Flexor Hallucis Longus muscle (and subsequent tendon) runs down the medial border of the tibia past the malleolus and inserts into the plantar surface of the foot and into the base of distal phalanx of hallux. Its role is to move the big toe downwards. FHL pain is usually characterised by medial ankle and foot pain. Sometimes, the discomfort can run up into the medial calf.