There was an article recently published in a major physical therapy journal where they studied the kneecap (patella-femoral) joint and the achilles tendon (the tendon at the end of the calf muscle) to determine how much stress these areas underwent during TREADMILL vs OVERGROUND (outside) running.
The runners were healthy, uninjured runners, and the findings were published in the May issue of the Journal of Orthopedic & Sports PT.
The findings found that both treadmill and outside running produced the SAME loading forces, but that the TREADMILL produced greater ACHILLES TENDON forces, which would suggest that treadmill running could lead to more achilles tendon injuries
Here’s my take on the findings.
The knee is a shock absorber and the shock forces change depending on several things, one of them being the running surface.
Cement is worse than asphalt and downhill is harder on the knees while uphill is easier on the knees but harder on the achilles. Dirt, grass or man-made tracks are all softer than cement .
Another consideration is this. I’m a FORM guy. I believe that RUNNING FORM is the ultimate variable in determining not only how much stress is placed on areas such as the knee or achilles but I believe that running form is one of the main (if not the main) reason runners get injured.
To find that the treadmill caused more stress on the achilles tendon makes sense. A treadmill belt moves under the runner and therefore it is moving the leg back behind the runner. The work of the achilles happens at the point where the leg is behind the runner and depending on how far behind the runner will determine how much work and stress this tendon will undergo.
It is up to the runner to remove the foot early off the ground to minimize this issue. An optimal cadence will reduce the workload on the achilles, while a slow cadence will increase the workload.
In the same way an optimal landing will reduce the workload on the patello-femoral joint regardless of the surface, but it will still be less stress on the knee if the surface is more cushioned (dirt, grass, track, treadmill VS. cement sidewalk) than less.
The conclusion I will make is this. If you run on a treadmill, while the surface is easy on the knees (and joints) the tendency is to have the leg go back too far which puts stress on several structures including the achilles tendon. If you run outside, the increased shock forces of cement and asphalt is harder on joints but may be easier on the achilles.
The bottom line is FORM. Knowing how to run optimally will help you stay in the safest running mode no matter what surface or environment you are placed in.