The large quadriceps muscle in the upper leg terminates at its lower end in a te

Question

The large quadriceps muscle in the upper leg terminates at its lower end in a tendon attached to the upper end of the tibia (Figure a). The forces on the lower leg when the leg is extended are modeled as in Figure b, where? is the force in the tendon, is the gravitational force acting on the lower leg, and is the gravitational force acting on the foot. Find T when the tendon is at an angle of phi = 25.0 degree with the tibia, assuming Fg,leg = 22.0 N, Fg, foot = 15.0 N, and the leg is extended at an angle theta = 40.0 degree with respect to the vertical. Also assume the center of gravity of the tibia is at its geometric center and the tendon attaches to the lower leg at a position one-fifth of the way down the leg.

Explanation / Answer

Here,
Net torque about knee is zero.

Tnet = 0

T*(L/5)*sin(phi) - Fg,leg*(L/2)*sin(theta) - Fg,foot*L*sin(theta) = 0

T*sin(25)/5 - 22*sin(40)/2 - 15*sin(40) = 0

T = 5*(22*sin(40)/2 + 15*sin(40))/sin(25)

= 197.7 N <<<<<<<<<<<<<<------------Answer

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