: Why is the single leg deadlift stiff-legged? I'm wondering why is the single leg version of the deadlift is usually performed with stiff legs. For instance, exrx.net and most of the youtube videos that teach the single leg

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The single leg deadlift is done with a stiff-legged due to the fact that it's biomachanically advantage:

Keep balance
Increase leverage
Natural movement pattern

From a Sports Physical Therapist standpoint, I prescribe all single leg exercises to include the single leg deadlift for a few reasons:

To maximize side to side muscle isolation
To decrease compensatory movement/dominance from one side to another
To be sports-specific and functional, since most sports and daily activities are single leg activities (walking, reaching, running, jumping, cutting, etc.)
To improve propriocenption or balance enhancement
Also, all of these reasons can later enhance injury prevention

As for as muscle recruitment goes, read this article. Basically, stiff-legged deadlift has been shown to recruit more hamstrings, glutes and paraspinal muscles.

There are risks for all exercises; however, with proper form and technique, the single leg deadlift is safe and effective for the above reasons. Hope this helps!

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A big portion of it is simply the mechanics of the lift. Just try to stand on one leg in a conventional deadlift stance (sumo would be impossible). You run into the big question of what to do with the leg you aren't using. If you stick it in front of you like a pistol, grabbing the bar becomes nearly impossible. If you stick it behind you there is a high likelihood that you don't have the flexibility to keep it behind you like a half split while you initiate the pull.

The stiff single leg deadlift is a natural movement
You have the advantage of the other leg acting as a cantilever to help you lift heavier
The stiff leg helps target the hamstrings and glutes more, which make it a good assistance exercise.

The second portion is understanding what the exercise is designed to target. The primary part of the posterior chain that single leg deadlifts are hitting is the hamstrings, and secondarily the glutes. As such, you need to follow these guidelines:

Don't lock your knee, that puts more stress on the tendons and ligaments rather than the muscle itself
Do try to minimize how much you bend the support leg. That increases the stretch reflex, and ultimately your flexibility. It also allows you to snap back more quickly.

In the video you linked to, the leg was bent too much to be of much use on the hamstring and glutes. Instead, it will be more focused on the hips. It also loses the stretch reflex making it less bio-mechanically advantageous.