Filaments of actin constitute the thin filaments.

Muscles which oppose one another: as one contracts, the other relaxes.

Adenosine triphosphate - is a source of energy for muscle contraction.  Energy is released when ATP is hydrolised (broken down) to ADP (adenosine diphosphate) and inorganic phosphate.

Shortening of the muscle

A very high resolution microscope which uses beams of electrons rather than rays of light to create a magnified image of a specimen.  It produces very detailed images at much greater magnifications than can be achieved with a light microscope.   Online dictionary

Filaments of myosin constitute the thick filaments.

Lengthening of the muscle as it relaxes

The theory behind muscle contraction, which involves thick and thin myosin and actin filaments respectively.  These filaments slide over one another within sarcomeres in the muscles.