- 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.
This article may be reproduced with biographical information intact.