Figure 3.

Pain referral from a myofascial trigger point (MTrP) in the soleus muscle to the sacroiliac (SI) joint. As shown in figure 2, referral of pain to the SI joint can be explained as follows: first, nociceptors in the trigger point induce local pain. The nociceptive impulses arising from the trigger point are then carried over spinal cord neurons belonging to the segments L5–S1, which are the normal relay stations for impulses from the soleus muscle. As excitation spreads in the spinal cord (in this case, mainly in the caudal direction), the normally ineffective connections between the soleus muscle and the neurons of the S2–S4 segments become effective. Impulses from the trigger point nociceptors can now activate neurons in S2–S4 that otherwise provide sensory innervation to the SI joint. The individual therefore feels pain referred to the SI joint.