Flexor hallucis longus
Flexor hallucis longus originates on the posterior surface of the fibula in a position lateral to flexor digitorum longus and tibialis posterior. Its tendon of insertion begins in the middle of the leg and passes across the posterior talar groove and the inferior groove of the sustentaculum tali as it courses distally. The tendon may be best palpated by asking your subject to plantarflex their ankle and flex their hallux against resistance. With the tendon under tension, it may be perceived as it passes across the posterior talar groove and as it courses inferior to the sustentaculum tali.