what are the interactions between actin and myosin filaments of the sarcomere responsible for?

This question probably would be better in biology than in botany.  Actin and Myosin are the two microfilaments that compose muscle fibers and allow for movement.  The actin filaments are much smaller and surround a bundle of myosin filaments.  All along the myosin bundle are what they call "myosiin heads."  (Picture a bundle of golf clubs, with the heads all pointing out towards the actin.)  On the actin filaments are myosin binding sites.  The myosin heads bend and contact the binding sites, sliding the actin filaments.  As the actin filaments slide, they shorten the sarcomere and the overall muscle fiber.  When they all shorten, it leads to muscular contraction.