Single-leg strength is a quality that has frequently been ignored in strength programs, but is essential to the improvement of speed, development of balance, and prevention of injury. Unfortunately, most strength programs focus on conventional double-leg exercises like squats, leg presses, or even worse, nonfunctional leg exercises such as leg extensions and leg curls. A closer look, however, reveals that most sport skills are performed on one leg.
Any sport that involves running, jumping, or throwing involves performance on one leg. In contrast, the double-leg squat distributes the load to two legs. While this is a good exercise to develop lower body strength, it does not contribute to single-leg strength and balance as much as purely single leg exercises, such as the single-leg squat, do. The actions of the pelvic stabilizers are different in a single-leg stance than in a double-leg stance. Single-leg exercises force the glute medius to operate as a stabilizer, which is critical in sport skills.
The one-leg squat places a high proprioceptive demand upon the body. It challenges the joint and muscle receptors of the lower body to provide feedback regarding joint and limb position and reposition them accordingly. The proprioceptors assist the body to generate movement in a form that is appropriate to the demands placed upon the body. If exercises are of low proprioceptive demand, the body will quickly adapt and will not be prepared for the greater demands called for in the game. That is why the single-leg squat has been called "the king" of all single-leg exercises.
Initially, many people will not be able to perform the single leg squat. This is mostly due to a lack of ankle, knee, and hip flexibility, balance, or leg strength. There is, however, a particular progression that can help anyone to perform the single-leg squat. While it will take some time, about three to four weeks on each exercise before progressing, the program should allow anybody, with no lower body injuries, to perform the exercise.
The first exercise in the progression is the split squat. In this exercise the athlete assumes a long lunge position with their hands on their head. In this position the athlete has two stable points on the ground. From this position the athlete attempts to touch the back knee to the floor while keeping the front knee over the ankle. Some things to look for are, to make sure the athlete can maintain an erect posture, proper balance, and control. An athlete that lacks control will not be able to lower themselves without banging their back knee into the ground. This is an indicator of poor yielding/eccentric strength. This will improve over time and with this exercise. This exercise then progresses to the lunge.
In a lunge, the athlete begins with both feet together and hands on their head. Next, the athlete should take a long step out with one leg and touch the back knee to the floor while keeping the front knee over the ankle. Finally, the athlete should push off of that leg and return to the starting position and switch legs. The things to look for in the lunge are similar to the split squat. The lunge then progresses to the single-leg bench squat.
When performing this movement the athlete will get into a position similar to the split squat, except that the back foot is placed on top of a bench behind them. From this position, the athlete descends into a position where the front thigh is parallel to the floor. Once again, this exercise is done with no movement and, as in the split squat, the dynamic flexibility of the hip flexors will be improved. Things to look for in this exercise are similar to the exercises above. You will notice that many people who are extremely tight through their hip flexors will exhibit a lack of range of motion.
The movement finally progresses into the single-leg squat. This exercise forces the athlete to use the leg without any contribution from the opposite leg for balance. The pelvic muscles must now function as stabilizers without the benefit of the opposite leg touching the ground or a bench.
This twelve-week progression, three weeks on each exercise, can help anybody to perform the single-leg squat. A well-designed program should also include other exercises to help strengthen pelvic stabilizers such as the glute medius, which will assist in the performance of the single-leg squat.
I would like to acknowledge Mike Boyle on his thoughts, ideas and progression of single leg training.