Shin splints are caused by repetitive stress on the shinbone and the tissue surrounding it, typically from running or other high-impact activities. Between 13.6% and 20% of runners develop the condition, and it accounts for up to 60% of all lower leg injuries in active people. The underlying problem is a combination of bone microtrauma and inflammation of the tissue lining the bone, triggered when your body can’t repair itself fast enough to keep up with the demands you’re placing on it.
What Happens Inside the Bone
Your shinbone (tibia) is built to handle impact forces, but it does so through a constant cycle of tiny damage and repair. Every time your foot strikes the ground during a run, the tibia bends slightly under load. That bending creates microscopic damage at the structural level, where small units of bone called osteons begin to separate from the surrounding tissue. In a healthy training cycle, your body rebuilds those micro-injuries between sessions, and the bone actually gets stronger over time.
Shin splints develop when that repair cycle falls behind. The micro-damage accumulates faster than your body can fix it. Researchers have found that people with shin splints show increased bone-building cell activity and new blood vessel growth at the injury site, signs that the body is trying to keep up but can’t. On top of the bone damage, the periosteum (a thin membrane wrapped around the outside of the bone) becomes inflamed and swollen. Connective tissue fibers called Sharpey’s fibers link the periosteum directly to the bone, so when muscles pull repeatedly on that membrane, the inflammation can spread in both directions.
People who develop shin splints also tend to have measurably lower bone mineral density and smaller bone cross-sections at the injury site compared to other active people. In other words, the structural capacity of the bone matters. A thinner or less dense tibia is more vulnerable to overload from the same training volume that someone with a thicker bone handles without trouble.
Training Errors Are the Most Common Trigger
The single biggest cause of shin splints is doing too much, too fast. A sudden increase in workout frequency, distance, or intensity gives your bones and surrounding tissue less time to adapt. This is why shin splints are especially common in military recruits, with incidence rates ranging from about 8% to 35% during basic training programs lasting 10 to 26 weeks. These populations go from relatively low activity to high-volume running and marching almost overnight.
The same pattern plays out in recreational runners. Adding extra days per week, jumping to longer distances, or introducing hill running all increase the cumulative load on the tibia. There’s no single mileage threshold that guarantees injury, but the principle is consistent: your bones need gradual, progressive increases in stress to remodel and strengthen. Skip that gradual buildup, and micro-damage outpaces repair.
Running Surface and Footwear
Concrete delivers higher impact forces to your legs than softer surfaces. Research measuring shin acceleration during running found that the number of high-magnitude impacts (in the 4 to 5 g range) was roughly 36 to 37% higher on concrete than on synthetic track or grass. That difference adds up over thousands of foot strikes per run. If most of your training happens on sidewalks and roads, your tibia absorbs more cumulative stress per session than it would on a track or trail.
Your shoes matter too, though not in the way marketing often suggests. The key factor is age. Most running shoes maintain their shock absorption for about 300 to 500 miles. Beyond that, the midsole foam compresses permanently and stops doing its job. If you’re running 20 miles a week, that means replacing shoes roughly every four to six months. Worn-out shoes transfer more impact force directly to your shins with every step.
Foot Mechanics and Body Structure
How your foot moves when it hits the ground influences where stress concentrates along the tibia. Overpronation, where the foot rolls inward excessively after landing, increases traction forces on the muscles and connective tissue attached to the inner edge of the shinbone. One study of cross-country runners found that those with a navicular drop (a measure of how much the arch collapses under load) greater than 10 millimeters were nearly seven times more likely to develop exercise-related lower leg pain than those with less arch collapse.
The muscles involved are the soleus (a deep calf muscle), the posterior tibialis (which supports the arch), and the flexor digitorum longus (which curls the toes). These muscles attach to the tibia along the area where shin splint pain typically occurs. When they pull repeatedly on the periosteum during running, they contribute to the inflammation and micro-damage cycle. Weakness or tightness in any of these muscles can shift stress patterns and increase the load on the bone itself.
How Shin Splints Feel
The hallmark of shin splints is a diffuse, aching pain along the inner or front edge of the lower leg. Unlike a stress fracture, where pain is concentrated at a single tender spot, shin splint pain tends to spread across several inches of the shinbone. Pressing on the area produces tenderness over a broad zone rather than one pinpoint location.
Another useful distinction: shin splint pain often improves or at least stays manageable once you warm up during exercise, then returns afterward. Stress fracture pain does the opposite. It persists or worsens with continued activity and is reproducible at the same specific location every time. If your pain is becoming more localized over time, or if it hurts during everyday walking, that progression can signal that accumulated bone damage is concentrating into a stress fracture.
Who Is Most at Risk
Several factors stack the odds against you:
- New or returning runners whose bones haven’t adapted to impact loading yet
- Military recruits facing sudden, high-volume training on hard surfaces
- People with flat feet or excessive pronation, particularly those with a navicular drop above 10 mm
- Women, who tend to have lower bone density and smaller tibial cross-sections on average
- Anyone with a history of shin splints, since the structural vulnerabilities that caused the first episode often persist
How to Reduce Your Risk
The most effective prevention strategy targets the root cause: managing cumulative load. A commonly cited guideline is to increase weekly running volume by no more than 10% at a time, giving bone tissue enough recovery time to remodel and strengthen. If you’re starting a new running program, alternating run days with rest or cross-training days keeps the repetitive impact from stacking up.
Varying your running surfaces helps distribute the mechanical stress differently across sessions. If you normally run on concrete, shifting one or two runs per week to grass or a synthetic track reduces the total number of high-force impacts your tibia absorbs. Tracking your shoe mileage and replacing them in the 300 to 500 mile window prevents the gradual loss of cushioning from becoming a hidden risk factor.
Strengthening the calf and foot muscles, particularly through exercises like calf raises and toe curls, builds the muscular support system that absorbs force before it reaches the bone. Stronger muscles mean less strain transferred to the periosteum and tibia with each step. For runners with significant overpronation, arch-supporting insoles or motion-control shoes can reduce the inward rolling that concentrates stress along the inner shin.