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I Intensity - Terminology

  1. May 28, 2016 #1
    Hello all,

    Great site...such a cool resource.

    I am a fitness coach...as in any profession basing what we do on solid science is vital....and exercise practice goes "some way" to doing this...but it has its flaws. Hopefully a coach will talk about work, power and torque etc correctly, and have his clients utilizing the terminology correctly as well.

    However, two terms in particular seems to be used totally incorrectly. I was hoping to tap into your physics minds to better understand the physics around these terms. I look forward to learning some cool stuff ;)

    The terms in question are "intensity" and "effort"

    Let me explain...
    As a exercise practitioner I might teach you a movement such as a back squat.
    Back Squat Example Link - Youtube (SFW)

    There are three variables I see here
    1 - What I currently call (likely incorrectly) effort
    2+3 - And the inverse correlation between weight and the number of repetitions of this movement it takes to exhaust the available energy in the system (the body)

    So in its most basic form, this may be the weigh and reps you can move before exhausting all the energy available (what's often referred to as working to fatigue or working to "Reps Max")

    1 - 100 kg
    5 - 87.5 kg
    10 - 75 kg

    So if we remove all of the names etc we are working with one variable that is constant (100% us of all available energy) and two other variables that are inversely correlated.....weight goes up - reps to fatigue go down.

    In fitness this is referred to as intensity.....and it would appear this is being used colloquially...as my understanding of intensity in physics is its a bit of a reach to apply intensity to this situation.

    The second term is when you program for an individual to not work to maximum effort. For example....I might get you to execute 5 reps of squat, but instead of 87.5 kg which would fully fatigue your energy (mainly the ATP energy system) I might get you to work at 70% of this.

    This tends to be called effort....i.e. I want you working to 95% effort, or 70% effort...again not a particularly well picked term.

    So I might ask you to work to 75% intensity (i.e. 75% of your maximum 1 rep weight) but instead of 100% effort (using all energy is system - going to full fatigue) which would be 10 reps, I might get you to work at 70% of this effort, namely 7 reps.

    From a physic point of view, we are now playing with what was a constant (100% usage of system energy).

    Can anyone talk intelligently about what to call these terms that are currently probably incorrectly called intensity and effort.

    I don't imagine there is currently a discrete term that covers this, however much as the term intensity has tried to be hijacked...I'd be open to understanding a physics principle that works in a similar way.....3 variables working together in work and system energy usage.

    I look forward to your thoughts.

  2. jcsd
  3. May 28, 2016 #2


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    Sounds to me like you should be using the word power rather than intensity.

    Power = work/time

    Looking at your data for reps vs weight (and assuming a rep always takes about the same time) it appears that the capacity of a human to do work is critically dependant on the power output (eg the rate of doing work). For example they become exhausted doing one rep at 1000N but can manage 10 reps at 750N which is 7.5 times as much work.

    I think in cycling they talk more about power levels than "intensity"
  4. May 28, 2016 #3
    Thanks so much for the reply C

    From a Fitness point of view....when we talk about strength, we're talking about what's often referred to as the slow lifts. And we tend to be using slow twitch muscle fibres as the work system and ATP as the energy applied to that system.

    In the "Fast Lifts" generally Clean and Snatch variations (Olympic Lifts) You use speed component to generate greater power using fast twitch muscle fibres. Therefore in fitness we refer to power as moving the bar quickly over time to create inertia against gravity to allow the bar to "float" and therefore buy time to position for the "catch".

    Anyone who lifts knows I just butchered that...but close enough.

    So definitely even strength is an incorrect term....virtually everything in fitness is power from a physics point of view...as all fitness is work over distance over time.

    Its just there are different systems providing and delivery that work.

    So back to your comment, intensity (probably used becuase the higher the weight and the lower the weights the more "intense" it feels) is actually referring to a one stable variable (100% energy usage), and two variables in inverse correlation Weight and reps.

    To reiterate, I havnt quite got my head wrapped around Physic Intensity...but Im pretty sure its being misused here. So I am looking for a term or an existing similar physics principal to use as an analogues term to replace intensity.

    Same with what we call effort....how much energy in the system you use before stopping work.

    Hope that Helps?

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