posterior intermuscular septum of leg posterior intermuscular septum of leg the posterior intermuscular septum of leg, or posterior crural intermuscular septum is a band of fascia which separates the lateral compartment of leg. posterior intermuscular septum of legcross-section through middle of leg.detailsidentifierslatinseptum intermusculare cruris posteriusta98a04.7.03.023ta22710fma58758anatomical terminology the deep fascia of leg gives off from its deep surface, on the lateral side of the leg, two strong intermuscular septa, the anterior and posterior peroneal septa, which enclose the peronæi longus and brevis, and separate them from the muscles of the anterior and posterior crural regions, and several more slender processes which enclose the individual muscles in each region.