Shin Splints are experienced as acute pain in the front of the tibia (shin bone). They are usually brought on by running. The term shin splints is used in different ways to describe different things. Usually the term shin splints is used to describe the condition Medial Tibial Stress Syndrome. However, sometimes shin splints is used as an umbrella term to describe a bunch of conditions that manifest as pain in the shin including stress fractures and compartment syndrome.
In this article, we’re going to dig into the pathology (disease process) of shin splints and why shin splints develop. In part 2 of this series we’ll dig into the treatment side of the equation.
Where is the Pain of Shin Splints Coming From?
To answer this question we have to define what condition we are talking about. I personally use the term ‘shin splints’ when I am talking about Medial Tibial Stress Syndrome (MTSS). MTSS is an inflammation of the periosteum (outer bone layer) of the tibia (shin bone). This inflammation is often called a stress reaction. The periosteum has loads of nerve endings in it so when it becomes inflamed it can be very painful.
We are going to think of a continuum here with a normal bone at one end, stress reaction (shin splints) in the middle and a stress fracture at the other. A stress fracture is when the injury not only affects the periosteum but also causes a crack in the cortical bone underneath it. This continuum represents the spectrum of Bone Stress Injury (BSI).
This all sounds a bit complicated and medical so here it is put more simply:
“Shin splints is inflammation of the outer layer of your shin bone. If it gets really bad it will become a crack and that’s a stress fracture.”
Why do I have Shin Splints?
As we land when we run our tibia absorbs some of the impact. The fancy name for impact is Ground Reaction Force (GRF). This GRF can be really high when we are running. Usually somewhere in the region of three times our bodyweight! That’s a lot of impact to absorb. Fortunately, our body is amazing at coping with and adapting to this stress.
Let’s say we run 25k one week. Every step we land some of that massive impact force will be absorbed by the tibia. On a cellular level this causes microdamage to the tibia bone. The body sends in the clean up crew to get rid of all the debris. This clean up crew is made up of cells known as osteoclasts. Following close behind the clean up crew is the repair team. These cells are osteoblasts and they rebuild the bone.
The repair team are really good friends with the clean up crew. They don’t want too much microdamage next time you go for a run or there might be too much for the clean up crew to handle. So the repair team make the bone even stronger than it was before. On the next run, the tibia will be stronger and there will be less microdamage. This is another amazing example of the body’s capacity for adaptation to stress/load.
The problem comes when there is too much microdamage for the clean up crew. Say the following week you run 35k. The repair team only made your tibia strong enough for 30k. Now there is a lot of microdamage for the clean up crew. They get out there and start clearing the debris but there is too much. They’re holding up the line and the repair team can’t get in there and start repairs until the debris is cleared.
When the clean up crew get out of the way the repair team get started. They’re in there working like crazy but there isn’t enough time to finish. The next week rolls around and you decide you’re going to run 40k. The repair team have not even finished making your tibia strong enough for 25k!
Now the tibia is not strong enough to withstand the impact being placed on it. The microdamage exceeds the repair and the result is inflammation. It’s the outer layer of bone known as the periosteum that becomes inflamed first. Now it hurts towards the end of your longer runs but only a little bit. It goes away as soon as you stop so you decide it’s just a niggle and you can continue training.
Over the next few weeks you don’t increase the weekly kilometres too much but it doesn’t matter. The clean up crew are still playing catch up on all the debris accumulated over the last few weeks. The repair team have barely made your tibia strong enough for 25k and you’re logging 30-45k per week. The inflammation of the periosteum worsens and eventually the layer of cortical bone underneath it starts to get inflamed as well. Now it hurts after only 5k running and lasts a few hours after every run. This continues another few weeks and then the cortical bone cracks a little. Now you have a stress fracture.
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Above is an example of overloading. In this example, we can see how it wasn’t a giant increase in running in one week that caused the problem. Rather, it was the cumulative loading over weeks and weeks that was just a little bit more than the clean up crew and repair team could handle. With demand on the tibia just exceeding supply a little bit it took quite a long time for the symptoms to become anything more than just a niggle.
