According to this website that the target muscles of this exercise is the Gluteus, and the hamstring muscles as secondary muscles. While doing this exercise I am obliged to bend my knees since the bench I have is not the same height as my legs, so the following is a picture that explains the situation.

enter image description here

My question is: does bending the knees affect the impact of the exercise on the mentioned muscles? Does this includes an effect on the lower back muscles?

Thank you in advance.

  • 1
    My first instinct is that it would take the hamstrings almost completely out of the exercise and more fully isolate the glute since they are bent across two joints, but I will have to read a bit. It's been a while since I had to consider that in an exercise.
    – JohnP
    Commented Feb 20, 2018 at 19:58
  • If you see this question again in the near future, I would encourage you to switch your acceptance to the other answer, as the parties involved have agreed it is a better representation of the information.
    – JohnP
    Commented Mar 2, 2018 at 14:35
  • @JohnP Thank you for pointing this, in deed I will take some time reading the new long answer and accordingly switch the acceptance.
    – Nizar
    Commented Mar 6, 2018 at 7:31

1 Answer 1


The accepted answer is not correct.

Not trying to start WWIII (and I think this is literally the first time I've disagreed with DarkHippo) stay calm. Everyone seems to freak out when they're questioned.

JohnP was on the right track with his first comment.

The Bottom Line

By bending your knee only slightly you're avoiding passive insufficiency of an anterior thigh muscle called your Rectus Femoris (which will tighten sooner limiting hip extension range of motion).

In addition, the slight bend in your knee also avoids active insufficiency of your hamstrings and maintains an optimal length tension relationship (see the bottom image).

Understanding What's Happening

To understand this we need first look at the "Thigh Muscles" that cross both the hip and knee. I created an image below showing these 4 muscles.

enter image description here

enter image description here

The Hamstrings are a multi-joint muscle since they cross both the hip and knee joint

  • Active Insufficiency Example: When the knee is flexed (shortening the hamstrings) the hip can't extend as far.

enter image description here

  • The Rectus Femoris is also a multi-joint muscle.
  • This muscle is an antagonist (performs the opposite motion) of the hamstrings
  • When the hamstrings are Actively Insufficient the Rectus Femoris is Passive Insufficiency, and vice versa.

Muscle Resting Length

  • Overlap determines a muscles ability to contract.
  • Actin (red) and Myosin (blue) = Collective Called Cross-Bridges

    • 1. Fully Contracted (Shortened) Muscle: Cross-bridges fully overlapped.
    • 2. Optimal Length Tension Relationship: Muscle can generate max power.
    • 4. Fully Stretched (Lengthened) Muscle: No Cross-bridge overlap (streched too far). Muscle cannot contract.

enter image description here

EDIT Sorry haven't logged on in quite a while. In regards to your question about what happens with the glutes / what muscle "takes over".

When you slightly bend the knee the hamstrings are at a more optimal length to contract since more cross bridges are overlapping. This overlap is required for a muscle to generate force. Similar to "A Tug of War" the more people pulling the greater the force.

If the knee is fully flexed when you're lying on your stomach (Figure 2) the hamstrings are maximally contacted and have no more cross-bridges that can "connect and pull" so they can't generate more force -- also less are available to connect (aka produce force) throughout the range of motion (ROM) due to this position.

Since the hamstrings are spending more time in a less optimal length throughout the ROM, your glutes are required to work harder to produce the force required to extend your thigh.

  • Having read into it a bit more, I agree with your answer, but I'm not sure what's the best course of action to rectify my incorrect answer (fitness.meta.stackexchange.com/questions/756/…)
    – Dark Hippo
    Commented Mar 2, 2018 at 9:43
  • @DarkHippo - Wow normally people get VERY defensive if you have differing information so first off - thanks. Your answer doesn't list anything dangerous so not a big deal - should be fine. Honestly just found the question really interesting so wanted to better clarify what was going on.
    – Mike-DHSc
    Commented Mar 2, 2018 at 9:57
  • I mean, er... how dare you correct me!? You take that back!!! I shall now question your family heritage and stuff. Seriously though, I don't know that much anatomy, I tend to answer things based on personal experience, so my answer was taken from mainly using a reverse hyper machine along with a dose of physics knowledge (increased distance from the pivot point requiring more force, etc). I'd rather help people than be right all the time, so I don't mind being corrected.
    – Dark Hippo
    Commented Mar 2, 2018 at 10:33
  • HA, see that sounds like the more logical way to go. You can't go wrong with personal attacks. I've literally said this before but your answers are almost always spot on (I remember saying this to you a few months ago...) and for having limited anatomical knowledge -- impressive. I wish there were more debates on some of these topics but tough to debate without any kind of research supporting another viewpoint....... Oh well.
    – Mike-DHSc
    Commented Mar 2, 2018 at 13:39
  • Thank you for this through response, however it is still unclear for me, what happens to the glutes in the case of bending knees ? and What are other muscles introduced?
    – Nizar
    Commented Mar 6, 2018 at 7:39

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