A limb acts as a class-3 lever: the muscle's pull (effort) sits between the joint (fulcrum) and the load, so the muscle must apply far more force than the load itself in exchange for speed and range of motion.
τ = F · r · sin(θ) (torque about the joint)
F_muscle = (m·g·L_load) / r_moment (equilibrium about elbow)
MA = r_moment / L_load (mechanical advantage, always « 1 for limbs)
- Load mass — weight held in the hand; scales the torque the muscle must resist.
- Muscle moment arm — perpendicular distance from the joint to the tendon's line of pull (changes with elbow angle in real anatomy).
- Rep speed — animation speed of the joint cycling through its range of motion.
- Preset — swaps the whole segment geometry/motion: curl (elbow flexion), throwing put (shoulder + elbow extension), sprint stride (hip/knee cycle).
Real-world relevance: this is why a short biceps moment arm (~4-5 cm) needs 8-10x the load force in muscle tension — the trade-off that lets a small muscle contraction move the hand fast and far.