Final answer:
Ib afferent nerve fibres from the Golgi tendon organ have sensitivity to muscle tension, which allows them to signal the force produced by isometric muscle contractions.
Step-by-step explanation:
The property that allows Ib afferent nerve fibres projecting from the Golgi tendon organ to signal the amount of force produced by isometric contraction is sensitivity to muscle tension. The Golgi tendon organ is a tension receptor that detects the force of muscle contraction. Unlike muscle spindles, which detect muscle stretch, Golgi tendon organs are specifically sensitive to the degree of muscle tension, which enables them to provide the nervous system with real-time feedback on the force being generated by the muscles. This is crucial for the coordination of muscle contractions and to prevent potential damage from excessive force.