The muscles consist of a multitude of elongated cells - muscular filaments capable of contracting under the influence of electric impulses that they receive from the nervous system. When stimulated, the muscular filament will contract only if the stimulating impulse is above a certain threshold of magnitude. This contraction will be maximal and will not change if the impulse becomes stronger. The modern theories state that contractions are caused by actin strings - one of the muscle string types - as they slide against the myosin strings. The energy required for this process to occur is provided by breaking down of ATP (adenosine triphosphate).
This model illustrates the process of muscular contractions under different levels of magnification. Press "Run" button to initiate the animation. Press "Stop" to bring it to a halt, and press "Reset" to return the model to its initial state.