From resting conditions....
Dynamic fusimotor stimulation causes local intrafusal contraction which stiffens the polar regions, but has little effect on the primary region.
Much of the external stretch is transmitted to the primary region which may extend by 15%.
When stretch is maintained the polar regions 'creep' and the primary endings unload slightly. This is the basis of slow mechanical adaptation.
Dynamic bag fibres are sensitive to change in length and this is enhanced by dynamic fusimotor stimulation. Secondary afferents are rather less important.
Primary ending (group Ia) and secondary ending (group II) afferent discharge frequency during stretch alone (black lines) and during stretch superimposed on tonic dynamic fusimotor stimulation (red lines) at 75Hz.