Can you measure the body's Center of Gravity (or Center of Mass) with stills?

[nwb]

Brian (anyone) - can you just explain what this one means please? I am not disagreeing i am just not sure of what this means we should be doing/thinking - cheers NWB

You also see folks drawing dots for body CoG's and trust me folks, the guys who work with 3D, and don't have a horse in the race, think this is a joke as well. You just can't tell unless you had multiple ultra-high speed cameras, and good CAD programs.

Man o'Man, just buy an AMM machine if you want to REALLY KNOW. Or a force plate system.

 

[Brian Manzella]

Brian (anyone) - can you just explain what this one means please? I am not disagreeing i am just not sure of what this means we should be doing/thinking - cheers NWB

You also see folks drawing dots for body CoG's and trust me folks, the guys who work with 3D, and don't have a horse in the race, think this is a joke as well. You just can't tell unless you had multiple ultra-high speed cameras, and good CAD programs.

Man o'Man, just buy an AMM machine if you want to REALLY KNOW. Or a force plate system.

Sure.

There are folks who like to guesstimate where the body's center of gravity is during the swing by using stills.

Or folks who like to show the "point between the shoulders."

I have done this for analysis on this site at times.

But, it is a BIG GUESS, and folks who know better, like the two companies that have a force plate system that measure the CofG suring the swing, and folks with good 3D systems, like the AMM and the MATT (no, not the K-Vest), know that you can not tell where these locations are using a damn still.

You can, just let everyone know it is a guesstimate.

At best.

 

[jaridyard]

I'm not sure anyone claims to identify the body's CoG's? That would be fatally flawed from the start as there is only one CoG (or CoM as I was taught to call it!)

But anyway, as an aside this could be a good discussion.

I was taught to find the body's CoM (Centre of Mass) using 2D images while studying for my under-graduate degree. If joint centre's are marked, stills digitised and the appropriate formulae applied a fairly accurate estimation of CoM can be produced. A brief explanation of the process can be found here..
Center of Mass

I'm equally sceptical of a Force Platforms ability to locate the CoM as anyone should be from 2D, Centre of Pressure absolutely but not CoM. When understanding the 2D method of finding CoM, applying that to a picture of Nicklaus from a period when he pretty much leaned left but still had almost his entire left foot off the ground you would see a CoP way right but a CoM more central maybe even slightly left (at the top). I was under the impression that the CoM was estimated from the CoP? That might have changed however.

Anyway, my point of view is that you can measure CoM quite accurately from 2D, if the correct protocol is followed. Good 3D systems that use joint location markers will trump that of course! Something like the Vicon system.
Vicon | Applications | Life Sciences | Sports Performance

I'm just waiting to hear from my University whether I can use their labs for some experimentation, especially on the Force Platforms, can't wait.

ETA: Upon further thought force platforms don't just measure the location of the CoP but also the direction the pressure is acting in. In the Nicklaus example the pressure would not be acting directly downwards but at an angle, In this case I assume the software would then calculate the likely location of CoM using this angle, the mass of the subject and CoP location. Not just vertically up from the CoP. Still a guesstimate though.

What a dummy.

 

[Brian Manzella]

I talked to Dan Goldstein, MS, PT, OCS, SCS, ATC, TPI, who is the Owner and Developer of the DBS System.

He doubted that stills could be used for this task.

He also said he will post on this thread very soon.

BManz

 

[matt kluck]

Another source

I would question Dr. Christian Marquardt on the COM/COG discussion.
He has information, I doubt, anyone has thought of dynamically measuring the golf swing and the acquisition of the skill as a combined skill set.
Just my honest opinion.
MK

 

[nwb]

Thanks Brian

 

[jaridyard]

Straight from a Phd at the UK's leading University Sports Faculty to whom I asked the question, 'Do you feel Centre of Mass can be accurately measured using a 2D still?'

A: 'From a 2D image where you can see all the limbs and have appropriate inertia data you can make a good estimate of where the mass centre is in that plane – certainly good enough for most things.'

I'm not saying it's better but it's more than possible as long as the criteria are met. Interested to hear Mr. Goldstein's thoughts, he surely has a horse in the race though doesn't he?

 

[Jay845]

Interested to hear Mr. Goldstein's thoughts, he surely has a horse in the race though doesn't he?

I was actually just going to post this same comment as well.

I am very interested in hearing his thoughts as well, but do agree with jaridyard that on the surface it would appear that he does have a horse in the race.

Looking forward to reading Mr. Goldstein's perspective.

 

[jaridyard]

Ok, I have another response from another Phd in Sports Biomechanics, this one from my old University.

'2D asessment of centre of mass is not going to be that accurate if the movement involves more then 1 plane.

In golf it is particularly problematic because of the rotational element.'

He's taken it a step further, expanding upon the point 'in that plane' from the other answer. As a golfer rotates the planes of motion also move.

So 2D can provide an acceptable level of accuracy when measuring CoM motion in one plane.

Looking forward to learning more about the process of CoM calculation from Ground Reaction Forces.