Several rowing bicycles are available aiming to improve upper body training. However, they are expensive, it is difficult to find local distributors and most of them are not really suitable for everyday use. This one is cheap, easy to drive and fulfils all traffic regulations that I’m aware of.
I open a blog for Spanish and other languages speakers: http://lamarianette.blogspot.com/
You may also check my second instructable on this subject, explaining some improvements over the original construction: "marianette update".
Find suitable bikes, in my case a folding bike and a child's bike:
Cut the rear of the child’s bike and mount it as a front wheel in the folding bike using the method of https://www.instructables.com/id/Front-Wheel-Drive-Center-Steer-Semi-Recumbent-Bicy/
To make place for the rear hub in the front fork, you may deform it using a dismounted hub as in the picture:
My folding bike (like most) has a hinge to fold the stem. If your bike doesn’t, then cut the stem and fix a suitable hinge (how to make a hinge is explained in https://www.instructables.com/id/mariannette-update/ ).
Fix a pedal on the stem or handlebar and link a bar from that pedal to one of the front pedals. You will have a crank mechanism: http://en.wikipedia.org/wiki/Crank_(mechanism)
I used a short crank with a crank shortener in order to make shorter rowing strokes:
Crank shorteners are sold in pairs. I used the second crank shortener and the saddle support of the child bicycle to fix the pedal to the stem.
It is difficult to pass the two dead centres of the crank, so I add an elastic rope to help. It would also be possible to freewheel while pushing and use only the pull motion, like rowers do.
Then you have a pushbike in which you can really push while riding: