Final answer:
Muscle contraction against the skeleton allows for body movements at joints, where muscles, generally arranged in antagonistic pairs, orchestrate bending and straightening actions. Muscle contractions are stimulated by the nervous system and involve the shortening of myofibrils within muscle cells.
Step-by-step explanation:
Movements in the human body are facilitated by muscles contracting and exerting force on the skeleton, specifically at the joints. The muscles around a joint are typically arranged in pairs, known as agonists and antagonists, that work opposite each other. For example, muscles that bend a joint are called flexors, and those that straighten it are called extensors.
During a movement, the agonist muscle contracts to create the desired motion, while the antagonist muscle stretches and yields to the contraction. Additionally, stabilizing muscles, or fixators, help maintain a posture or limb position. Muscle contraction itself is triggered by signals from the nervous system that cause myofibrils within the muscle fibers to shorten, pulling on tendons and moving the bones they are attached to at the joint.
It's also essential to recognize the role of connective tissues such as tendons, which connect muscle to bone, and ligaments, connecting bones, in maintaining joint stability and enabling movement. The entire process results from the coordinated efforts between muscles, connective tissues, and neural inputs.