All the muscles in the body make up the muscular system. All the bones of the body make up the skeletal system. There are three types of muscle - skeletal, heart and smooth muscle. Skeletal muscles are anchored on either end by tendons to bones. Skeletal muscle is voluntary muscle - this means contraction (shortening) of the muscle occurs at will. When a skeletal muscle contracts, it pulls on the bones, resulting in movement of the body.
Skeletal muscles are responsible for body movements and posture control. If the skeletal muscles were to fail (stop working) a person would not be able to move their body in any way - no walking, jumping, etc. Even simple things like sitting upright or standing in place could not be done. It may not feel like sitting up requires much work, but a great number of skeletal muscles are involved.
Smooth and cardiac muscle are involuntary muscle. Involuntary means that you cannot consciously cause this muscle to contract. Cardiac muscle composes the heart. Smooth muscle is present in many organs (such as the stomach and intestines) and the circulatory system. Neither smooth muscle nor cardiac muscle are involved with the skeletal system.