The Stiff Leg Deadlift, unlike other deadlift variations, begins and ends in the standing position. So you need to begin the exercise a little bit differently than you would the other deadlift variations.
There are a few ways to set up for the stiff leg deadlift.
Set the safety bars or hooks in a squat rack just below knee height. From here, you will squat the bar up and out of the safeties. Take a couple of steps back and then initiate the set.
Start from the floor. Perform a standard conventional deadlift to get the weight into the starting position.
Once you have chosen one of those two ways, and gotten into the starting standing position we can continue the rest of the set up.
Use a shoulder width stance with your toes facing forward. Either a double overhand or mixed grip is OK. There is some risk with the mixed grip variation. On very rare occasions a lifter will pull their bicep on the arm with the underhand grip. So be careful.
You want to avoid doing any shrugging of the barbell. This is a posterior chain movement not a trap movement.
Begin the set by pushing your hips straight back. Your knees should stay mostly extended during the set. You can have a slight bend in the knees, but only slight. As you push your hips back, imagine trying to push your hips upwards towards the ceiling as well. This will help you keep your knees in a more extended position.
The Stiff Leg Deadlift is a movement that does something unique. It lengthens the hamstring under a load. During a Stiff Leg Deadlift the tension is at its highest when your hamstring is at its most lengthened position (90° at the hips with extended knees). Therefore, the most important part of this exercise is feeling the stretch in your hamstring on each trip.
You can also use this to dictate your range of motion. Some people will be able to go all the way to the floor while maintaining a flat back. While others will only be able to get to a 90° hip angle. A rare person with hypermobile hips may even need to consider a Deficit Stiff Leg Deadlift. In which they stand on a platform, letting them go even lower than the barbell and plates would allow for. Again, look for the stretch on each rep and use that feeling to dictate when you initiate the concentric portion of the movement. When you feel the stretch, time to come back up.
It's of the utmost importance you keep a flat back so you do not put any unnecessary stress on your lumbar spine. This does not mean you will not feel this exercise in your lower back. The opposite is true. You can and most likely will feel the Stiff Leg Deadlift working your lumbar spine. This does not mean you are doing anything wrong. The lower back has muscles just like everywhere else in your body. These muscles can also fill with blood giving you a pump in your lower back. Unlike having a pump on your biceps or pecs, a pump in your lower back is very uncomfortable because the spinal erectors swell and begin to press into your lumbar spine. Again, this does not mean you were doing anything wrong. You can even expect some soreness in your lower back after performing this movement
The concentric portion of this movement is relatively simple. Just go to a standing position to complete the rep. Do not force your hips all the way forward as this will hyperextend your lumbar spine. Hyperextension is just as dangerous as hyperflexion.