Final answer:
External forces generally have the longest moment arms compared to muscles, ligaments, and tendons, particularly when the force exerted is a long distance from the joint, providing greater mechanical leverage.
Step-by-step explanation:
The moment arms are longest for external forces applied at a distance from the joint. This is because forces in muscles and joints are largest when their load is a long distance from the joint, creating a mechanical advantage. For example, playing sports like tennis involves a racquet that has a considerable moment arm, making it essential for players to use the heaviest racquet they can handle efficiently to exert sufficient force to the ball while trying to minimize fatigue and potential long-term damage to the joints and tendons.
Muscles attach to bones via tendons close to joints, thereby creating mechanical advantages much less than one. As a result, the input force exerted by muscles is significantly greater than the output force that limbs apply to the outside world. This concept implies that moment arms for muscles and tendons are relatively shorter compared to those of external forces or weight acting at a distance from the joint.