CTA Thought: Keiser, Tubing, and Force

Image courtesy of wickedlocal.com

I had this progression of thoughts on the CTA earlier this week when I was thinking about force.  It isn't earth shattering by any means, but I haven't really heard it talked about much so it was new for me to mull around in my head a little bit.

Image courtesy of columbia.edu

I was thinking about the equation for force--force equals mass times acceleration; F=ma--which I've looked at or thought about or seen in my head thousands of times over the past year.  But I was thinking about it in terms of Keiser equipment, with which you increase or decrease the amount of air pressure as one way of altering the resistance force.  Keiser equipment is often said to be inertia-free because inertia is a property of mass and because the resistance force of an exercise while using Keiser equipment is most often dictated by air pressure, the only mass usually involved with the exercise is the mass of the involved limb/body part and whatever mass is associated with the handle, etc of the machine.But it's not zero mass, because you need mass in order to have force, based on the equation for force.  And when you press the yellow buttons on the Keiser machine and you make the numbers go up or down, the resistance for any given exercises seems to increase or decrease respectively.

In RTS™ we talk about how during an exercise the challenge to any given joint and the tissues that control that joint is not necessarily the resistance force, but rather the torque created by the resistance force and its associated moment arm to any given joint.  Torque equals force times moment arm (?=F(MA)), so if you perform two reps of an exercise exactly the same such that there is no change in the moment arms of the resistance force created to each respective joint at any given point in the motion of the two reps but there is an increase between the first and the second rep in the torque production requirements of the tension generating tissues around each joint in order to overcome the torque created by the resistance force (holding fatigue and speed throughout the motion constant between the two reps), the increased torque has to be coming from an increase in the resistance force.

To simplify, if you perform two reps of an exercise exactly the same but one rep challenges you more than another, holding everything else constant, that increased challenge has to come from increasing the resistance force.

Actually, I'm not sure the simplified explanation is completely accurate, which is why I went with the wordier more technical explanation first, but hopefully my point is being communicated clearly.

Okay, back to Keiser.  So you press the yellow button associated with the (+) sign and the number on the screen goes up and the motion is more challenging to perform and/or the position(s) is/are more challenging to maintain.  As I tried pointing out above, this increased challenge is coming from an increase in the resistance force.  Going back to the beginning, F=ma, but there is not an increase in the mass  you are having to control after you press the (+) sign, so the increase in the resistance force has to be coming from an increase in the associated acceleration.

Image courtesy of medco-athletics.com

In other words, when you change the torque production requirements of any given exercise while using a piece of Keiser equipment, you are actually changing the magnitude of the acceleration with which the machine is trying to bring the handle, pad, etc. back to the starting position as opposed to the magnitude of the mass you are trying to control.  This is distinctly different than, say, using a dumbbell, where gravity is a constant acceleration on the surface of the earth but the mass of the object must change if the torque production requirements of the tension generating tissues are to change and the moments to each respective joint remain the same throughout the motion.  Admittedly, you can manipulate the acceleration with dumbbells as well, based on how fast you are moving them, but as in the examples before, I am looking at this from the perspective of keeping that variable a constant relative to the two reps.

Image courtesy of fitness-equipment-info.com

Then I was thinking about tubing and bands and how the measure of the ease with which each is deformed from its resting position to a given percentage change is more or less a measure of its force.  But, while the thicker tubes and bands may offer a slight bit more mass than the smaller ones, the magnitude of that difference is not nearly enough to equate for change in the magnitude of torque that has to be produced at each respective joint in order to perform the exact same motion as the thickness of the tubes/bands increase.  So, this change torque production requirements due to a change in the resistance force must also be coming from a change in the acceleration associated with how fast one thickness of tubing/band is attempting to return to its resting length versus another thickness of tubing/band.

Just something I was trying to work through my head the other day.  I don't think it has necessarily been specified like this in class before but has certainly been alluded to simply via discussions of force and torque and their respective equations.

This post ended up a little wordier than I was hoping.  What did I not say clearly enough and/or what part of my thought process is not exactly accurate?  Or am I just completely off on this, which is why it hasn't been discussed in detail before? Let me know below!

Your body.  Your training.

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