Sissy Squat Machine
A sissy squat machine (or sissy squat bench) is a piece of equipment with pads that lock your legs below the knees. With this apparatus, you can perform a squat motion by bending only at the knees and hips. Your back stays straight.
One benefit of this variation is that it’s more stable, so you don’t have to worry about balancing. And it doesn’t require as much quad strength, so it’s a good starting point.
On the other hand, you will notice some significant differences in body position and loading.
First, the range of motion is often stopped at 90o due to the pad. This means you’re only loading the quads through a very limited ROM.
Second, bending at the hips shifts your center of gravity forward. So there is less distance between the load and the pivot point. And you actually get less resistance on the quads.