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Fitness Theory and Practice. CrossFit's rationale & foundations. Who is fit? What is fitness?

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Old 08-22-2011, 02:25 PM   #1
Jay DaSilva
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Physics of Fitness Help

So at the Level 1 cert I was at one of the trainers explained a method to measure one's fitness level, or something along those lines. The explanation included the equation for average power which I think is:

p=(F*d)/t

I've never taken a physics course so I'm hoping I've got this right. Anyways with the average power that you calculate and the time of a WOD (I believe that was the variable on one axis of the chart and avg power on the other) you make a scatter plot. Once there is a scatter plot you create a trendline. Once there is a trendline you calculate the area beneath the line and this number will represent your fitness level.

So my questions are the following:

1. Does this sound familiar to you? Is it in fact avg power on one axis and time on the other?

2. How do you calculate average power when your not holding any weight? (eg. pull ups, air squats, running, rowing, etc.)

3. How do I create a trendline? How do I find out the area under it?
(this question isn't that important to me as I do all my calculations on excel anyways. let the computer do the hard part. :P)

Those are the main concerns I have right now. If anybody has already done this I'd love some input. I'm just attempting to create my own charts from scratch so anything you think I might like to know please don't hesitate to tell me.

Thanks!

Jay
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Old 08-22-2011, 05:14 PM   #2
Robert Fabsik
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Re: Physics of Fitness Help

From my understanding of the Level 1 Cert I attended and my understanding of physics, you have things down right, but I also think you are taking them too literally.

They were using the basics of physcis to define a way to approach physics.

In general CF wants to define fitness as your ability to perform at different time domains/energy systems. How much average power can you generate in maximal effort lifts? How much average power can you generate in a 200m sprint, a 400m sprint? How much average power can you generate doing Fran? How much power can you generate rowing for 20 minutes?

Now imagine graphing these points on an average power vs. time of event chart. Connect all the dots for the various points and the area under this curve represents your fitness at this relative time.

Next, in the next year work on your fitness, improve your Fran, improve you max squat ad press. Improve your rowing and long distance. Regraph all those points with new data. Did everything go up? Then the area under the curve will be greater and your fitness will have improved. Did some go up and others down? Then your overall fitness might have remained the same but at the cost of some other ability (say I could squat 900lbs and run a mile in 15 minute, and now I can squat only 300 but run the mile in 4minutes--area under the curve might be similar but how my fitness across time domains has changed).

The concept is the average power you can generate at different time performance points is a measure of fitness. In reality you would need to do a lot of work to graph this. For barbell lifts you'd have to measure how much work (FxD) is done per rep in addition not only for moving the barbell but for a % of your weight. So I wouldn't do this literally.

Instead, I'd keep an eye on some benchmark workouts, lifts and activities and see how they respond to my training program, diet and rest. If I do a strength program and forget metcons for 3 months, what went up and what went down? If I do a GPP program like CF main page, do things go up but how fast?

Hope this helps
Bob
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Old 08-22-2011, 06:40 PM   #3
Jay DaSilva
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Re: Physics of Fitness Help

Thanks Bob.

It is a big help. You've got the right idea. I think I was taking it too literally. Still there are certain benchmark WODs I'd like to be able to have a calculation for. Fran for example. I figured the avg power for the pull ups and avg power for the thrusters than the average of those two.

I also like to have an average power for some other benchmark workouts like dips, the powerlifts, and runs. I know it won't be too precise but thats fine with me. As long as I calculate it the same way now and later I think it should represent a general reflection of my progress.

It seemed simple enough. I calculated my avg power for fran was something like 126 ft-lb/s. Thrusters were p=(95lbs x (3 feet x 45 reps) / 4:01. Something similar for pull ups except I used my bodyweight. Again, I don't know anything about physics but this seemed like a good number to start from. I became confused when I used the same formula to calculate my avg power for running. The result I got (using my bodyweight over 1.5 miles in 9:01) was like 2000-something ft-lb/s.

Is that waaay off or is it supposed to be such a huge difference? Honestly I'm a lot more gassed after doing Fran than I am after that run. lol.

Anyways, I really appreciate your help. It's nothing to stress over. I just really enjoy looking at this stuff this way. Being able to look at results on a chart or having a general number to reflect fitness is really interesting too me. Thanks again.
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Old 08-22-2011, 07:27 PM   #4
Robert Fabsik
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Re: Physics of Fitness Help

I don't know all the physcis of running. But think about amount of effort as a guideline for power. The more anaraerobic, typcially the more power involved. So Fran should be done with more power then running 1.5 miles.

Now when the time domains start to get close together the power used should be close, but might not be exact based on your strengths and weaknesses.