It’s a bit like money. If you spend a little bit more than you earn, you will be okay for quite a while. However, if that behaviour continues then the debt will slowly accumulate. One day you realize you’re in a financial crisis. Now you have big fat whopping debts. The important part here is that it will take quite some time to pay off those debts and get back in the black. Even more importantly, you will only start to pay off the debts if your behaviour changes.
It’s the same for your tibia. If you have been loading it a little more than it can handle for a long time, eventually it will start to hurt. Unless your behaviour changes it will start to hurt more and if it continues you will end up in a crisis. The trick is to ‘spend less than you earn’. Then you can build up some savings. We’ll return to this in more detail in the following article on shin splints treatment. For now though, let’s get back to the pathology.
We’ve talked above about how the tibia becomes overloaded in shin splints. A lot of this has to do with applying too much load to the tibia too quickly. This is to do with training, running technique, strength and things like that. We’ll dig into those more in the next article. For now we are going to talk about the other side of the equation. Not so much ‘How much load do you put on your tibia?’ but ‘How much load is your tibia able to handle?”. This brings us to bone density.
A bone will bend just a little when placed under load. When you are running you will load your tibia as you land and it will bend just a tiny bit. Softer bones will bend a little more and experience more strain. More strain = more microdamage. Conversely, harder or denser bones will bend a little less and so experience less strain. Less strain = less microdamage.
So denser bones will experience less microdamage and be less likely to develop shin splints or stress fractures. Anything that increases bone density will help protect you from shin splints and stress fractures. Anything that decreases bone density will make you more vulnerable to shin splints and stress fractures.
Physical Activity History
A long history of physical activity seems to be protective against shin splints and other bone stress injuries (Warden 2014). This makes sense given what we discussed above. The repair team make bones stronger in response to microdamage (as long as there is not too much). This will increase your bone density. So if you’ve spent years running around and loading your tibia they are going to have adapted to that loading and become as strong as steel.
Many new runners experience shin splints and this can be one of the reasons. A new runner who has a long history of physical activity is less likely to develop shin splints than one who has been relatively inactive.
A long history of physical activity will also mean you’re more likely to have strong muscles. The muscles can help manage some of those impact forces coming up through your leg as you run. The stronger your muscles the more impact they will absorb so the tibia doesn’t have to (we’ll talk more about this in the next article).
Those osteoblasts in the repair team need materials. They have to build stronger bones out of stuff and if there is no stuff available then they can’t build good strong bones. This happens when the energy output is above the energy input and so we have an energy availability problem. In short, you’re not eating enough to fuel your training.
This can happen because you are training too much, not eating enough or a combination of the two. Often, people take up running to help them lose weight. These people are prone to shin splints for a number of reasons:
- They are eating less (less energy intake)
- They are exercising more (more energy expenditure)
- If they are trying to lose weight they may have a higher BMI (a risk factor for shin splints)
Shin splints and other bone stress injuries are also problematic for high level runners. Female distance athletes in particular are prone to these issues as a part of something called the Female Athlete Triad. This is a syndrome involving three interrelated conditions; Energy deficiency (with or without disordered eating), menstrual disturbances/amenorrhea and bone loss/osteoporosis.
Calcium and Vitamin D Status
Lower calcium intake seems to be related to increased risk of stress fractures in distance runners (Nieves 2010). Low vitamin D levels have been correlated with increased risk of stress fractures in military recruits (Ruohola 2006). This makes sense because we need calcium to build our bones out of and we need vitamin D to help us absorb more calcium.
I’m not a nutritionist by any means, but I do make a hobby of reading up on nutrition. To say that nutrition is a controversial topic is an understatement. Should we supplement Vitamin D? Does dairy cause osteoporosis? Where should we get our calcium? This stuff is widely debated and I’m not qualified to comment. All I’m going to say is that we need enough calcium and we need enough vitamin D. You will have to do your own research.
The pain of shin splints is caused by inflammation of the periosteum (outer layer) of the tibia (shin bone). If it progresses a crack will develop in the cortical bone underneath known as a stress fracture. This happens due to overloading of the tibia.
Anything that reduces bone density will make you more likely to develop shin splints or stress fractures. This includes a history of low levels of physical activity, low energy availability and vitamin D and/or calcium deficiency.
By now you should understand more about why people get shin splints and how they can be prevented. In Part 2 we talk about how to treat shin splints if you already have them.