This video is going to be pretty math-heavy. Since you asked for it, don't fall asleep or click away.
When you hold the human flag position, there are two main physics conditions that are required.
First, the upward force coming from the bar needs to be the equivalent to the body weight. Let's say
you push the bar with your bottom hand with force F due to Newton's third law, which is the law
of action and reaction. It states that when two bodies interact, they apply forces to each other
equal in magnitude and opposite in direction, which means the bar will give us an opposite force
with the same magnitude F. Similarly, when you pull the bar with your upper hand with force F,
the bar will give us back an opposite force with the same magnitude F. Now we can simply extract
the upward portion of these two forces from the bar by multiplying a cos theta. The sum of these
two needs to be the same as your body weight. This is not the only physics condition that needs to be
satisfied, otherwise according to this equation, the smaller the theta is, the easier the human
flag will be since less force would be required to be generated by the arms. However, there's a
reason why we have never seen someone do something like a human flag iron cross to make theta
completely zero. It is because there's a second physics condition that needs to be satisfied,
which is that the net torque for both points of contact between two bodies should be equal to
zero. The definition of a torque is the moment arm times the effective force. The effective
force is the force perpendicular to the moment arm. As you can intuitively see, if the force
is parallel to the moment arm, it won't cause the object to rotate. Let's take a look at the
point of contact at the upper hand. The force the bar gives at the upper hand directly goes through
the point of contact, so the torque is zero. As for the force the bar gives at the bottom hand,
the effective force is F times sin theta, and the moment arm is the distance between your two arms,
so the torque from the bottom hand is F sin theta times the distance between your two arms in the
counterclockwise direction. Now let's take a look at the opposing torque in the clockwise
direction from our body weight. The effective force is our body weight times cosine phi,
and the moment arm is the distance between your center of mass to the upper hand, so the torque
from the body weight is body weight cosine phi times the distance between your center of mass
and the upper hand. The magnitude of two torques should be exactly the same and cancel each other
out, otherwise you will be spinning around. Since the body is symmetric, the equation will
be exactly the same if we look at the point of contact at the bottom hand. In order to further
understand it, we need to plug in some real numbers. All the numbers in these equations
can be easily measured except the location of your center of mass. Fortunately, I did a DEXA scan;
therefore, I have a pretty good estimate of where my center of mass is. I am not going to repeat how
to calculate the center of mass in this video. If you are unclear about how the calculation is done,
you can check out my previous front lever DEXA scan video. Alright, this is what it looks like
if I plug in the real numbers from my DEXA scan data and generate a plot for the first physics
condition. The x-axis is the angle between the arms and the bar, which is theta. The y-axis is
the force you have to generate in terms of body weight multiplier, which is F. As you can see,
the smaller the angle is, the easier it will be to meet the first physics condition. However,
if I generate a plot for the second physics condition, you can see that both large
angles and small angles will make the human flag difficult. It's the medium angle that makes the
human flag the easiest, which is no surprise because everyone who can do the human flag does
it with a medium angle. Now I can simply combine these two plots and get the overall strength
requirement for me to do the human flag at various angles. According to the plot, the easiest angle
for me would be at 41 degrees, which is exactly the angle I had when I achieved the human flag,
and the strength requirement for me at 41 degrees is to push and pull 0.86 times of my body weight.
I know what you are thinking. You might think that generating this amount of force per
arm doesn't sound easy at all. It's actually not that hard. First of all, we are talking
about straight arm strength here instead of bent arm strength. For the pulling portion, you don't
have to be able to do a one-arm pull-up. All you need to do is be able to hang with one arm on the
bar for 5 seconds. If you can, congratulations, you just smashed the 0.86 times of body weight
strength requirement for the straight arm pull. Similarly, for the pushing portion,
if you can do a wall-assisted one-arm handstand for 5 seconds, congratulations, you just smashed
the 0.86 times of body weight strength requirement for the straight arm push. I know some of you are
going to say, "but the wall must've taken out some weight during the one-arm handstand." Yes,
but the wall actually only takes out only around 3% of the body weight. The main purpose of the
wall is not to take out weight, but to let you not have to worry about balance, which is the main
difficulty of the handstand. Addtionally, if you can't hold a full wall-assisted one-arm handstand,
but you can hold it with the fingertips of the other hand on the ground, chances are pretty high
that you will still meet the strength requirement for the straight arm push as well. Therefore, if
you can hang on a bar with one arm for 5 seconds, and also do a wall assisted one-arm handstand with
the fingertips of the other hand on the ground for 5 seconds, you have the strength to do the
human flag. From my point of view, these strength requirements are probably even easier to meet
than the bent arm pull strength required for the muscle-up. Now it's time to play a game of true
or false on some statements about the human flag. Number 1, it's harder for people with long arms to
do the human flag. This is absolutely false. If I increase the length of my arms by 10 centimeters
and plot the physics requirement graph again, you can see that I can simply decrease the angle to 39
degrees and the push and pull strength requirement will drop down to 0.82 times of my body weight.