So that a max effort deadlift power would be close to the power of a max effort squat (same time demands), but would differ from a clean since it is done in a shorter time span. But these would be much closer than Fran, but closer to Fran than running 1.5 miles.

Now I don't know exactly how to calculate the work done by running, but 2,000 ft-lbs/sec is way off. A horse power is 33,000 ft-lb/min or 550 ft-lb/sec. And to do 1 HP of work is tough. Your Fran time might be a rough estimate. Maybe someone with some deeper physics training can help us out.
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Old 08-22-2011, 09:46 PM   #5
Ben Moskowitz
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Re: Physics of Fitness Help

Here's a website that calculates the power output of some of the major movements, even as programmed in a metcon: http://www.cathletics.com/resources/powerOutput.php I also recommend reading the article that is linked there ("Crossfit Criteria") to learn about some of the limitations of this approach.

link wfs
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Old 08-23-2011, 03:38 AM   #6
Jay DaSilva
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Re: Physics of Fitness Help

Awesome link! Thanks so much. Definitely helps to put things in perspective so I know whether or not I'm on the right track. I'm at work now but I'll check out that article when I've got some time.

I'm definitely going to continue to try and find a way to add running and rowing into this. In the meantime maybe I'll just make up a formula to use. For example, instead for the Force part of the equation maybe use 50% bodyweight, or even 10% or 5%. So long as the result is in the same ball park as other workouts and wods.

Keeping my eyes peeled for anybody whos got this mastered. Thanks again, both of you, for all the help.
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Old 08-23-2011, 07:10 AM   #7
Brendan McNamar
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Re: Physics of Fitness Help

beyondthewhiteboard.com also can calculate power output. You have the ability to put in your body dimensions to improve accuracy.
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Old 08-23-2011, 07:40 AM   #8
Ash Smith
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Re: Physics of Fitness Help

Calculations get really tough with bodyweight. Pullups, for example...you're not really lifting your forearms. Your upper arms, very little. Everything else is true dead weight. So a pullup might be 90% bw.

Squats, same thing with calves vs thighs vs rest of body. Probably 75% bw (I'm just guesstimating). When you can squat your bodyweight, you're really generating a lot more than 2x the power of a bw squat...and you can probably tell by your perceived exertion.

Pushups are often estimated at around 60% bodyweight, which can easily change based on your height, feet vs knees, weight of head, etc.

I'm a numbers wonk myself, but I would say you shouldn't get too hung up on any numbers except for those most commonly tracked (time, reps, weight). Soon you'll have a giant data set of very little use, IMHO, but it will look really cool
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Old 08-23-2011, 09:24 AM   #9
Jay DaSilva
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Re: Physics of Fitness Help

Thanks, Brendan, for the link. I'll check that one out too. Thank you Ash too. I appreciate the input. The more I think about how hard it would be to have accurate calculations the more I think this has to be a personal project. Something I can use for myself and anybody I train.

I just really love the idea of an overall general fitness "rating". I think the goal now isn't to be accurate as far as physics are concerned but rather to create a formula where all the numbers are in the same area. For example the results for Fran for a beginner will be in the 50 - 100 range and the Fran for an elite crossfitter will be in the 150 - 250 range.

If I can tweek different bodyweight percentages for bodyweight exercises so that they also end up in this general area I'll be happy. For example, I used 5% for my 1.5 mi run and got a number around 150. It won't be scientifically correct but I'll have a good scale that I can use to illustrate my general fitness level.

I'd love to be able to do this so it were scientifically correct but for now this will have to do.

Thanks again everyone for the help.
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Old 08-23-2011, 10:49 AM   #10
Ben Moskowitz
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Re: Physics of Fitness Help

Here's a paper that models the energetics of running: http://www.physfarm.com/govss.pdf (WFS). I didn't look at it in too much detail but I imagine you could take some of the equations given, determine the coefficients of your specific location, and then plug it in and get a power output. This is just a regression model; as I understand it the biomechanics/physics of running is pretty complex.

I would say that guessing 5% of distance for a run offers no advantage over simply recording the time. If run on a treadmill, you could talked your energy expenditure calcuated by the machine (in calories), convert to the appropriate units, and then divide by the total time. Of course, it would be foolish to forgo running outside for running on the treadmill for redundant information (all you need to track is your time!). The rower will give you an average power output.

I wouldn't say that work calculations for bodyweight exercises are that complex. You just determine the displacement of your center of mass, which will vary according to the orientation of the body but is generally just above the hips. I'm pretty sure this is how the power output calculator I linked to figures in bodyweight exercises. Comparison to bodyweight is a bit different since you are talking about force and not work.

Finally, I really recommend reading the article Crossfit Criteria because it highlights some of the problems a focus on externally measured work and power can have on effective training.

link wfs

Last edited by Ben Moskowitz : 08-23-2011 at 10:53 AM.
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