Some people put their foot in their mouth; this psalmist puts his foot in his heart — or does he have his heart in his foot? Notice these words:
“My heart is set on keeping your decrees
to the very end” (119:112).
And v. 111 ended with this: your statues are “the joy of my heart.”
The heart that is joyous is the feet that are guided by the light of the Torah.
The Torah gives light for the feet as they walk the path; because his feet know the way, his heart is joyous and now resolute to keep the Lord’s decrees to the very end.

Easy to resolve to go all the way; hard to do. The psalmist’s history shows his future: the best way to predict a person’s future is to know their past. This psalmist’s past was a clear path of following the light the Torah had provided for his feet.
The heart of the psalmist was in his feet — his heart was resolved to walk in the ways of the Lord.