Why must muscels cross a joint to be involved in bodily movement?
Muscles contract. If they are tied to the same bone, the bone bends. If you want to move something else, one end of a muscle has to be tied to something else.
Muscles must cross a joint to be involved in bodily movement because the contraction of a muscle causes it to shorten, which in turn pulls on the bones to create movement at the joint. When muscles contract, they generate force that is transmitted through tendons, which attach the muscle to the bone. This force exerted by the muscle on the bone helps produce the desired movement at the joint.
To visualize this, you can try a simple experiment: Bend your arm at the elbow. As you do this, you can feel the muscles in your upper arm contracting. These muscles, such as the biceps and brachialis, cross the elbow joint, attaching to the bones on either side of the joint (the humerus and radius/ulna). When these muscles contract, they pull on the bones, causing the forearm to flex and the arm to bend at the elbow joint.
Similarly, when you straighten your arm, the triceps muscle (which also crosses the elbow joint) contracts, pulling on the bones and extending the forearm at the elbow joint. This demonstrates how the muscles crossing a joint are essential for generating movement.
In summary, muscles need to cross a joint to create movement because their contractions exert force on the bones, resulting in joint movement.