What is the flexor hallucis longus, or FHL? It is the fancy anatomy name for the longer of the two muscles that flexes (pushing down towards the ground) the big toe. You may think that this would be a pretty small muscle, considering it moves only one of your toes. However, this muscle runs all the way from the back of your fibula (lower leg bone on the outside of the leg), down towards the inside of the lower leg (tibial side), and then through the arch of the foot, all the way to the furthest away bone of the big toe (the base of the distal phalanx).
So what does this muscle have to do with running?
First, this muscle is a stabilizer of the foot. It helps maintain the medial longitudinal arch of the foot, due to its location in the foot. Strengthening this muscle and other foot muscles to better support the arch can help with pronation control while running.
Second, this muscle helps with the last bit of push-off during running. We often talk about “triple extension” of the hip, knee, and ankle when discussing the push off phase of running gait. While the glutes, as the largest of these muscles, should be the primary driver of this motion, the FHL provides great toe flexion as the final movement that occurs before toe off.
Pain related to the FHL may present as pain when performing heel raises, pushing off running or walking, or when pointing the toes downward. This pain may be felt in the ankle, especially near the inside ankle bone, or in the bottom of the foot along the big toe. FHL tendonitis or dysfunction is most common in people who point their toes frequently, or use the muscle to forcefully push off the ground a lot such as runners, dancers, and gymnasts.
Treatment of the FHL usually involves some amount of rest or decreased loading, and then strengthening of the muscle as well as the other foot and ankle muscles.
There are many ways to strengthen the FHL. To activate the muscle, the big toe pushes into the ground. This can be combined with a single leg balance exercise, like a kettlebell pass, to work on foot and ankle stability and strength. The Mobo board is another of our favorite tools for the FHL. In addition to being a balance board, it has cut outs where the 4 small toes go, forcing you to push through the big toe for balance.
To stretch the FHL, bring your big toe back towards your shin. This should be performed with the ankle dorsiflexed, which means that the foot is also pulled back towards the shin.
Thanks for reading! If you have pain that sounds like it might be your flexor hallucis longus, or any pain that is keeping you from enjoying your sport, come see one of our physical therapists who will get you back to doing what you love!
Dr. Elizabeth Karr PT, DPT