SLDL vs RDL: Kinetic Chains and Biomechanical Differences

Published by Debbie Luna
Last Updated: March 27, 2021

The deadlift is one of the most basic yet most effective compound exercises. It trains various muscle groups in both the upper and lower body. Several variations of the deadlift such as the Romanian Deadlift and the Stiff-Leg Deadlift improve the engagement of specific muscle groups, particularly the hamstrings and the lower back. 

The Romanian Deadlift (RDL) and the Stiff-leg Deadlift (SLDL) differ primarily in the knee angle and bar placement which causes a change in the kinetic chain when performing the two deadlifts. The RDL targets more of the hamstring and gluteal muscles while the SLDL targets the glutes and lower back.

RDL vs SLDL vs Conventional Deadlift

conventional deadlift technique
Conventional Deadlift Technique

Deadlifts improve muscle mass and strengthen the back, thighs, and hips. It improves body coordination and balance as well as improves jumping ability. The amount of weight endured for deadlifts also improves bone mineral density which decreases the risk of leg injuries.

Other types of deadlifts are considered supplemental lifts to target muscle groups that are less prioritized in the conventional deadlift. The Romanian Deadlift (RDL) and the Stiff-leg Deadlift (SLDL) are supplementary lifts that aim to change some of the fundamental mechanics of the conventional deadlift to shift the weight to certain muscle groups. These two lifts also have less knee flexion when compared with conventional deadlifts.

The RDL is considered a basic exercise for more complex movements. It reinforces the lifter's back by preventing the bending of the knees during the lift. However, some coaches would prefer more knee flexion during a RDL.

The primary benefits of the RDL are better glute and hamstring engagement and increased hip strength. This exercise also has great applicability in other lifting sports such as the Olympic weightlifting. It also helps to prevent hamstring injuries which is one of the most common injuries in weightlifting.

The SLDL is very different from the RDL specifically in its basic mechanics and range of motion. Performing a SLDL, however, does not necessarily require locked knees. It is often advisable to unlock the knees when performing a SLDL for a greater transfer of force and to keep the hamstrings activated throughout the motion.

RDL and SLDL also have distinct differences in basic mechanics and range of motion. The weight when performing RDL is usually lower than when performing SLDL. A RDL is usually performed with 30-40% of one-rep max (1RM) or the maximum amount of weight that can be lifted in 1 repetition. A SLDL can reach up to 70% of the 1RM for deadlifts. 

Kinetic Chain and Target Muscles of RDL and SLDL

Both the SLDL and the RDL target similar muscles. This includes the muscles that conventional deadlifts engage such as the quads, inner thigh, hamstring, glutes, traps, lats, and abdominal muscles. However, the priority muscles for the RDL and the SLDL when compared with the conventional deadlift are the glutes, lower back, upper back, hamstrings, and gripping muscles.

A RDL activates more of the gluteal and hamstring muscles when compared with the SLDL. The SLDL, on the other hand, engages more of the glutes and the lower back, and the spinal erector muscles which increase back strength.

Romanian Deadlift (RDL)

SLDLs when done incorrectly (i.e. with locked out knees) are unable to engage the hamstring muscles at all. The lower back in a neutral position forces the lumbar erectors to carry most of the load especially when lifting the weight up and away from the ground. To remedy this, one can focus on the eccentric contraction of the hamstrings while lowering the weights by controlling the overall downward motion. Meanwhile, the RDL does not present this kind of problem. All in all, the optimal way to train the hamstring muscles both concentrically and eccentrically is to perform RDL over SLDL.

Biomechanical Differences Between RDL and SLDL

stiff leg deadlift form
Stiff-Leg Deadlift (SLDL) Form

Aside from the difference in target and priority muscles, RDL and SLDL also differ in basic mechanics. The RDL has a greater knee angle compared with the SLDL. Most coaches, however, would incorporate a slight angle in performing a SLDL but not to the extent of the knee angle when performing a RDL.

The greater knee angle of the RDL engages the hamstrings and the glutes. The RDL engages the hip muscles by pulling the hips backward and pushing the hips forward during the eccentric or downward motion and concentric or upward motion, respectively. The range of motion linked to the hips during the RDL minimizes the engagement of the spine resulting in minimal spinal flexion.

The shoulder placement is also a major difference when performing the movements. The shoulders are more in front during a RDL. This is largely due to the greater emphasis on the eccentric motion as well as in the greater knee angle.

Another major difference between the RDL and the SLDL is the range of motion. The RDL only lowers the barbell to as low as the shin before going back to the concentric or upward motion. The SLDL can go as low as the barbell touching the ground. The barbell is also allowed to disengage from the body during SLDL. This, along with the lesser knee angle, allows the SLDL to have a greater engagement of the lower back muscles.

The SLDL also has a greater engagement of the lower back muscles or the lumbar spine because the barbell comes off the floor before the concentric motion. The RDL is only lowered just below the knee which produces an elastic effect when going back to the concentric motion.

The RDL is better and safer for developing the posterior chain of the lower body such as the hamstrings and glutes because of the assistance from the knees during the exercise. The greater knee angle also helps in improving timing when transitioning between the concentric and eccentric motion.

A RDL is also easier to perform for all body types and easier to learn biomechanically. The heavier emphasis on the eccentric motion when performing the RDL promotes strength and muscular development and increases flexibility for injury prevention.

Final Thoughts

RDL and SLDL are good complementary exercises for conventional deadlifts. Each of these variations of the conventional deadlift aims to shift the kinetic chain by altering the biomechanical form when performing the deadlift.

Debbie (Deb) started powerlifting and Olympic lifting in High School as part of her track team's programming; She continues to train in order to remain athletic. Inspire US allows Deb to share information related to training, lifting, biomechanics, and more.

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