What is it? 
Speculation is rampant, but in reality no one really knows.  As with any "syndrome" exactly what is causing the pain is unclear or at least not conclusively proven.  It may be irritation of the outer covering of the tibia itself, tendinitis of the muscles of the lower leg, or a combination of both.  It also could be any of several other possibilities.  

It hurts where? 
In most situations the athlete will have pain on the posterior-medial aspect of the tibia.  The pain is usually in the lower 1/2 of the leg.  Normally the painful area will be 2-5 inches in length.  It will be sensitive to the touch just over the edge of the bone and or the muscles/ tendons in that area. 

How does it happen? 
TSS is an overuse injury.  It will occur gradually over a period of time.  True TSS will not occur from one day of training.  It most often occurs from repetitive running and on occasion jumping.  The direct causes can vary greatly including, shoes, the running surface, increasing training too soon, compensating from other injuries, overtraining, or from structural problems (such as over pronation of the feet, leg length discrepancy).  Structural problems may need to be corrected with orthotics. 

Injury Progression ... 
Initially the athlete will have pain only while running, this is considered stage 1.  If the problem progresses pain will be present while running and last for a period of time after running is concluded, this is stage two.  Progression past this stage is bad news.  The athlete will have pain before, during, and after training.  Or it hurts all the time.  Athletes that truly reach stage 3 warrant referral to an orthopedist. 

Similar Injuries: 
Any athlete with TSS must be monitored for a stress fracture of the tibia.  Pain along a specific area of the tibia to palpation covering only 1/2" in length (or less) is a sign a stress fracture should be suspected.  TSS will not present with any type of altered sensation or numbness or tingling in the leg or foot.  These symptoms require referral to an orthopedic surgeon. 

Treatment: 
Wrapping ice bags over the painful area for 20-25 minutes after training is a good place to start.  At Stockton we’ve had a great deal of succes with the following, strengthening all of the ankle musculature (including the soleus), working on proprioception (wobble board), increasing range of motion (plantar/ dorsiflexion).  Time off from aggravating activities is also usually necessary, including additional days off from training and substituting non-impact activities for running (biking, swimming, etc.). 

Participation Status: 
Athletes in stage 1 can continue to participate as tolerated implementing ice and a rehab protocol.  Athletes in stage 2 can also continue to participate but require rehabilitation and additional days off from training and days of training with alternate activities.  The athlete that gets to stage 3 should be seen by an orthopedist before regular running is permitted.  If a stress fracture is found by the physician, the athlete will be out of competition for about 4 weeks.