they contain thick, noticeable myofibrils.  A notable feature of these muscles is that they do not require stimulation for every muscle contraction. Therefore, they are called asynchronous muscles because the number of contractions in these muscles does not match (or synchronize) with the number of action potentials. For example, a muscle in the wing of an attached fly can receive action potentials at a frequency of 3 Hz, but it is able to beat at a frequency of 120 Hz.  High-frequency beats are made possible by the fact that the muscles are connected to a resonance system that is trained at a natural vibrational frequency.
A description of the structure of skeletal muscles, including thick and thin filaments of sarcomeres. University of Houston: Department of Health and Human Performance. grants.hhp.coe.uh.edu/clayne/6397/unit3.htm How do the bones of the human skeleton move? Skeletal muscle contracts and relaxes to move the body mechanically. Messages from the nervous system cause these muscle contractions. The whole process is called the mechanism of muscle contraction and can be summarized in three stages: Figure 3. A sarcomere is the area from one Z line to the next Z line. Many sarcomeres are present in a myofibrill, which leads to the characteristic scratch pattern of skeletal muscles. Muscle contraction is the response of a muscle to any type of stimulus, where the result is a shortening of the length and development of strength.
Their muscles contain fiber called myosin. Depending on how you need to use your muscles, myosin fibers tighten and shorten or relax and expand. Myosin is also responsible for muscle contractions such as your heart rate, which occurs at regular intervals. Muscle contraction can be affected at various levels. Neurological problems, autoimmune diseases, infectious diseases, and spinal cord injuries can all contribute to impaired muscle contraction. Each I band has a dense line that runs vertically through the center and is called a Z disk or a Z line. Z disks mark the edge of units called sarcomeres, which are the functional units of skeletal muscle. A sarcomere is the space between two consecutive Z disks and contains an entire A band and two halves of an I band, one on each side of the A band.
A myofibril consists of many sarcomeres that run along its length, and when the sarcomeres contract individually, the myofibrils and muscle cells shorten (Figure 3). .