In the skeletal muscles, there are alternating bands of thin filament fibers called actin.  Myosin serves as a "ratcheting" mechanism between the filaments of actin, causing a paddle-and-grab type of motion, allowing the actin fibers to shorten.  As the filaments shorten across one another, force is generated, which is transferred to the ends of the sarcomeres.  This force is not evenly distributed, so uneven tension may be noted in less-thin areas of the skeletal muscle in question.  It should also be noted that skeletal muscles work in pairs, when one skeletal muscle is contracting in this fashion, the partner skeltal muscle is relaxing, rebuilding energy for the next contraction.  Skeletal muscles may be trained to generate increasing amounts of force by using resistance training, where some type of resistance, such as weights, are used to develop more of these actin filaments along which the myosin may work its ratchet effect.