Automobiles, trucks and motorcycles burn calories but bikes rowing machines and treadmills do not people who use them burn calories. How many calories a person burns doing any particular activity is based on a number intrinsic and extrinsic variables.
The most important single factor in how many calories can burned by an individual is their VO2 max- capacity. "It is well understood in all exercise physiology and nutrition texts that for every liter of oxygen consumed, approximately 5 calories are burned".(Exercise After-Burn: Research Update By Chantal A. Vella, Ph.D. & Len Kravitz, Ph.D.).
Except for anaerobic work, which is always limited in endurance activities,
the amount of calories an individual burns in any givien activity is directly proportional to the amount of oxygen consumed during the activity by that individual.
How many calories a person burns when rowing, cycling or running on a treadmill will be determined by the intensity with which they perform these activities and not the machine.
An individual who performs each of the above activites at the same intensity will burn the same amount of calories. Outside the metabolic lab the best way to gauge the intensity level is your heart rate. So if an individual performs each of the above activities at a heart rate of 150 he/she will burn the same number calories in each activity.
The heart rate is driven by oxygen demand and there is a linear relationship between heart rate and oxygen consumption up to the 100% VO2 max- which coincides with 100% maximum heart rate.
The individuals ability to train at or near their VO2 max in each activity will be determined by the individuals training in each activity.
How many calories an individual can burn doing each of the mentioned acitivitis will not only be determined by the intensity but by how long they can perform thes activities at any given intensity.
I have been riding a bicycle since 1984 and when I have been training seriously on my bike I have no difficulty in riding 100 miles in the hilly terrain of New England. But I have only been rowing for the past two years and rowing 1-2 hrs is difficult for me. (I have no trouble spinning for 1-2 hrs) Yet there are rowers who are able to row continuously for what I consider ridiculous amounts of time. It has taken me 2 years to row 4 million meters and presently there is one individual on the annual meter board who has already rowed 4 million meters since the beginning of May and is projected to row 24 million meters for the year.
Some individuals have no trouble running marathons and some even run ultramarathons of up to > 200 miles. At present I don't think I could run two miles since it has been several years since I last used a treadmill.
As for which activity has the leanest athletes I guess it depends on your definition of lean. According to Websters dictionary (
http://www.m-w.com/dictionary/lean)
Lean: 1 a : lacking or deficient in flesh b : containing little or no fat. SKINNY means thin because of an absence of excess flesh whereas LEAN stresses lack of fat and of curving contours. I suspect that John Rupp is using lean to mean skinny. There many elite athletes who are very muscular yet lean (meaning low % body fat). Rowers tend to fall into the latter category since irowing considered a power-endurance activity.
I am sure that the arguments will continue to rage on about which activity burns more calories but I personally think that the argument is probably a waste of energy which could better be utilized to row your erg, ride your bike or run on your treadmill.