Muscles can contract for different durations and at different speeds and forces. Such regulation does not take place at the level of each individual myofiber, because each fiber is contracted or relaxed. Instead, this regulation takes place at the level of motor neurons connected to myofibers. Motor neurons are connected to a myofiber by the neuromuscular connection, also known as the neuromuscular synapse. This synapse functions as a neuron-neuron synapse, in which the action potential of an axon is transferred to a downstream neurite. At the neuromuscular junction, the myofiber is located downstream of the axon, and the action potential of the axon leads to the depolarization of the sarcolemma, which moves rapidly along the sarcolemma and tubules T, resulting in a contraction of the myofiber. Each motor neuron is connected to one or more myofibers and forms a motor unit (English and Wolf, 1982). However, each myofiber has only one neuromuscular connection, which is connected to a single motor neuron. Therefore, muscle length and speed are regulated by the number of activated motor units. The more the motor units are activated, the higher and faster the contraction.
On the other hand, the rate of stimulation by the motor neuron controls the force generated by each myofiber in a motor unit. Mado, K., Chekulayev, V., Shevchuk, I., Puurand, M., Tepp, K. and Kaambre, T. (2019). On the role of tubulin, lectin, desmin and vimentin in regulating mitochondrial energy flows in muscle cells. On the. J. Physiol. Cellular physiol.
doi: 10.1152/ajpcell.00303.2018 Rassier, D. E. (2012). Improvement of residual strength in skeletal muscles: one sarcomere after another. J. Muscle Res. Cell. Motil. 33, 155–165. doi: 10.1007/s10974-012-9308-7 Myofibrils in the muscle fibers (muscle cells) of the skeleton and heart muscle have thick, thin filaments that overlap to create patterns called I-bands, H-zones, A-bands, Z-discs, and M line.
Thin filaments contain two strands of protein called actin, which are wrapped in a spiral structure with a strand of two other proteins called troponin and tropomyosin. Thick filaments contain many small filaments of proteins called myosin filaments, which consist of a head and tail. These patterns give the skeleton and heart muscle a “scratched” appearance. During the contraction of the skeleton and heart muscle, the I-band shortens, while the other ligaments and areas remain the same length. Traditional rehabilitation programs have often skipped eccentric training. Although there are no definitive studies that support eccentric training as an absolute prerequisite for returning to athletic play,19 there is research that supports its use, particularly for the rehabilitation of microtrauma/overload injuries. For example, Roos and Kollegen20 developed a prospective randomized clinical trial to test the hypothesis that eccentric calf exercises relieve pain and improve function in patients with Achilles tendonitis. After 12 weeks, members of the group who performed eccentric exercises reported significantly less pain, and more patients in this group returned to exercise after 12 weeks.20 Muscle contraction has fascinated researchers since primary physiological studies.
Along with ATP, calcium was identified early on as one of the most biochemically important participants (Szent-Györgyi, 1975). In its soluble form, calcium exists as a cation (Ca2+), which can enter into electrostatic interactions with negatively charged molecular parts (e.B. carboxyl groups). Variation in Ca2+ levels can induce conformational changes in proteins and thus act as a regulator of their function. This is the case of muscle contraction machines, which contract at Ca2 + binding. There are two types of heart muscle cells: autorythmic and contractile. Autorythmic cells do not contract, but determine the rate of contraction of other heart muscle cells that can be modulated by the autonomic nervous system. In contrast, contractile muscle cells (cardiomyocytes) make up the majority of heart muscle and are capable of contracting. Gerber, J. P., Marcus, R.
L., Dibble, L. E., Greis, P. E., Burks, R. T. and Lastayo, P.C. (2007). Safety, feasibility and effectiveness of negative work exercises due to eccentric muscle activity after reconstruction of the anterior cruciate ligament. J. Orthop. Sport Phys. Ther. 37, 10–18.
Chen, T.C., Tseng, W.C., Huang, G. L., Chen, H. L., Tseng, K. W. and Nosaka, K. (2013). Low-intensity eccentric contractions alleviate muscle damage caused by subsequent maximal eccentric exercise of knee extenders in the elderly. Euro. J. Appl. Physiol.
