Problem 2.2 - Consider an athlete doing lower leg flexion-extension exercises to strengthen quadriceps muscles (Fig. 3i).
Determine the net moment generated at the knee joint (around point O) in any of the following positions:
Position 1 - leg is fully extended horizontally ($\theta$ = 0°, Fig. 3ii)
Position 2 - leg is flexed and makes an angle of $\theta$ = 30° with the horizontal axis (Fig. 3ii)
Position 3 - leg is flexed and makes an angle of $\theta$ = 60° with the horizontal axis (Fig. 3ii)
Position 4 - leg is flexed and makes an angle of $\theta$ = 90° with the horizontal axis (Fig. 3ii)
Assumptions:
• The weight of the athlete's lower leg W1 = 50N
• The center of gravity of the leg (A) is located at the distance of a = 20cm from the knee center of rotation (point O)
• The weight of the training boot W2 = 100N
• The center of gravity of the training boot (B) is located at the distance of b = 50cm from the knee center of rotation (point O)
Definition of Net joint moment: The summation of all moments applying at the joint. If for example different forces F1, F2, and F3 would generate moments (torques) M1, M2 and M3 around the joint, the net joint moment M would be:
M = M1 + M2 + M3.
