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Shin Splints – Part 1 – What are shin splints? What causes shin splints?

Shin splints, or the more medical name for it: medial tibial stress syndrome (MTSS) is a common running injury. It’s usually placed in the top 5 lists of prevalent running injuries. It’s probably number 2 on the list of what I see in clinic, just behind PFPS (patella femoral pain syndrome or runners knee). Studies have reported shin splints to occur in 4% to 20% of the running population. Out of all of the running injuries that I see, I think this one, is the least well understood, in terms of exactly what’s going on. It’s an injury that I have personal history with. When I took up running again, after a long absence, it was the first injury I got, and the resulting frustration, is what kick started my journey into understanding running injuries.

Like many running injuries, shin splints is classed as an overuse injury. It does appear in other sports, but is much more prevalent in runners and in jumping sports (I see a few cases from high impact aerobic classes). It definitely seems to be a condition that effects novice runners more than fitter, more experienced athletes, the reason for this should become clear later on.

A trip around the lower leg

Before we get into the symptoms of shin splints, let me orientate you to a few important structures.

Tibia of a right leg. Draw your focus to about a 2/3rds of the way down the tibia, where the bone is narrowest. We'll be referring to this area a lot as we continue, it's where all the action is taking place.
Tibia of a right leg. Draw your focus to about a 2/3rds of the way down the tibia, where the bone is narrowest. We’ll be referring to this area a lot as we continue, it’s where all the action is taking place.
Deeper, posterior (behind) tibial muscles – tibialis posterior, flexor digitorum longus, flexor hallucis longus
Deeper, posterior (behind) tibial muscles – tibialis posterior, flexor digitorum longus, flexor hallucis longus
Soleus lies deeper to gastrocnemius and orinates above the knee. Gastrocnemius originates above the knee. Both muscles converge to form the achilles tendon
Soleus lies deeper to gastrocnemius and orinates above the knee. Gastrocnemius originates above the knee. Both muscles converge to form the achilles tendon

 

This view gives us a better view of the how the muscles sit, in relation to the bone. Note the lack of muscles on the medial (inside) tibia. You have some muscle bulk to the front and lateral tibia but most of the muscle bulk is behind the tibia.
This view gives us a better view of the how the muscles sit, in relation to the bone. Note the lack of muscles on the medial (inside) tibia. You have some muscle bulk to the front and lateral tibia but most of the muscle bulk is behind the tibia.

Symptoms of Shin Splints

In shin splints, pain is usually felt 2/3rds of the way down the tibia, on the inside (see fig. 6). In the early stages of the condition, pain is felt at the beginning of exercise and then subsides during the training session. It also tends to settle a few minutes after the session has finished. If the condition progresses, pain is felt in less intense activities and can be present at rest. It is painful to palpate the distal medial third of the tibia and the soft tissues around this area (red section in figure 6). Because the antero-medial border (front/inside) of the tibia is subcutaneous (directly under the skin), pitting oedema, and even callus formation, can be felt on firm palpation in some cases. This is thought to be a response of the periosteum to micro-fracture formation.

Common site of pain in MTSS
Common site of pain in MTSS

 

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Theories

So what exactly is going here? As I mentioned earlier, the exact causes of MTSS, and what structures are involved, is still up for debate. Lets look at the more plausible theories.

The soft tissues of the tibia

One of the suspects in this case, are the soft tissues of the tibia. The muscles of the calf, and the smaller muscles of the ankle, have their attachments along the tibia. Specifically, the tibialis posterior, flexor digitorum longus and soleus muscles (fig. 2), have been implicated as possible sources of injury. The theory being, that tightness, weakness, or excessive movement at the ankle causes traction or tugging at the site of their origin on the tibia, leading to an irritation of the periosteum (Fig. 4).

However, this theory doesn’t really stand up to anatomic scrutiny, in that, all of the above mentioned muscles, have their origins above the site of pain. It seems unlikely that traction of an origin site, higher up the tibia, would produce pain at the distal, medial shin. There is however, another soft tissue that does attach to the medial border of the tibia, in the location of the symptoms. That structure is the deep crural fascia (fascia is a thin sheath of fibrous tissue enclosing muscles and organs). This structure has deep insertions to the medial tibial border, finishing at the medial malleolus (inside ankle bone). It’s highly likely that the above mentioned muscles will be continuous with this fascia. Therefore, from an anatomical point of view, the deep crural fascia could tug on the periosteum, in the location of symptoms, and create the traction mechanism of injury detailed above.