Number 2, if you spread your arms as wide as possible, it will be easier to do the human flag.
I heard this repeated a few times in popular human flag tutorials, but by now you should know that
this is obviously false. Spreading the arms wide means theta will become smaller. The human flag
is very difficult to perform when theta is too small. Number 3, big hypertrophied upper body
muscles will make the human flag easier. This is only somewhat true. If I add 20 lbs to my upper
body and plot the physics requirement graph again, the easiest angle would become 40 degrees and the
push and pull strength requirement will drop down to 0.84 times of my body weight. However,
if we factor in the allometric scaling, which I covered in great detail in my previous video,
the difficulty of a 154-lb guy generating force equal to 0.84 times of body weight might not be
necessarily easier than a 134-lb guy generating force equal to 0.86 times of body weight. It
depends on how much of these extra 20 lbs are muscles that can generate force for the human
flag. Sorry, guys, big beautiful muscles give you a gigantic advantage for the front lever,
but there is barely any for the human flag. Number 4, Taller guys will have a more
difficult time learning the human flag. Yes, this is true, but it's not because of physics.
If I increase my height to 6 feet and keep the proportion of my body exactly the same,
the physics requirement graph will be exactly the same. However, again, due to allometric scaling,
the 6 feet version of me will have a harder time pushing and pulling 0.86 times of body weight
than the current me pushing and pulling 0.86 times of body weight. Taller guys will have a
more difficult time not only in the human flag but also in calisthenics in general. I also
want to give my take on a question that everyone cares the most, which is, how should you actually
train for the human flag? As I mentioned in my previous human flag video, conventional training
ideas fall into 3 main lines of thought, using the resistance band, training tuck variations,
and training dynamics. I explained why I don't like any of them in my previous human flag video.
Common sense tells me that lacking straight arm push strength is what the bottleneck will likely
be instead of the straight arm pull strength. Therefore, I believe the best exercise to train
for the human flag is, you got it, wall-assisted one-arm handstand with as minimal assistance from
the other hand as possible, and eventually work your way towards with only the fingertips of the
other hand on the ground. Once you can perform this reliably for 5 seconds, you should be able
to perform the straddle human flag, bypass all the dubious training methods, and start getting your
body to be used to being in the sideways position. Full disclaimer, my take is only based on logic
and physics, I don't have any empirical experience to back it up. I was fortunate enough to be able
to do the straddle human flag immediately once I get the technique down, so if some of you
decide to try training the wall-assisted one-arm handstand for the human flag, let me know how it
goes. I am curious to know whether or not it works for you. Lastly, there is one last nuance that I
want to point out. When we look at the physics requirement graph, there's an assumption that the
difficulty of generating force in all theta is roughly the same. This is venturing into
biomechanics territory, which I am not an expert in, but based on my intuition and experience, I am
fairly confident that this assumption is true. It feels natural to push overhead, push 45 degrees,
push to the side, and so on. In contrast to the front lever, the small angle in between the arms
and the torso not only causes the effective force to counter the body weight smaller. Moreover,
the force you can generate also drastically decreases because of how unnatural pulling is at a
smaller angle. In any case, I will stop the front lever talk right here as this video is about the
human flag. Thanks for watching. If you don't want to bother doing the math, and you want to simply
input your DEXA scan data on a website and get your own personalized strength requirement for the
human flag, I can easily make it happen because after all my core competence is software and math
instead of climbing and calisthenics. However, I want to see some support from you guys for my
latest 5Bar venture. You will not only get awesome healthy premium bars to eat but also support the
channel at the same time. If I see enough support, I will be happy to spin up a website for free to
the public and 5Bar customers will have early access to beta test it. Make sure to check out
the link in the video description below to order some 5Bars. See you in the next video.