The stiff leg deadlift primarily works your hamstrings, glutes, and lower back, with your hamstrings doing the heaviest lifting. EMG studies show the hamstrings activate at roughly 50-55% of their maximum voluntary contraction during the movement, while the glutes contribute around 42-51%. What makes this exercise unique compared to other deadlift variations is how much it isolates the posterior chain by keeping the knees nearly straight throughout.
Hamstrings: The Primary Mover
Your hamstrings are the star of the stiff leg deadlift. Because your knees stay in a fixed, slightly bent position, the hamstrings are stretched under load through a long range of motion as you hinge forward, then contract forcefully to bring you back up. This makes the exercise one of the better choices for targeting the back of your thighs.
What’s interesting is which part of the hamstrings works hardest. The inner hamstring muscle (semitendinosus) gets more activation during the stiff leg deadlift than the outer hamstring (biceps femoris). This difference becomes even more pronounced if you perform the exercise on one leg. So if you’re trying to build the inner portion of your hamstrings, the stiff leg deadlift is a strong choice. EMG data shows the semitendinosus activating at around 38-50% of its maximum capacity, depending on the load and whether you’re measuring average or peak values.
Glutes: A Strong Secondary Driver
Your gluteus maximus works hard during the stiff leg deadlift, particularly during the lifting (concentric) phase. Studies measuring muscle activation found the glutes firing at roughly 42-51% of their maximum capacity on the way up. That’s meaningful engagement, though not as high as what you’d see during a hip thrust or a conventional deadlift with more knee bend.
The glutes are responsible for the final “lockout” at the top of the movement, where your hips push forward to bring your torso fully upright. If you’re not feeling your glutes during this exercise, you may be cutting the range of motion short or not driving your hips through completely at the top. The deeper your hip hinge (torso approaching parallel to the floor), the more your glutes have to work to reverse the movement.
Lower Back: Stabilizer or Active Muscle?
This is where the stiff leg deadlift gets a bit different from its close cousin, the Romanian deadlift. Your spinal erectors (the muscles running along both sides of your spine) play a major role, but exactly how they work depends on your form.
If you keep a flat, neutral spine throughout the entire movement, your erectors work isometrically. That means they’re contracting hard to hold your back in position, but they’re not moving through a range of motion. This version shifts more of the work onto your glutes and hamstrings.
If your lower back rounds slightly at the bottom of the movement (which naturally happens for many people as they reach the limits of their hamstring flexibility), the erectors now work dynamically. They lengthen on the way down and contract on the way up, making the stiff leg deadlift more of a direct lower back exercise. This is one of the key differences between the stiff leg deadlift and the Romanian deadlift, where the back stays rigid throughout. Both versions load the spinal erectors similarly in terms of overall EMG activity, but the nature of that load changes.
This distinction matters for your spine. Deadlifts in general produce significant compressive and shearing forces on the lumbar spine, with shear forces at the L5 vertebra reaching around 1,900 newtons in moderately trained men lifting at 75% of their max. Competitive lifters can see shear forces above 3,000 newtons. Reported injury thresholds for lumbar shear sit between 1,000 and 2,000 newtons, which means the lower back is genuinely working near its limits during heavy stiff leg deadlifts. Keeping a neutral spine reduces how those forces are distributed across your vertebrae.
Supporting Muscles You Might Not Expect
Several muscles contribute without being the main drivers:
- Adductor magnus (inner thigh): The posterior fibers of this large inner thigh muscle assist with hip extension. It’s one of the reasons you may feel soreness on the inside of your upper thigh after heavy hinge work.
- Calves: Your gastrocnemius and soleus stabilize your ankle joint and help maintain balance as your weight shifts during the hinge. They’re not doing heavy work, but they’re active.
- Core muscles: Your obliques and deep abdominal muscles co-contract with your spinal erectors to create a rigid torso. Think of them as a natural weight belt, resisting the forward pull of the barbell.
- Grip and upper back: Your forearms, traps, and rhomboids work to hold the bar and keep your shoulder blades from rounding forward. These muscles are under constant tension but aren’t producing the movement.
How the Stiff Leg Deadlift Compares to the RDL
People often use these names interchangeably, but there are real differences in muscle recruitment. The Romanian deadlift typically produces greater semitendinosus activation during the lifting phase compared to the stiff leg deadlift. For the biceps femoris, no significant difference exists between the two. On the way down, both exercises activate the glutes and hamstrings equally.
The erector spinae activation is statistically similar between both exercises in both directions. So if lower back work is your concern (either seeking it or avoiding it), neither variation is meaningfully different in terms of raw muscle activation. The real distinction is mechanical: the stiff leg deadlift allows some lower back rounding, which changes how force is distributed across the spine, while the RDL keeps the back locked flat with a slight knee bend that limits depth.
How Form Changes Which Muscles Work Hardest
Small adjustments to your setup shift the emphasis considerably. Here are the key variables:
Knee position. The straighter your knees, the more your hamstrings are stretched at the bottom and the harder they have to work. Adding even a modest knee bend shifts some load to the glutes and reduces hamstring stretch. If you feel the exercise primarily in your lower back, try a slightly wider stance with a soft bend in the knees to redirect tension to the posterior chain.
Depth of the hinge. Aim to bring your torso close to parallel with the floor. Your shins should stay roughly vertical, and your hips should end up at about the same height as your shoulders at the bottom. Going deeper than your hamstring flexibility allows forces your lower back to round, which turns this into more of a spinal erector exercise and increases shear loading on your lumbar vertebrae.
Foot pressure. You’ll naturally feel your weight shift toward your heels as you hinge. That’s normal, but pushing too aggressively into the heels lifts the front of your foot and reduces stability. Maintain contact through three points: big toe, little toe, and heel. This keeps your balance and allows your entire posterior chain to contribute.
Head and spine position. If you tend to round your upper back, looking up slightly can help you maintain extension. If you’re naturally more arched (common in athletes), tucking your chin slightly keeps your spine in a neutral position. The goal is a spine that doesn’t move during the lift, so the work stays in the hips.