Benefits of Using the Seated Leg Curl Machine

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            You can search the Internet and find articles about the positives, the negatives, “machines are bad!”, or “here is the argument to use machines...”, until your hands are sore from all your searching.  This article, about exercise machines, is exactly that, an article about an actual machine.  Specifically a seated leg curl, more specifically the Cybex seated leg curl like this one shown below.

            The seated Cybex leg curl is a machine designed to work muscles that primarily bend the knee (functional training for people in Game Of Thrones).  Essentially, you sit in the machine, this one has a thigh pad that lowers down for added support, rest your legs on the pad at the lower end, and then bend your knees tucking your ankles under the seat.  Now we can discuss doing one leg or both at the same time, alternate legs, tempo, adding pauses, pointing toes up or down, and all that, is all dependent on your goals, and not the point of this article.  

            So let’s look at this machine.  We have the seat, the back pad which adjusts to move you forward or backward, that thigh pad that provides additional support, and the input pad where your legs push on.  The back pad should be adjustable, as that way you are able to line your knee joint up with the axis of the motion (denoted by a black dot on this particular machine).  Find an approximate middle, as actually you tibia rolls around the end of the femur, creating something called an instantaneous axis, as opposed to an actual hinge, like on a door. 

            This machine also has a weight stack, which has a belt attached to it, that runs up and over a pulley, then is attached the this round disc called a cam.  As you tuck your ankles underneath, the cam rotates counter clockwise and pulls the belt, which lifts the weight.

            Now here is where the science and physics of exercise, machines, the human body, and understanding it all comes together.  First let’s look at the machine.  When the legs are straight, look how far away the belt is from the axis.  That distance is called a moment arm.  As we bend our legs, that distance gets less. The shorter that distance, the less the torque (how hard it is).  Now that doesn’t mean that is feels better, however, mechanically speaking, we have less torque around that joint.  This is called the resistance profile of the machine.   It’s hard in the beginning, then easier at the end.  This is important to understand in this machine because of how our knee benders work.  In the two photos below, you can see the blue line on the left has a longer distance between the axis (black dot) and the black line, than the one on the right (the belt was hidden by the cam, so I drew it in).

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Now this is important to understand how this machine works.  Where the machine is mechanically challenging or mechanically easy, is called a resistance profile.  With this particular machine, it is more challenging in the beginning than at the end, mechanically speaking.  The reason for that is when the hamstrings, calf muscles, and all the other knee benders work to lift the weight, as they go through the knee bending range of motion, they get stronger and then drop off at the end and get weaker. This is called a strength profile.  Where are the muscles strongest, or have greatest mechanical output, versus where the muscles have a limited ability to produce force.  So it would be advantageous that the machine we are choosing to use has a similar profile.  Keep in mind, that this is how this specific type of hamstring curl machine works.  Some others are different, based on the cam or your position.  Which is also why the prone (or laying down) hamstring machine feels harder, because everything is different in those positions and something to tackle in an upcoming post. 

            If this seems too deep, “Oh I didn’t take physics in high school.”(neither did I), “It’s just weight lifting, this is not important.”, I implore you to check out the Resistance Training Specialist program run by Tom Purvis.  While these terms like moment arm, profiles, are engineering based, they are important to understand, and Tom brings it to our fitness world and explains why it is important to understand these concepts.

            So whether you enjoy using machines, or not, feel they are a necessary part of your workout, or should be banished from every gym, or are just curious how these heavy pieces that are still in some gyms work, I hope you have found this article helpful.  If you are curious, ask a trainer at your gym how a machine works, or contact us here.  At AIM Human Performance, machines are tools in our tool box to use when appropriate with our clients, which may be helpful in helping them and you reach your goals.  All our machines are hand picked, to be some of the most mechanically sound pieces in the industry.  If you have any questions, feel free to reach out to us and we are glad to help!  Happy knee bending!