It is explained through Sliding - Filament theory proposed by A.F. Huxley and H.E. Huxley in
1954

• Muscles are made of filament like structure called myofibrils, it is packed parallel with myofilament or protein thoughts it's length such as actin and myosin 

•Actin is made up of f-actin in a monomeric ‘G’G' actins and Myosin is made up of heavy meromyosin and light meromyosin  
 According to this during muscle contraction, partially overlapped actin filaments are slide over to
the surface of myosin and gets inserted into the middle of “A-band”
 At the beginning of contraction a signal sent by the CNS via a motor neuron
 A motor neuron along with the muscle fibres connected to it constitute a motor unit.
 At this motor unit area synaptic knob has a close contact with sarcolemma of muscle fibre to form
as a neuromuscular junction or motor end plate.
 When the neural signals reaches motor end plate then a neurotransmitter called Acetyl choline
releases which generate an action potential / depolarisation in the sarcolemma
 Signals passes through the T-tubules into the sarcoplasm to reach the part called Triad.  From Triad, action potential spreads into SR and cause release of Ca+
ions
 Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filament
 Now troponin displaces tropomyosin and mask like Troponin-I laterally to expose the active sites
of F-actin.
 Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed myosin
binding site on actin to form as a Cross bridge or Actomyosin complex.  Rotation or tilting of cross bridges causes sliding of thin filaments towards the centre of A-band
 The Z-lines attached to these actins are also pulled inwards thereby causing a shortening of the
sarcomere. ie, contraction.
 During contraction, the width of ‘I’ bands gets reduces, whereas “A-band” retains the same length.
 The myosin releasing the ADP and Pi goes back to its relaxed state. A new ATP binds and the cross bridge is broken
The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and
breakage is repeated causing further sliding.
 In complete contraction I bands reduces its width, H-zone disappears, Z-lines come to lie on either
side of A-band but length of thin and thick filaments does not changes.
 The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae, resulting
in the masking of actin filaments
 This causes return of Z-lines back to their original position ie, relaxation.