The Chachamim say
"When one bows at the word baruch (in Shemoneh Esrei), he must bow low enough to see and issar a coin opposite his heart (Brachot28b). This can be interpreted as "Bow and lower yourself to the point where you will see issur-borbidden actions and sin in your heart"...
This could be what Chazal alluded to when they taught that "one who does not bow during Modim (in Shemoneh Esrei) after a while his spinal column is transformed into a serpent"(Bava Kama 110a)... That is, if he does not bow low enough to see an issar opposite his heart. As we explained, this alludes to the ability to perceive one's shortcomings, and this is achieved through bowing and humbling oneself rather than imagining that one is perfect a lacking nothing. Such a person never exerts effort to work through the seven levels of divine service, and such a person's spine transformed into that very serpent ( serpent in Gan Eden the original cause of sin ) that is the source of his deficiencies because he didn't humble himself. MePeninei Naom Elimelech pg 262
