Moment Arms

Image courtesy of chegg.com

**While all of this information can be found throughout various books and other texts, my experiences with the information have all come through different RTS lectures, study groups, workshops, courses, and discussions.  It is because of these experiences that I am able to write this post, in addition to many of my past and future posts.**

In Monday's post I wrote about the importance of seeing moment arms as they relate to joints and tissue when discussing joint motion during exercise, but I never actually defined what a moment arm is.  So, today I will give you a very brief overview and introduction to moment arms.

Image courtesy of http://en.wikipedia.org/

A moment arm is defined as the shortest distance from the axis of motion to the line of force that is perpendicular to the line of force and runs through the axis.  It is the measure of a force's ability to produce rotation of a lever around an axis, i.e. the greater or longer the moment arm the greater the force's ability to rotate a lever around an axis.

If we are going to divulge into moment arms then I should also explain what a line of force is.  A line of force is 1) a line, that is 2) created by a force (internal--mainly muscles, although other tissues can/may play a role; external--machines, free weights, tubes, bands, etc).  Remember, a line is infinitely long and goes in both directions, meaning the line of force created by gravity goes both up and down even though the vector will only go straight down.

One of the main reasons for knowing the length of a moment arm is to be able to calculate torque, which equals the force times the length of the moment arm an is a measure of rotational force.  This would be considered the resistance for that weight in that one position.  Once the weight starts to move you need to not only recalculate any changes in the length of the moment arm but also take into consideration any changes in the force, as well, which equals mass times acceleration.  That is beyond the scope of this post, though.

So, when looking at moment arms in relation to joints in the body you need to find the axis of the joint in the desired position, the line of force, and then draw a line that is perpendicular to the line of force and have it intersect the axis.  An example of the moment arm (rough estimate) to the coxofemoral joint (hip) at the starting position of a deadlift can be seen below:

Image courtesy of http://colinhaller.blogspot.com

Actually, you would need to lower the red line until it actually intersected with the axis of the coxofemoral joint, but hopefully the visual gives you a slight understanding of how moment arms can be found relative to the body.

Now, what if there is no moment arm to the joint, meaning the line of force goes directly through the axis of the joint?  A basic but crude/not completely accurate example of this would be at the top of a seated dumbbell shoulder press.

Image courtesy of broncofit.blogspot.com

In the picture on the right you can see that if you were to draw a straight line going up and down (ceiling to floor) through the middle of the handle of each respective dumbbell, that line would almost go right through each  respective axis of the glenohumeral (shoulder) joint.  This means that in this position there is almost no torque on that joint.  There are still joint forces, but there is almost no force that is creating rotation around the axis of that joint.
Compare that to this video:

 

Here we also have a shoulder press movement, but a completely different moment arm to the glenohumeral joint (GH).  Instead of there being a moment challenging humeral abduction et. al, there is now a moment challenging predominantly the external rotation capabilities of the GH, even though the motion is the same.  Can you see how the two moment arms are different?
Drop a comment below if I need to still clear some things up.

Your Body.  Your Training.

Get big or die tryin'.

Charlie Cates, MATs, CSCS
Self Made_®, Owner and Founder

Charlie Cates is a Muscle Activation Techniques_® specialist and a strength and conditioning specialist.  He is the owner and founder of Self Made_® (http://selfmadefitness.com/) in Chicago, IL.  He has worked with competitive and everyday athletes of all ages and ability levels, from 9-year-old kids to NFL MVP’s.  He can be reached via e-mail at charlie@selfmadefitness.com.
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