While in Greece in 2000, I climbed Mount Olympus with a group from my university. One of the professors at the time was a triathlon competitor and stated that he locked his knee, fully straightening his leg after each step. The reason, and this is also practiced with bicycle seat height (i.e., legs locked straight when that foot's pedal is down) is that it allows the thigh muscle to fully relax on the return from each steap, or for a cyclist, on each rotation of the pedals.
In practice, it does not come naturally to me, so I tend to use it when I feel that I may be getting tired as a way to conserve some energy/muscle strength. Does anyone here follow this practice, or are you familiar with similar approaches. I tried to search, but the terms were a little obscure to get any good results.