Occupy. Physiol. 113, 1005–1015. doi: 10.1007/s00421-012-2517-3 Paulsen, G., Mikkelsen, U. R., Raastad, T., and Peake, J.M. (2012). Leukocytes, cytokines and satellite cells: what role do they play in muscle damage and regeneration after eccentric training? Exercise Immunol. Rev 18, 42–97 Myosin heads involved in the formation of transverse bridges have different properties in slow-twitch and fast-twitch muscle fibers. In fast-twitch muscle fibers, the formation of transverse bridges between actin and myosin filaments occurs more rapidly due to the higher ATPase activity of myosin, and the binding capacity of their binding sites is significantly high.
After contraction, ca ions are brought back to the sarcoplasmic reticulum by active transport, and troponin C returns to its resting state, allowing the muscles to relax. Chaabene, H., Prieske, O., Negra, Y., and Granacher, U. (2018). Directional Change Speed: Towards a strength training approach with accentuated eccentric muscle actions. Sports Med. 48, 773–779. doi: 10.1007/s40279-018-0907-3 Muscle contractions can be described using two variables: length and tension.  Muscle contraction is described as isometric when muscle tension changes but muscle length remains the same.     In contrast, estotonic muscle contraction when muscle tension remains the same throughout the contraction.     When muscle length shortens, the contraction is concentric;   When muscle length lengthens, the contraction is eccentric. In the natural movements that underlie the activity of the musculoskeletal system, muscle contractions are diverse, since they are able to produce changes in length and tension in a way that varies over time.
 Therefore, it is likely that neither the length nor the tension in the muscles that contract during the activity of the musculoskeletal system remain the same. LaStayo, P.C., Woolf, J.M., Lewek, M. D., Snyder-Mackler, L., Reich, T., and Lindstedt, S. L. (2003b). Eccentric muscle contractions: their contribution to injuries, prevention, rehabilitation and sport. J. Orthop. Sport Phys. Ther. 33, 557–571.
Hortobagyi, T., Lambert, N. J., and Hill, J. P. (1997). Greater cross-training after training with muscle lengthening as shortening. Medical Sci. Sport Exerz. 29, 107–112. Huxley, A.
F., and Simmons, R.M. (1971). Proposed mechanism of force generation in striated muscles. Nature 233, 533–538. doi: 10.1038/233533a0 LaStayo, P.C., Pierotti, D. J., Pifer, J., Hoppeler, H., and Lindstedt, S. L. (2000). Eccentric ergometry: Increased size and strength of the musculoskeletal system at low training intensities. On the. J.
Physiol. Regulate. Integrate Comp. Physiol. 278, R1282 to R1288. 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. Abbott, B.C., and Aubert, X.M. (1952).
The force exerted by the active striated muscles during and after the change in length. J. Physiol. 117, 77–86. Campbell, K. S. (2009). Interactions between connected semi-sarcomeres produce mechanical behavior emerge in a mathematical muscle model.
Calculation pLoS. Biol. 5 November:e1000560. doi: 10.1371/journal.pcbi.1000560 Overgaard, K., Lindstrom, T., Ingemann-Hansen, T., and Clausen, T. (2002). Membrane leakage and increased content of Na+-K+ pumps and Ca2+ in the human muscle after a 100 km run. J. Appl. Physiol. 92, 1891-1898. Mizumura, K., and Taguchi, T. (2016).
Delayed onset of muscle pain: involvement of neurotrophic factors. J. Physiol. Sci. 66, 43–52. The SR network is interrupted by T-tubules, which are intussusceptions of the sarcolemma. T tubules can be found near the Z disc or A-I compound, depending on the body and muscle type. The region of the SR, which is located next to each T-tubule, is called the terminal tank. A T-shaped tubule is surrounded by two SR terminal tanks that form a triad (Table 2). Triads play an important role in controlling muscle contraction. The T tubule has voltage-sensitive Ca++ channels called dihydropyridine receptors (DHPR) that are physically connected to the Ca++ channels in the SR called the ryanodine receptor (Table 2; Franzini-Armstrong et al., 1998).
The depolarization of the tubular membrane T due to the action potential generated at the neuromuscular synapse causes a conformational change of the DHPR on the tubule T.