Chronic is a synonym of Continuing. Continuing is similar to Long. Long is related to Leg. you are likely to find Muscle in/at/on Leg. 
Mechanic is similar to Mechanical. 
Chronic <--Synonym--> Continuing <--SimilarTo--> Long <--RelatedTo--> Leg <--AtLocation-- Muscle 
Mechanic <--SimilarTo--> Mechanical 