For the health of it

Tensing the muscles of your legs against your foot pegs does not consume anything close to the amount of oxygen that is consumed through the full range of hip and knee flexion and contraction that occurs with rowing on a sliding seat or sculling.

Add that to my upper body movements of torso, arms, & hands. Both apps have me burning much more calories paddling. My legs more than tense on the forward stroke. They are moving although not like a cycling.

Yes, and when cycling you use your biceps, triceps, and shoulders to a considerable extent as well, especially when hill climbing, although the range of motion is not so great.

I’m not sure what apps you are referring to. Obviously the calories burned per unit time in either activity is going to depend on a lot of factors such as equipment, wind direction and speed, current direction and speed, gradient, and the intensity with which you approach the activity. There is no doubt that you can burn more calories kayaking very vigorously than bicycling at a leisurely pace. But there are many comparison tables that show bicycling consuming more calories than canoeing or kayaking when done at similar levels of intensity and I will link you to another. This is a long table that shows energy expenditures for many different activities in terms of METS. It is a long table so I will list some extracts from it below.

Note that for all of the bicycling activities the energy expenditures are for a flat surface. Energy expenditures are much greater if any hill climbing is required (and it usually is).

Bicycling: leisure, light effort, 10-11.9 mph - 6.8 METS
Bicycling: leisure, slow, moderate effort, 12-13.9 mph - 8.0 METS
Bicycling: racing or leisure, fast, moderate effort, 14-15.9 mph - 10.0 METS
Bicycling: very fast, not drafting, 16-19 mph - 12.0 METS
Bicycling: racing, not drafting, > 20 mph - 15.8 METS

Water activities:
Canoeing on camping trip - 4.0 METS
Kayaking: moderate effort - 5.0 METS
Canoeing, rowing, 2.0-3.9 mph, light effort - 2.8 METS
Canoeing, rowing, 4.0-5.9 mph, moderate effort - 5.8 METS
Canoeing, rowing in competition, or crew, or sculling - 12.0 METS
Canoeing, rowing, kayaking, competition, >6 mph, vigorous effort - 12.5 METS

So by this table the only time that the energy expenditure for kayaking, canoeing, rowing approaches or exceeds that for bicycling at even a fairly leisurely speed of 14-15.9 mph is when done at a competitive level sculling, crewing, or kayaking at a speed of greater than 6 mph. In no case do these water activities approach the energy expenditure required to bicycle at 20 mph or greater, which is a speed competitive cyclists can maintain.

As for myself, when I cycled regularly although I could not maintain a 20 mph pace very long when not drafting, I could certainly maintain a pace of 16-19 mph for some hours which is listed in the table as requiring an energy expenditure of 12.0 METS and a pace that I personally would not describe as “very fast”

As for kayaking, a pace of greater than 6 mph (12.5 METS) would exceed maximum theoretical hull speed for any of the kayaks I own (longest being 17.5 feet) and I would ask how many individuals could maintain that pace on non-moving water for any length of time kayaking.

Strava and MAPMYRUN

As I understand it, apps of that type make estimates of caloric expenditure while bicycling based on factors like body mass, average speed, distance, and elevation using GPS to determine distance and elevation change. I could be wrong because I haven’t used them. But they can only give general estimates based on population averages.

Of course, the same is true of the tables of caloric expenditure and METS that I presented. The best way to estimate your energy expenditure for any activity is to track your heart rate. Of course a determination of VO2 or calorimetric determination of energy consumption would be more precise still but that is impractical or impossible for most. Heart rate is a reasonable estimate of oxygen consumption and caloric expenditure. There are quite a few bicycle computers that will track your heart rate while simultaneously tracking average speed and distance. That is the most reasonable way to estimate your personal energy expenditure for the activities you engage in.

I know that I consume far more calories riding a road bike on a relatively flat course on a good surface at 18-19 mph for 90 minutes than I do paddling a kayak on flat water at a speed of 5 mph for 90 minutes because I have tracked my heart rate doing both. And I could ride a road bike at 18-19 mph longer than I could paddle a kayak at 5 mph on flat water.

By the way, this question has been debated a number of times. You might want to peruse this old thread: