From resting conditions Static fusimotor stimulation causes intrafusal contraction which imposes a significant stretch on the primary ending, which may extend by up to 20%.
Much of the external stretch is taken up at the polar regions, and the primary region may only extend by 5%.
When stretch is maintained there is no further mechanical change, however there is a reduction in afferent discharge due to electrical adaptation.
Static bag fibres provide a strong positive bias to primary endings whilst reducing sensitivity to length changes. Effects on secondaries are similar but smaller.
Primary ending (group Ia) and secondary ending (group II) afferent discharge frequency during stretch alone (black lines) and during stretch superimposed on tonic static gamma fusimotor stimulation (red lines) at 75Hz.