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Barefoot Running Blisters: Interpreting My Pattern and the Importance of the Flexor Hallucis Brevis?

Posted on August 26 2011

Ever since running with my friends Mark Cucuzzella and Blaise Dubois out in Colorado a few weeks ago, I have been giving another go at a bit of barefoot running. Mark is now running a solid percentage of his weekly his miles fully barefoot (see video below), whereas Blaise supplements his regular training with small amounts of barefooting here and there.

Like most of what I do when it comes to running, my main reason for trying a bit more barefooting is simply to see what happens – yet another of my little experiments on myself to satisfy my curiosity. I like pushing my limits and seeing what my body can do, and quite honestly I’ve been enjoying the little bit that I have done so far.

When I was out in Colorado, I ran a mile fully barefoot on the Boulder Canyon trail, and it felt great. My feet handled it fine, with the exception of small blisters that started to form on the soft skin at the base of my big toe. Since returning, I’ve continued to end runs a few times a week with a bit of barefoot walking or running, always on asphalt sidewalks. Yesterday, I was running in Vivobarefoot Neos for a five miler, and about a mile into the run the insole on the right side started to scrunch up under my big toe, and over the next few miles I couldn’t seem to keep it flat. It was irritating the skin just behind the pad of my big toe. I stopped about a mile from home and figured I’d go the rest of the way barefoot. Unfortunately, the insole rubbing had done some damage in the same spot as where I had developed the (healed) blister in Colorado (see second photo below).


Note the blister at the base of my big toe. The red, irritated part further anterior is because I stupidly pulled the dead skin off too far.


Yuck. The real damage done to my big toe here was a loose, bunched up shoe insole moreso than barefoot running.

Anyway, blisters are a part of acclimating to barefoot running (just as they were when I started running high mileage in shoes – I have the calluses on the inside of the balls of my feet to show for it!). By and large, my feet have done great, and the only blisters of any consequence that I have developed were in this spot on each big toe. I’m barefoot most of the time at home during the summer, so my soles seem to be pretty well adapted to the condition. This got me to thinking – aside from the obvious point that I was running without shoes on asphalt, why did I get blisters in this particular location during my runs? My curiosity as an anatomy prof with an interest in running mechanics was piqued.

A few weeks ago I posted a link to an article by Jay Dicharry from the UVA Speed Lab where he discussed factors that are important for making a safe transition into barefoot or minimalist running. One of those was the the ability to isolate the flexor hallucis brevis muscle during stance phase of running (see photo at left). Here’s what Jay wrote about this:

“A key factor that distinguishes humans from primates is our medial longitudinal arch. This arch is actively stabilized by the flexor hallucis brevis (FHB). While standing, try to drive the big toe (1st MTP) into the ground (plantar flexion) while slightly elevating (dorsiflexing) the lesser toes. Make sure not to roll the ankle in or out. This test enables screening of muscles inside the foot that stabilize the arch. The FHB can be easily distinguished from the longus (FHL), as the FHL crosses another joint in your big toe (1st IP joint), resulting in your big toe curling. Spend some time getting to know your foot. Aim to drive the big toe down while lifting the little toes (without curling the big toe!), and lift the big toe up while driving the little toes down. It’s the best way to work on coordination of muscles that actively stabilize the foot in stance. It’s your foot – control it! If you can do this, it’s a sign that you can keep the rear foot stable on the forefoot when the body sees the greatest amount of pronation (which is just slightly after midstance and AFTER the heel is off of the ground by the way. Midstance is when forces are highest throughout the body- about 2.5x’s your body weight. You need the internal strength to be able to respond to these forces to keep things in alignment.”

Flexor Hallucis BrevisThe flexor hallucis brevis (FHB) muscle that Jay is referring to is an intrinsic foot muscle (meaning that it originates and inserts within the foot) that functions to flex the big toe downward (see red muscle in photo at left  – image via Wikipedia and Gray’s anatomy). It attaches to two small bones called sesamoids, and then continues to attach to the base of the proximal phalanx of the big toe (put simply, it attaches to the base of the big toe and helps to drive it into the ground during stance). The flexor hallucis longus (FHL), on the other hand, is an extrinsic foot muscle (meaning that it originates outside of the foot) that attaches to the fibula in the lower leg and inserts on the bone at the tip of the big toe. As a result, the FHL curls the big toe as it flexes it.

Given Jay’s thoughts on the importance of the FHB to stability during stance phase of running, my hypothesis would be that this muscle is partly responsible for my developing blisters in this particular location. Contraction of the FHB holds the big toe in a very stable position against the ground during stance by forcing the base of the big toe downward, and as I push off I suspect the FHL begins to pull the big toe back a bit, creating friction leading to the blister. My guess would be that if the FHL was playing the only role here (e.g., if my FHB was weak), then my blistering would be located further forward on my big toe, as I have seen in other images posted online, but just guessing wildly at this (and please, point out the error of my ways if I’m totally off on this!). I have no intention of injecting Botox to paralyze my flexor hallucis brevis, but it would be cool to see what would happen!

In a nutshell, I think this is telling me that my big toe is doing exactly what it should be doing while I run (i.e., stabilizing during stance), and that my skin in that spot just needs to toughen up a bit.  Incidentally, I can feel the tenderness under this spot during single leg balancing, but it alleviates if I attempt to curl my big toe while keeping it flat on the ground. All of this makes me suspect a the combined role of the FHB and the FHL here in causing the pressure that creates the ground friction needed to cause blistering under my toe (the abductor hallucis could also be involved, but let’s keep this simple for now…).

Anyway, the take home message is to not forget that you do have muscles in your feet, and they can be doing important jobs when you run. Couple this with shoe induced deformities like some cases of hallux valgus (inward bending of the big toe), and feet/toes adapted to sitting on stiff shoe soles, and you can see how it might take some to adapt to something less (or nothing) underfoot.

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