A shin splint or medial tibial stress syndrome (MTSS) is an inflammation of the tissue running along the shin bone (tibia), whereas a stress fracture is a very small crack or group of cracks that form in the bone itself.
With a shin splint, if you run your fingers over the shin, it’ll usually hurt all along the bone. According to physio Emma Deakin “many people describe it as a diffuse, dull ache along the inner border of the shin (tibia). It’s normally worse after running or weight-bearing activity and some triathletes will report feeling the same type of pain when out of the saddle on the bike. But if you ignore the pain it can become sharp and acute, limiting your ability to train.”
A stress fracture is a partial fracture (or break) of the bone that usually only goes a little way through. There’s usually one specific ‘spot’ or multiple spots that hurt really badly. If you have a stress fracture, you’ll also experience pain when walking and sitting.
It is an overuse injury, where the bone is unable to withstand a repetitive mechanical loading. This is different from a full fracture or break of the bone caused by a sudden traumatic mechanical stress.
In triathlon, most injuries are in the lower limbs and can be attributed to the running discipline. Commonly they occur in the tibia, fibula, metatarsals (small bones of the foot), navicular and femurs.