<P> In addition to the actin and myosin components that constitute the sarcomere, skeletal muscle fibers also contain two other important regulatory proteins, troponin and tropomyosin, that are necessary for muscle contraction to occur . These proteins are associated with actin and cooperate to prevent its interaction with myosin . Skeletal muscle cells are excitable and are subject to depolarization by the neurotransmitter acetylcholine, released at the neuromuscular junction by motor neurons . </P> <P> Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum releases ionic calcium (Ca2+), which then interacts with the regulatory protein troponin . Calcium - bound troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the myosin - binding sites on actin . This allows for myosin and actin ATP - dependent cross-bridge cycling and shortening of the muscle . </P> <P> Muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length . The torque around a joint, however, is determined by a number of biomechanical parameters, including the distance between muscle insertions and pivot points, muscle size and Architectural gear ratio . Muscles are normally arranged in opposition so that when one group of muscles contracts, another group relaxes or lengthens . Antagonism in the transmission of nerve impulses to the muscles means that it is impossible to fully stimulate the contraction of two antagonistic muscles at any one time . During ballistic motions such as throwing, the antagonist muscles act to' brake' the agonist muscles throughout the contraction, particularly at the end of the motion . In the example of throwing, the chest and front of the shoulder (anterior Deltoid) contract to pull the arm forward, while the muscles in the back and rear of the shoulder (posterior Deltoid) also contract and undergo eccentric contraction to slow the motion down to avoid injury . Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder . </P> <P> Contracting muscles produce vibration and sound . Slow twitch fibers produce 10 to 30 contractions per second (10 to 30 Hz). Fast twitch fibers produce 30 to 70 contractions per second (30 to 70 Hz). The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist . The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example . The sound is usually described as a rumbling sound . Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear . The rumbling sound can also be heard when the neck or jaw muscles are highly tensed . </P>

What are two other names for a muscle fiber