The tibia is the large bone in the lower leg. Pain on the inside of the lower third of the tibia that happens with exercise and sport activities is known as medial tibial stress syndrome, or MTSS. This condition is often called "shin splints."
The exact cause of MTSS is unknown. Imaging studies show an increase in skeletal metabolism in the area of pain, along with a decrease in the density of the bone. Scientists do not know which comes first: the MTSS or the changes in the bone.
Researchers have discovered that these changes in the bone are present in both legs, even when only one side is painful. This finding suggests that not enough bone is being deposited during the early phases of growth. It seems that exercise triggers the symptoms, but no one knows if exercise causes or just contributes to the changes in bone density. More studies are being done to answer these questions.