Pistol squats are an excellent unilateral (one-sided) movement designed to increase strength in the quads and hip flexors and mobility in the ankles, hips and knees. This movement is performed on a box (although a step or bench could be used too) in order to allow the leg to hang off the side. With further progression, pistol squats should be performed without the box.
Standing tall on a box or step, begin to slowly descend into a squat, dropping your hips towards your heel, lifting the opposite leg straight in front of you, trying not to let it touch the floor. Go as fat down as your flexibility will permit. Reach the arms straight out in front of you as a counter balance.
If the ankle mobility is decreasing movement, then slide a weight plate or wedge under the standing heel.
Pause briefly at the bottom of the squat, then drive through the standing heel to stand up. For additional help, you can grab onto a bar, but try not to alter the movement drastically.
Repeat for the desired number of repetitions, then switch sides.