Concept2 Forum

An engineering question:

I expect someone has looked at this relationship somewhere or sometime.

I want to use my erg to replicate an exercise my physio guy gave me using a leg bench press machine. At least I think thats the name of it... He set it at 40 pounds 'push'. Wimpy, but then my knee surgery is less than 2 weeks ago.

My parameters:
Erg on slides
A 'stop from coming all the way back' in the front slide so that I can't overcompress the knee by accident. Its about 18" long.

I did 500m at low df and a quick pace, as I don't want to do a very long stroke yet. It felt ok, but I would be more comfortable if I knew what I was doing to myself.

Is there some way of roughly approximating my leg pounds of push? I know of one I used 35 years ago to figure out my bicycle forces-a piece of surgical tubing. Measure the stretch and then calibrate the tubing.

I was kind of hoping someone had already looked at this, but if not I'll give it the 'primitive physics' test.

grams

grams wrote:I was kind of hoping someone had already looked at this, but if not I'll give it the 'primitive physics' test.

I haven't looked to see if anyone else has looked at this, but here's my primitive physics test.

energy = force x distance

power = energy / time
power = (force x distance) / time
power = force x (distance / time)
power = force x velocity

Now, let me assume a few things:

• You apply a constant force throughout your stroke. If you were to look at your PM3 force curve, it would be a perfect rectangle - not a triangle, not a left leaning gumdrop.
• Because of the stops you put in, your center of gravity is moving 0.5 m (~ 20 inches).
• You take one stroke every 0.5 s (or 30 SPM).
• Your instantaneous pace is always 2:20. That works out to 128 W.

First of all, your velocity would be 1 m/s.

If your velocity is 1 m/s, then your force would be 128 N. That force is equal to the normal force of a 13 kg mass at 1 G. 13 kg is 28.6 lbs. When pounds are used to indicate force, I believe they mean to indicate the normal force of that many pounds of mass at 1 G.

Now, given that the force you apply is not constant, you might apply more than 128 N at some point of your stroke and still have an instantaneous pace of 2:20.

The average force you are applying to the handle is:

F (in Newton) = 60.W/(L.R)
Where W is the power in Watts, L is the distance the handle travels each stroke during the pull, R is the rating in strokes/minute.
1 pound is apx 4.5N, 1kg is 10N.

However this calculates the average force, not the peak value, and it might be this you should beware of. Also, presumably, leg force = pull force.

As a numerical example, 20kg average pull force @ 25, pull length 80cm, means 65W.

Careful when climbing stairs too: you may find yourself lifting your body weight one leg at a time.

Edit: this calculation gives the average net force tending to turn the flywheel; net of the chain tension due to the chain return system and of any force due to climbing the rail slope. For small handle forces, these two are a non-negligible proportion of the total. Anyway I still guess walking produces much larger forces.

Thaks everyone. I'll take your info and also Pauls comments and present them to my mechanical engineer husband. We can have some togetherness time trying to work it all out.

My therapist guy says really I can't 'hurt' my knee splice job with the erg, the most I can do is set myself back a bit. In the long run it comes down to 'if it hurts, ease off'.

grams