Final answer:
To maintain an extended position at the knee against an external torque of 5 Nm, the knee extensor muscle group must produce a torque of 5 Nm in the opposite direction.
Step-by-step explanation:
The question asks, how much torque must be produced by the knee extensor muscle group to maintain an extended position at the knee if the weight of the knee is producing an external torque of 5 Nm. To maintain an extended position at the knee against an external torque, the knee extensor muscles must produce an equivalent but opposite torque to achieve a state of equilibrium. Since the external torque exerted by the weight of the knee is 5 Nm, the torque produced by the knee extensor muscle group must also be 5 Nm, but acting in the opposite direction to balance it out. Therefore, for the leg to remain in an extended position, the knee extensors must exert 5 Nm of torque.