Explore BrainMass

Exercise Physiology/Kinesiology/Physics Problem

I am trying to calculate the true weight effects of a supine leg press, standing squat, and bench press in supine.

Below is communication between my previous physics professor and myself, but it really didn't answer my question. I guess I want to know if there is mathematical way to get 'true' numbers for the actual weight involved considering: gravational aspects, body positioning, energy utilization, energy economy and anything else that you may know that I am missing.

Here is the conversation:


HI Dr. M.

In lab today we did what is called 1RM (one repetition max) of a couple of exercises. Some positions were parallel and others perpendicular, meaning with gravity or against gravity and gravity assisted.

Leg Press, recumbent (sitting) position, this is parallel to earth, meaning the movement is sitting on the machine and pressing the foot rest in and out but gravity eliminated positioning (↔ ). I leg pressed 410lbs (the max of the machine J). It is a CAM mechanism. So what I am wondering is since the exercise was performed in gravity eliminated, with a CAM mechanism, do you have any idea what the TRUE weight was actually pressed?
The same muscles (hamstrings, gluts and quadriceps) also get used performing a "standing squat". Meaning, in standing an individual places the bar bell with the weight on the top of scapulas. To perform the squat the individual has to control the weight in a downward motion, which is with gravity (↓), fully controlled till hips and knees are 90 degrees flexion, if this is not controlled the wt would drop to the floor. After the individual reaches proper depth, the person then has to move the weight up, against gravity (↑) to the starting position.

Now taking all this information, and the fact that gravitational pull is 9.8m/s2, What would then be the weight with this form? Because there is no way I could put 410lbs on my back and squat then stand with gravitational pull. So I am wondering what would have been the weight if I had chosen the squat vs the leg press.

Bench press, person is supine (laying on back on bench), and barbell of 80lbs is 1RM, same as the above going with and against gravity, but this time the persons trunk is stabilized. Would there be any difference from the squat where the person is co contracting the whole body, vs spine supported by the bench?

DR. M.

Hi Cheryl,

Let's see what I can suggest for your questions:

1) In this case your legs, etc. are producing a FORCE (not a weight) of 410 pounds. This has nothing to do with weight but perhaps you could lift something that heavy with your legs, etc. if you were lying on your back and pushing straight upward.

2) This one is not so simple. Your legs, midsection, back etc. (and presumably NOT your arms) provide a force equal to your weight plus the bar bell (as you move your center of gravity up some distance). However, this seems to imply that if you weighed 100 pounds that you could lift about 300 pounds of bar bell -- sounds like a stretch to me! It would be interesting to know the bend of your legs for part 1) as in part 2) it varies quite a bit for the rep.
Also when you are lifting the bar bell I would think that a lot of the lift is coming from the back muscles as much of your body is not directly above your feet and you need to supply a torque to bring that part up vertically above your feet.

3) I'm not sure of your question in this one. To bench press you must use muscles in your back, neck, and arms - I think, so this doesn't seem to relate to 1) and 2). Basically, you are just lifting the bar as the bench is supporting your weight.

Those are my thoughts at the moment. Hope they inspire some thoughts - you are the one who knows how the body works.

Solution Preview

Hi there. Thank you for posting your question to Brainmass.

Please see attached for solution.

Check ...

Solution Summary

The solution provides detailed explanation and calculations to explain the true weight effects of a supine leg press, standing squat, and bench press in supine.