The first line of advice would be to consider how to better avoid falling in the first place. I have run over two hundred trail races and fallen twice. Both occurred under the following conditions:
- (a) later in the race when already tired
- (b) steep downhill sections on technical terrain and
- (c) using bad form.
What is bad form ? May be one of a number of traits: here are some I had been guilty of:
- pointing your toes downward.
- Not being "light" on your feet.
- "Committing" yourself to big steps - instead of taking more smaller ones: What happens if that "big" step hits a rock? Take more and smaller steps. One should strive to use several small/quick steps instead of one "heavy" one.
Now .. to your original question about knee pads. They will not "save" you if you take a bad fall -especially if you were taking big heavy steps as described above.
However they can certainly be helpful. Look into either
Volleyball knee pads
Skating knee pads: many varieties are available
of lesser or greater protection - and corresponding
greater or lesser flexibility.
I have worn
volleyball knee pads in trail races - and won handily. They can make little or no difference in your performance: in one 8Km race I tied my own course record wearing the pads.
Update (two weeks later..). The "light on your feet" just saved me in a 10K trail race. I hit an uneven portion of the trail badly and started to fall forward. But I had been using the lighter steps and keeping the knees a bit raised.
Therefore, the other foot stayed underneath me and several small and quick strides later I recovered proper form. The arms went wide and the body went nearly parallel to the ground but the face plant was avoided. That all happened at about a 6:30 minute/mile running pace on a fairly steep downhill: so hitting the dirt would not have been pleasant.
The added safety of putting more effort into each downhill stride does come with a cost: I find my downhill pace is slower. It also does use more energy: just kind of "falling" downhill is easier on the legs and faster (*). But - as just shown - the extra few seconds is well worth the slight performance hit.
(*) This approach is useful for higher downhill speeds on smoother surfaces such as pavement or well maintained fire roads.