A value less than twice the force exerted by the biceps muscle group.
This scenario describes what is occurring when a person holds a mass steady with a bent elbow, preventing the forearm from moving either up or down. The triceps, having one-half the lever arm, must exert twice the force of the biceps in order to steady the mass. However, the mass also acts down on the forearm at a lever arm of 40cm, and the forearm itself has mass which can be assumed to act downward at approximately the center of the forearm. Thus the bicep must counteract the torque due to the weight of the mass, torque due to the weight of the forearm itself, and whatever force/torque the tricep is exerting. Therefore, when in equilibrium, the tricep must be exerting some force that is less than twice the force of the bicep—due to the co-directional torque resulting from mass m. If the tricep were exerting exactly twice the force of the bicep, the two muscle-based torques would be balanced, but as a result of the mass m and the mass of the forearm itself, it would move downward.