Torque equilibrium at the elbow joint

Steps:

• Treat the elbow as a pivot for rotational equilibrium.
• Muscle tension T acts at a short lever arm (e.g., 5 cm) from the pivot.
• The 40 N weight acts at a longer lever arm (e.g., 35 cm) from the pivot.
• Set torques equal: T × 0.05 m = 40 N × 0.35 m, so T = (40 × 0.35) / 0.05 = 280 N.

Why C is correct:

• Torque balance (∑τ = 0) requires muscle tension to counter the weight's moment arm, yielding 280 N exactly.

Why the others are wrong:

• A: Underestimates lever arm ratio (ignores mechanical disadvantage).
• B: Partial calculation, like force without full arm length.
• D: Overestimates, perhaps confusing tension with total joint force including bone compression.

Final answer: C