Final answer:
The resistance torque changes as a limb moves due to shifts in the pivot point and the perpendicular lever arms associated with joints, which affect mechanical advantage and the force needed to counteract an external load.
Step-by-step explanation:
The magnitude of a resistance torque changes as a limb moves through its range of motion (ROM) due to the variations in the lever arm and mechanical advantage intrinsic to our musculoskeletal system. When considering a joint like the elbow, the pivot point may shift as the joint is flexed, altering the perpendicular distance (lever arm) from the pivot to the line of force exerted by the muscle. This change in distance affects the resistance torque, as torque is calculated using the relation № = r₁ F, where r₁ represents this perpendicular distance, and F represents the force applied. Therefore, as a limb flexes, the necessary muscle force to counteract an external load, such as holding up a book, can increase or decrease based on the angle of the joint and these changing lever arms.
The practical implications are seen in activities such as cycling, where proper adjustment of the bicycle seat to match the rider's leg length results in optimized lever arms, reducing leg strain and making pedaling more efficient.