I have to disagree with one point of pkt's otherwise excellent and detailed analysis:
The 90-degree angle should be between the front calf and the floor. This provides the best foundation for recovering from the lunge.9. at the end of the lunge the front thigh and the front calf should form a 90-degree angle any thing greater than 90-degree, you're under-lunging, anything smaller than a 90-degree angle, you're overlunging;
The angle between the thigh and calf can and should vary from 90 degrees, depending on how flexible the fencer is. Insisting on a right angle at the knee could risk injury to beginning fencers who don't have that flexibility yet, and discourage more flexible fencers from getting that bit of extra distance.

Maybe he's just recovering.. 