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I do bodyweight circuit training where isotonic and isometric exercises are mixed. So I want to know how to convert repetition number for dynamic exercises to hold-time for static ones.

For example, I want to perform, for lack of a better term, push up and plank equivalently. Then how many seconds of plank hold equals ten repetitions of push up?

The system I currently follow without any educated method is: 5 seconds hold is equivalent to 1 repetition.

Edit: By 'equivalently' I mean that, say for some reason I do 10 push ups, 10 crunches, 10 squats; same repetition for every exercise despite each focuses on different muscle groups. Now if I want to include plank in this circuit, I want to hold the pose the amount of time which somehow is equivalent to 10 repetitions if it were a dynamic exercise. For lack of better term I can't express the matter clearly other than stating 'equivalently'. I guess I am talking about time under tension.

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    Push-up and planks are different muscle groups so I don't know if that's a logical link. If you were, say, going from squat to pause squat, you might have a correlation.
    – C. Lange
    Oct 19 at 18:05
  • I understand that. But still say for some reason I want to do same amount of 'activity' for different exercises. That is if I do 10 push ups, I also want to hold plank that amount of time such that, it is equivalent to doing 10 repetition of plank if it were a dynamic exercise.
    – Kawrno
    Oct 20 at 4:07
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Unfortunately calculating equivalency between two different movements or even the same movement by one person on different days is near impossible. You would also have to define exactly what you meant by equivalent. Amount of ATP burned, number of protein molecules generated, amount of work done, effort required, etc. I suppose the most common way to compare exercises is through muscle/strength gained by comparing different workout routines.

Effort?

Based on how long you have been doing an exercise, your body will have different neural adaptations, meaning certain exercises will be easier for you because your body is optimized for what you have already done. Combine this with the fact that everyone's bone structures and muscle/fat distributions are different. The same exercise on different people is incomparable, or even the same exercise that you performed two days ago is incomparable due to your most recent adaptations in terms of effort.

Work Done?

Hypothetically if you get a detailed body scan, you could do the math to see how much 'work' is required for a rep based on where your weight is distributed, the angle of your pushup, etc. The problem with this, is you run into a bit of an issue with terms from different worlds when you try to calculate work done in static holds. You could try going down the rabbit hole and ask something along the lines of Do static holds build power, or static work done during a Bulgarian split squat answered on both the Physics Stack Exchange and here, or Calculating amount of work done in gym workouts. Work in physics is Force * Distance. Static holds don't move, so technically there is no work being done according to physics.

Muscle Generated

This is likely the most common way of determining how effective certain exercises are compared to others. You can find study after study comparing how well different subjects responded to different types of workouts, movements, etc. In a review of current literature of isometric strength training, they mention that in order to increase hypertrophy, you should perform isometric training at 70-75% of of maximum voluntary contraction between 3-30 seconds per pre, at a total of 30-90s per session. This is not saying that different rep ranges don't improve neural adaptions or strength gains, it is saying that hypertrophy is optimized at that range.

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  • Thank you for your thoughtful reply. I have just wrote in my post what I mean by 'equivalently'.
    – Kawrno
    Oct 20 at 3:59
  • @DaveNewton Nice find on another physics link of the same question. I hadn't thought about the muscle slightly drooping and reflexing to maintain position. It is a bit out of my wheelhouse, can anyone here confirm that is what is happening? Oct 20 at 16:48
  • Ultimately it comes down to “work” in the physics sense not being a complete picture. Isometric exercise does no “work” in the physics sense, but it’s not helpful here since muscles use energy to stay contracted. You could calculate the force needed to maintain a straight body, I suppose. Oct 20 at 17:25
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    @Kawrno I don't think you are going to be able to find a good conversion. Personally, I don't think you should be finding a conversion for this anyways. When working out, you should be maxing out volume on each muscle group. Different muscle groups have different total capacities, and as such, doing 'X' units of each exercise means you are leaving a lot on the table for some movements and maxing on others. Different movements will progress at different speeds, and each workout you should do your best to improve any rep/weight ranges from the last time you worked out. Oct 20 at 18:48

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