Gets its name from the position you are in at the bottom of the movement. With 90 degrees at both the front and back leg this can be used both as a teaching progression for single leg work or a strength exercise for the quads. Can be loaded in multiple ways, most commonly it is used with either dumbells or a barbell.
With DB’s down by your side or a bar placed behind the neck set yourself with your chest up, shoulders pulled back and abs engaged. Step one foot back behind you so that you are in a split stance (the correct stance is one that allows 90/90 at the bottom position) and on the tip toe of the trailing leg. Squeeze the glute of the trailing leg so to set the pelvis to neutral. From here lower the back knee down to the floor keeping the rear glute engaged throughout. No forward movement should occur and should be a straight up and down movement. Once your knee makes gentle contact with the ground push yourself up to the start position and repeat.
As with any unilateral leg exercise the 90/90 split squat is used to develop balance through the hips and is also phase 1 of my single leg progressions when teaching single leg work. It encourages dynamic stability of the glutes and lower abs to maintain pelvic alignment through movement.
When used as a teaching progression higher reps (8-20) and lighter loads are used. For strength purposes reps between 5-10 work best.