Guess the centre of gravity

Yes, it's gravity again, as I am still working through the importance of the centre of gravity and how the weight of the human body is supported. Knowing the centre of gravity turns out, is just very useful, for balancing, trying to decrease muscle strain, prevent injuries e.g. through lifting or excessive forces on joints, predicting how a body moves through space ....

So I searched the internet for a source of an approximate weight of the individual body parts of a human body to help me demonstrate where the centre of gravity is.

In general, the centre of gravity, is the point, where there is the same amount of weight above and below, on either side and front and back, i.e. if you draw lines through the centre of gravity, you will cut the body in equal halfs of weight left and right, front and back, up and down. Also if you were to suspend a body from the ceiling, attaching the rope at the centre of gravity, the body will stay just as it is, no rotation occurs, as the weight of the body is equally balanced.

So here is an example body with each of the body parts drawn according to its weight. Larger areas = more weight, smaller areas = less weight. Different densities of the body materials, bone, muscle, water and fat have been considered in this example.

 

Weight per body part:

Head and Neck:           8 %

1 Arm:                          5 %

Torso and Pelvis:         50 %

1 Thigh:                       10 %

1 Lower leg and foot:    6 %

Head/Neck/Arms/Torso

=

Feet/Lower leg/Thighs/Pelvis

Toezeren, Aydin (2000) Human Body Dynamics: Classical Mechanics and Human Movement. Springer: New York. (pg. 302)

I used percentages rather than actual numbers, as it then can applied to different body weights. For a 60 kg person the head and neck weighs 8% of 60 kg = 4.8 kg. So you can calculate your own approximate body part weights.

The percentages are rounded to nice whole numbers from a range of values given in Toezeren's book. These values are an example only. Everyone will differ according to their measurements. Men tend to be more top heavy with a slightly higher centre of gravity and women bottom heavy.

So having an idea on how the body weight is distributed throughout the body, where would you guess the point of gravity of the human body is, in the following pictures? You can compare your guesses with mine later.  Remember, at the centre of gravity the weight below and above is the same and the weight left and right, i.e. from a side view of the body, the body is cut into four equal-weight quaters. 

I     I

I     I

I     I

I     I

I     I

I     I

I     I

I     I

I     I

I     I

I     I

I     I

V    V

HERE ARE MY ANSWERS (the CoG is where the lines cross):

How did you do? Surprising, how little the point of gravity changes with different postures, even extreme postures. I suppose it shouldn't be, as most of the weight is carried in the trunk/pelvis. Useful though, as the point of gravity always has to be vertical above the body support (contact point with the floor or space between contact points to the floor) for us to be able to balance. 

This means, the centre of gravity has to be above our feet/foot or in the space between our feet, when we are standing, for us not to fall over. As we have only little feet compared to the length of our body and tend to keep the feet close together, it's a good thing that the point of gravity doesn't change noticably with the movement of limbs for example. A lot easier to balance like this.

I have a couple of blogs lined up on squatting and lower back pain and knowing the point of gravity of a human body gives greater insight into the issues in the upcoming blogs.