Can you clarify on the following:
1) What is a restricted squat?
2) Do you like single leg squats or not? I was initially under the impression that you did and your last post seems to make me think otherwise.
I personally don’t think RF needs much special attention. In my opinion it seems to be overworked compared to the posterior chain in most programs.
The restricted squat is one where the knee is restricted from moving beyond the toes. Nothing new really just has a name. Perhaps it encourages a bit more rotation about the knee.
In terms of single leg squats I like it only if it is restricted and even then I prefer the body position to replicate that of the second pull phase of an OL clean. Also it has to be mainly concentric (i.e. off a box) or else it will be too quad and glute (medius) dominant. I feel this trains the muscles in the co-ordinated fashion that replicates that of the sprint block start plus first three steps. The RF generally gets a lot of work in knee extension but during the early part of the race it must do more hip extension. It is based on strengthening in the specific patterns required in the actual event (Newton & Kraemer 1994, Schmidtbleicher 1992, Young 1993). The advantage of doing this specific type of resistance training as opposed to just sprinting is that we can promote adaptation in strength because we can increase the load.
Otherwise I like the (double leg) squat though I feel it facilitates stiffness endurance mainly in the later part of the race but also helps out a little in the initial acceleration phase with one-quarter the effect that plyometrics can.
In regards to posterior chain, if it being neglected by coaches they may be interested to know that the biceps femoris may actually play an equal if not greater role than the RF or Vastus muscles in the blocks and first few steps of sprinting (according to EMG in Mero et al. 1992 and studies by Jacobs, Van Ingen Schenau and Bobbert). For the posterior chain I favour the Russian hamstring curl (albeit quasi-isometric) with a good morning. If that is not clear enough, it is a good morning from a very bent knee position.