It's a very common complaint of an individual with irritation of the ITB to be fine with running uphill, but immediately feel lateral knee pain running downhill. Why is this?
When running downhill, our biomechanics are altered in that we spend more time braking and less time in propulsion. This can drastically increase knee loading.
The impact force peak has been shown to increase by 44% on a 9 degree decline (downhill), compared to level, with no impact peak at all when running on incline (uphill).
"Downhill running increases normal impact force peaks and parallel braking force peaks. In constast, uphill running decreases impact force peaks and increases parallel propulsive force peaks" (Gottschall & Kram, 2005).
Gottschall & Kram, 2005
So therefore, with such a higher impact peak and more shock absoprtion requirements, it's imperative that our lower limb has an excellent ability to absorb shock (eccentric strength and control). If this doesn't exist, we will call much more on the passive structures - such as the ITB - to take that increased impact force and load.
Therefore, eccentric lower limb strength is integral to handle the demands of downhill running for individuals with lateral knee pain.
Gottschall, J., & Kramm R. (2005). Ground reaction forces during downhill and uphill running. Journal of biomechanics, 38, 3, 445- 452.