One leg squats should not be confused with pistol squats. I see this happen all the time. We (MBSC folks) talk about one leg squats and, the conversation immediately turns to pistols.
Let me be very clear. We neither do nor endorse pistol squats. (do you agree? Tell us about it in this thread) Although these two exercises seem very similar they are not necessarily interchangeable. In a one leg squat there is significantly less stress on the hip flexors and subsequently less stress on the lower back than in a pistol squat. In a one leg squat the non-working leg is not required to be parallel to the floor and, various set up options can be used. This decreases the use of the hip flexors and decreases the potential for back pain.
A pistol squat is generally done standing on the floor and, in my opinion, can be more a circus trick than an exercise. Have you noticed how people love to be able to show you that they can do a pistol! I must admit, being able to do a pistol is a nice party trick but, in my mind it is not great training.
Here is the fundamental difference. Performing a one leg squat without the requirement of holding the free leg parallel to the floor drastically changes the exercise.
Standing on a box may be the least stressful on the back as this position allows the free leg to fall to any angle, even potentially perpendicular to the floor.
In a one leg squat to a bench or box the free leg can generally be held at about a forty-five-degree angle to the floor vs, the parallel-to-the-floor relationship necessary for the pistol.
Because of the position of the non-squatting leg, pistol squats can often cause low back pain due to overuse of the hip flexors. Holding the free leg extended and parallel to the floor can cause significant low back stress and subsequent low back pain, particularly in athletes or clients with longer legs. In addition, there is no added benefit provided to the working leg from holding the non-working leg parallel to the floor.
In addition, the one leg squat is done to a femur parallel position. No attempt is made to go below parallel. Below parallel squatting often results in lumbar rounding. Although this may not be particularly dangerous with no load, it can be a potential problem as load increases. In addition, below parallel squatting may cause the posterior aspects of the medial meniscus to be compressed in the joint line. Remember, during flexion of the knee the meniscus moves forward in the joint. In below parallel squatting the posterior aspect of the meniscus (the posterior horn) can be “pinched” in the back of the joint.
Bottom line, we love one leg squats, but we don't even like pistols.