04/18/2018
Rhys Manchester:
I wrote this as a comment on a friend's post, but it was a couple of days old and in case it just kinda fades I wanted it for posterity as it took too long to write, LOL. PS, for context Nicholas Rolnick posted a really good question about which hip extensor initiated extension at the bottom of a squat. And to Nicholas's credit, he was looking for the an answer based in mechanical rationality (adductor magnus) due to moment arm length. However, it got me thinking.
I have a question for anybody who is willing to entertain it and provide feedback:
TL;DR - the quads might initiate extension
As a preface, I need to provide a procedural and pseudo-experimental frame for the question:
Envision, if you will:
1. Seated position, with approximately 90 degrees of hip flexion and approximately 120 degrees of knee flexion (a leg extension machine might serve well for this)
2. A physical restraint, or a barrier, placed on the anterior tibia just proximal to the T-C joint completely prohibiting motion of the tibia on the femur (lock down the weight stack on the leg extension)
3. A voluntarily contraction of the quadriceps with enough force to elicit the inversion of joint action (aka reverse action) concentric knee extension, in this case, femurs on tibias.
If one were to create a restraint to the tibia in terms of motion in the sagittal plane, given enough force generation by the quads, one’s knees would still extend concentrically with the relatively “lighter” end moving (femurs et al. above them) due to the tibia being anchored. In this case the femur will move on the relatively fixed tibia causing the mass of everything superior to the knee to move as well. However, the way in which that mass moves (excepting the femurs of course) is a matter of choice.
If one were to choose to maintain the 90 degrees of flexion hip position, they would likely fly off the machine (see 24s mark [https://www.facebook.com/OldSchoolGymm/videos/1849367015121434/]). If enough force is generated by the quads causing enough torque to move the femurs et al., in order to prevent flying off the machine, the COM vector of the segmental mass superior to the knee joint axes would have to be maintained in a relationship that is posterior to the knees. In order to accomplish this, varying amounts of unfolding (lesser degrees) from the 90 degrees of flexion hip position would be required. As the pelvis is lifted off of the seat, the amount of concentric hip extensor activity in terms of force would be determined by the position(s) of the COM of the segmental mass above the hips. If the COM of the mass is maintained with a zero moment arm to the hip axes, then little more than the force (possibly minimal) than that which is required for the steering necessary to maintain the relationship of pelvis on the femoral heads is needed.
If the inferiorly directed vector of the COM superior to the hips (much of the pelvis, trunk, arms, head, etc) falls anterior to the hip axes, the concentric hip extensor force requirement increases as a response that is dependent on the moment arm length of said COM to the hips (which might be changing). Regardless, in this scenario, the angular impulse needed to “lift” the body off of the pad is generated by the quads as concentric knee extensors.
Although a barbell squat is a different mechanical scenario, there are enough similarities between the two tasks that it might be worth committing the fallacy here of, “begging the question.” Isn’t it possible that the quadriceps might be thought of as the initiators of extension at the change in direction of a squat in that they are concentrically extending the knees (femurs on tibias as it is the floor now that provides the restraint)? If so, might the concentric hip extensors have more of a role in maintaining the hips’ positional relationships due to foldability requirements and also fighting the resistance from the COM’s moment (body mass and barbell). If so, the concentric hip extensors might actually have less of a role in concentrically extending the femurs on the ilia and more to do with dealing with the load line of the mass above?
One of the ways I have tried to think about this is if there was no involuntary center of mass maintenance (the innate desire not to fall over) (Purvis, 2006) and at the bottom of Nicholas Rolnick's squat we were to faradize his concentric hip extensors with a cattle prod, what would happen? Would his hips extend causing him to shoot up to an erect position or would the hips extend causing a loss in the geocentric trunk angle forcing him to thrust the bar behind him (his shoulders grossly moving backwards)? I dunno, just my two cents FWIW, feedback welcome.
Reference:
Purvis, T. (2006). Syllabus for Part 3 of the RTS 123 Course: The Exercise Experience (2006 ed., Vol. 3, RTS123). Oklahoma City, OK: Tom Purvis, RTS LLC.
