The chronic exertional compartment syndrome (CECS) consists of a disturbance of the muscular function of a compartment related to a rise in the intramuscular pressure of the aforementioned compartment, during an exercise. The most frequent location of this pathology is the leg. The 2 parts of this article are treating the subject. The clinical presentation is often not very specific and the realization of complementary investigations is essential to confirm the diagnosis. Among those, the measurement of the intramuscular pressures of all the compartments potentially accused, at rest, during an effort and especially in phase of recovery, represents the determining criteria. The physiopathological mechanisms implied in the syndrome and currently recognized, are also the object of this 1st part.