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

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07042007, 12:42 PM  #1 
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I appreciated seeing Lon Kilgore’s article on power in the CFJ. I especially appreciated his differentiating between “burst” power and “sustained” power because I have been looking at these myself for the last few months. (I entered the highland games and throwing is definitely a matter of “burst” power.) While I am hesitant to contradict someone with his experience and background, I have to disagree with the math that Lon Kilgore used.
We should all be familiar with the equation POWER = WORK/TIME that is used to evaluate power output for a workout, but the WORK in this equation is actually the sum of all of the reps in the workout. For example, the work done in Fran is 45 times the work of a single thruster plus 45 times the work of a single pullup. This is important because the work of one rep is “force x distance that the force is applied through” not “weight moved x distance moved”. Force is equal to “mass x acceleration”. Since measuring the actual acceleration of the bar is impractical I’ll assume that average acceleration is an acceptable estimate. Average acceleration is “2 x distance / time^2” (that’s time squared). The final equation for work then is “work = 2 x mass x distance^2 / time^2”. Note that there is very definitely a time element in work. The implications here are huge. Both the distance (range of motion) and the time (for the individual rep) are squared. If you cheat the range of motion by 5% you’re reducing the work done by 10%. In order to reduce the time per rep by 5%, you need to put out 10% more work. While I don’t think that most of us can do very much to increase the speed of our ballistic lifts (such as the clean and the snatch), we can definitely try to move faster through other movements such as squats and pushups. I agree wholeheartedly that both types of power are important to CrossFitters, but I think that if we don't recognize the importance of speed in the lifts that we're losing a very important tool in maximizing force and power. If anyone sees that I’m wrong on any of this, please say so! Doug 
07042007, 02:53 PM  #2 
Member

Maximizing force (and thus acceleration in lifts) has always been the *best* way to add strength/power/muscle mass. Any strength coach could tell you that. That's why most lifts are done at X010 where the 1 is supposed to be with constant acceleration.
(I haven't read the article in the CFJ so I don't know what you're referring to but you're spot on it seems) 
07042007, 09:14 PM  #3 
Member

Think of work as a measure of change in energy. Energy can be in kinetic energy (speed) or potential energy (height). In a lift, the weight starts with at the bottom from rest giving 0 potential energy and 0 kinetic energy. The weight ends at a higher position, again at rest giving higher potential energy and no kinetic energy. Since there is no change in kinetic energy, the only work done is the change in potential energy, or weight*height. It doesn't matter what path the bar took in getting to this point when talking about energy.
Now if the weight truly was accelerating the entire time there would be a kinetic energy component, but you would also be leaping on all your squats and you would have to push yourself down when doing pullups to keep from going too high. Speed and time are definitely important, but they only affect power. The whole article was about power so I don't think it was trying to say speed is unimportant. 
07052007, 05:08 AM  #4 
Member

Craig:
The acceleration the whole time is not meant literally... just to get the lifter to accelerate the bar as hard as he can through the movement to maximize the force the lifter puts on the bar. With lifts like squats and cleans, there is curvature in the lift which means it's not just straight up and down. It accounts to a different amount of work overall, but the potential vs kinetic is a decent approximation I would say. Lifting the weight for potential and kinetic energy is a good estimation, but it doesn't take into account speed which is vastly important especially for strength, power and muscle.. and type of muscle. It really depends on what your goals are but if it's a high amount of fast twitch fibers you better be lifting as hard as you can every rep. 
07052007, 07:28 AM  #5 
Member

Steven,
I was only commenting on Doug's assumption of constant acceleration and time being important in determining work done. I agree completely with you that speed is important for strength, power, etc. 
07052007, 07:32 AM  #6 
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I've seen conflicting formula's that would calculate amount of power created throughout a distance run.
Are there any concrete calculations? i.e. the idea of weight times distance divided by time does not jive with calculating joule's and/or newtonmetres, watts. Thoughts? 
07052007, 08:52 AM  #7 
Member

Gravity applies a force which is directed straight down, so from a physics standpoint there is no work done on an object in carrying it in a horizontal path. Your body is definitely still performing work though, and weight, distance, and time can be used for relative measures of work and power. They just won't translate to joules or watts.
In running, the calories burned (work done) is dependent on a person's weight and efficiency in running. Efficiency is affected by things like the amount of bounce in each stride (higher bounce is less efficient), unnecessary motions(head bobbing,side to side motion, etc) and the amount of energy which can be stored and returned in the achilles tendon and arch of the foot. Efficiency is affected to a degree by speed. A decent estimate of calories burned in a run is : calories burned = weight in kg * distance in km This is really only important if you are worried about losing weight. Otherwise, just worry about speed. 
07052007, 11:33 AM  #8 
Member

My disagreement with Lon’s article is where he says that “work = weight moved x distance moved”, and a few lines down where he says that work does not have a time component. The units of work are “mass x distance^2 / time^2”, the time element is inescapable and critical.
I'd prefer to use something more accurate than average acceleration, but with out some rather expensive equipment it’s the best that I have. Since F=MA (Force = Mass x Acceleration), for a given mass the way to maximize force is to maximize acceleration. I don’t need to be able to measure it to see that the faster I move it, the more force I’m using. More force yields more work and therefore more power. Physics won't answer every question, as the question on running points out, but it is a good tool for illustration. 
07052007, 04:28 PM  #9 
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The running work thing is interesting in that some work is performed as the body bobs up and down. There's also the horizontal force component, hence work in the horizontal plane.
As far as the work calcs, I agree with Doug. There has to be a time component to know what the force component of work is. W=F*d. Without a time one can not know the force and one can not know the work. A lot of the work calculations for exercise assume that the force is just opposing gravity, but the faster one moves through the exercise the more inaccurate that assumption becomes. 
07062007, 12:07 PM  #10 
Member

I have to agree with Lon, at least for lifting an object. The weight of an object is equal to its mass x the acceleration due to gravity. F=M*g. Since the object starts from rest and ends at rest, we know that the average acceleration is exactly 0. If it wasn't, the weight would still be moving at the top. This means that the average force applied to the object is exactly equal to its weight. The work done is then weight times height. W = Fd or Mgh. There is no time component.
Now if we're talking about something like biking where there is a drag force which is dependent on speed, then the work done will depend on the speed/time. 
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