Control of muscle stiffness

The muscle spindle reflex for maintaining muscle length and the GTO reflex for maintaining steady tension frequently work in opposition. Whenever the loading on a muscle is modified either the muscle is stretched, in which situation its passive tension increases (in the same manner that tension increases in a rubber band as it is stretched), or it should contract isometrically to maintain a steady length, in which case the active tension rises due to contraction. It is impossible for both tension and length to be held steady at similar time. This recommends that overall, motor systems do not control length or tension separately, but muscle stiffness, k, a constant which describes how much the length of a muscle is modified, ΔL, by a change in load, ΔF.

                                                           k = ΔF / ΔL

It emerges that the muscle spindle and GTO reflexes between them compensate for the complex way in which muscle stiffness changes over the normal working range of muscle. This permits the organization of supraspinal motor systems to be simpler.