Cytoplasmic calcium binds to troponin C and moves the tropomyosin complex from the actin binding site so that the myosin head can bind to the actin filament. From this point on, the contractile mechanism is essentially the same as in skeletal muscle (above). In short, using ATP hydrolysis, the myosin head pulls the actin filament towards the center of the sarcomere. When the stimulation of the motor neuron, which gives the impulse to the muscle fibers, stops, the chemical reaction that causes the proteins of the muscle fibers to rearrange is stopped. As a result, the chemical processes in the muscle fibers are reversed and the muscle relaxes. An isometric contraction of a muscle creates tension without changing its length.     An example can be found when the muscles of the hand and forearm grasp an object; The joints of the hand do not move, but the muscles generate enough force to prevent the object from falling. Although the muscle performs a negative amount of mechanical work (the work is done on the muscle), chemical energy (originally released by oxygen, by fat or glucose and temporarily stored in ATP) is still consumed, although less than would be consumed during a concentric contraction of the same force. For example, you use more energy when you climb a flight of stairs than when you go down the same flight. Muscle contractions can be described using two variables: strength and length.
The force itself can be distinguished either as a voltage or as a charge. Muscle tension is the force that muscle exerts on an object, while a load is the force that an object exerts on the muscle.  When muscle tension changes without muscle length changing accordingly, muscle contraction is described as isometric.     When muscle length changes while muscle tension remains the same, muscle contraction is estotonic.     In isotonic contraction, muscle length can either shorten to create a concentric contraction or lengthen to create an eccentric contraction.   In the natural movements underlying locomotor activity, muscle contractions are diverse because they are able to produce changes in length and tension in a way that varies over time.  Therefore, it is unlikely that neither length nor tension will remain constant when the muscle is active during locomotor activity. Muscle contraction is the tightening, shortening or lengthening of the muscles when you perform an activity. This can happen when you hold or pick up something, or when you stretch or exercise with weights. Muscle contraction is often followed by muscle relaxation when the contracted muscles return to their normal state. The myofibrils in the muscle fibers (muscle cells) of skeletal and cardiac muscles 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 along 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. An analysis of the evidence in support of the slip wire theory. University of Tennessee, Knoxville: Institute of Environmental Modeling. www.tiem.utk.edu/~gross/bioed/webmodules/muscles.html Unlike skeletal muscle, smooth muscle and heart muscle contractions are myogenic (meaning they are initiated by the smooth muscle or heart cells themselves, rather than being stimulated by an external event such as nerve stimulation), although they can be modulated by stimuli from the autonomic nervous system. The contraction mechanisms in these muscle tissues are similar to those of skeletal muscle tissue. Zeitschrift für Grundlagenforschung in der Kardiologie: «Der Mechanismus der Muskelkontraktion.
Biochemical, mechanical and structural approaches to elucidate the action of the transverse bridge in the muscle. After systole, intracellular calcium is reabsorbed into the atticulum sarcoplasmic atticulum pump (SERCA) by the sarco/endoplasmic reticulum, ready for the next cycle. .