Decreased calf strength

Some studies have suggested that smaller calf size, and decreased calf strength, could be connected with shin splints. The theory being, that it is the tibia itself, that is the tissue source involved in the pain response. Bigger, stronger calf muscles encourage the tibia to become stronger, and therefore, able to take greater loading. Additionally, the calf muscles themselves being stronger, would mean they would be able to absorb impact forces directly, again taking the load off the tibia.

Tibial bowing/bending

This alternative explanation, again implicates the tibia itself. With this hypothesis suggesting that the problem arises via micro-trauma to the bone, due to repeated bending or bowing of the tibia. It is well known in engineering, that when you load a long, narrow structure (like the tibia) axially e.g. place a force through the centre of the object, lengthwise, it will result in bending moments, and at the structures lowest cross sectional area (the narrowest part – going back to Fig. 1, the narrowest part of the tibia is the distal 3rd where shin splints are felt).

It is generally suggested by biomechanists, that the tibia bends in the saggital plane (forwards and backwards plane of movement) when running, placing most force at the distal anterior section of the tibia. But this depends on where you land. Think of it a bit like a pole vaulters pole. As they approach the jump, they plant the pole ahead of themselves (similar to an over stride when running) so you see this big, anterior bowing of the pole, great for pole vaulting but not so good for tibas!

Shin splints - Overstride
picture taken from http://triathlon.competitor.com/2014/07/training/hit-your-stride_57577 Fig. 7 – Anterior tibial bowing. If you land way ahead of your centre of mass with your tibia in front of your knee you will experience an anterior bending moment in the tibia, similar to the pole vaulters pole (although nowhere near this dramatic)

If we factor in 3 potentially important elements, we see that this anterior bowing force, in many runners, will be shifted medially, to where the symptoms of shin splints are reported. Firstly, pronation occurs, (we all do it to varying degrees) which will shift the ground reaction force more towards the medial shin, and cause more of a varus (sidebending) force in the tibia. Pronation is usually coupled with medial rotation of the tibia, which also places more stress medially. Thirdly, when we run, we tend to land more towards the midline than directly in line with our hip. This is because it’s easier to get our centre of mass over our base of support (one of the reasons we have a medially angled femur – check out my evolution blog for more details). The consequences of this, are that our tibias will absorb the ground reaction force at an angle, and therefore a varus (sidebending) force, will again, be applied to the medial tibia, at the part of the tibia with the least cross sectional area e.g. where most people with shin splints have their symptoms.

Pronation of the right foot, calcaneous everts, tibia rotates medially (inwards)
Pronation of the right foot, calcaneous everts, tibia rotates medially (inwards)

Additionally, the muscular contractions of our plantar flexors, namely soleus, can also cause a tibial bending moment. If you imagine the attachment site of soleus, at the top of the tibia, and its insertion at the calcaeneus. As we go through midstance, and the soleus begins to contract to slow the acceleration of the tibia, the origin and insertion sites will be pulled towards each other, again causing an anterior bending in the saggital plane.

It is important to note at this point, that bone stress is not a problem, if its applied at a gradual rate. Bone is an organic and dynamic structure. Like all living structures in the human body, it responds to stress by remodelling itself, to ensure it can cope with the demands being placed on it. The problem comes, when the accumulated stress of training, outstrips the body’s ability to remodel the area. This gives us an incite into why this condition is more prevalent in novice runners, or less fit runners, because their bones have not adapted over time to the stress of repetitive, high impact exercise. It also gives us another layer of evidence to illustrate that, when it comes to injury prevention, it may not be the sexiest intervention, but the most fruitful therapy is to obey the law of adaptation, and accumulate the stress slowly, giving your body’s tissues the chance to adapt and get stronger and therefore be able to withstand greater load.

The tibial bending theory has not been proven yet, but it certainly seems to me, to be the most persuasive of the current theories on offer. As always i’ll allow this to guide my practice, whilst keeping an open mind and following any further developments in the literature.

Differential Diagnosis

    • Stress fracture – Interestingly, some authors feel that stress fractures are on a continuum with shin splints, whereas others feel they are similar but different conditions.

    • Chronic exertional compartment syndrome or anterior biomechanical overload syndrome. 

    • Popliteal artery entrapment

    • tibial tumours

    • Bone infections/osteomyelitis

In part 2 of this blog, we’ll again look at how I treat this condition in clinic, and some cool tricks to get you running pain free…

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