Equinox

Still not sure I understand. Thinking back to when I ran, my HR profile was not THAT different from riding. With respect to muscles involved, I might argue that I use more of my musculature on the bike. My HRmax is higher when I XC ski, But my average is in the ballpark.

Carbonfiberboy 

I'll come back, maybe it'll help some other captain.

I had a research physiologist I ride with comment on my consistent seeming acidosis after our group rides, IOW fast breathing long after the ride was over. On a long hard tandem ride, I'm almost always between VT1 and VT2. My understanding is that blood lactate levels begins to rise at approximately VT1. Because I'm ~10 years older and less talented than the fast boys we ride with, I've trained myself to run at a higher percentage of VO2max than most riders. I climb in Z4 and recover in Z3. So maybe I simply have higher blood lactate levels than most folks. Perhaps that's all there is to it, not a metabolic malfunction.

There have been discussions on the Road Cycling forum about whether a heavy bike can help one train harder. The cognoscenti say that's not true. However my experience captaining a mixed tandem is that it's definitely true. I can't work this hard on a single bike. Back in '09 when we got our tandem I was a well-trained endurance cyclist. Our first tandem ride was only 20 miles, but my legs hurt like the devil afterwards. We've since done doubles and long hard rides in the mountains on our bike. Training is good. I put a great deal of my cycling longevity down to the tandem experience.

conspiratemus1

Given that you can ride like that, I'm sure there is nothing wrong with you. Perhaps you could put some of it in a bottle or an air-mail envelope and send some my way. At a lower level of achievement than you, I would credit my cycling longevity to tandeming also. It's fun (being with your partner is part of that), it's hard work (or it can be), and it's good for you. What more could a might-have-been or never-wozzer athlete want?

Remember that the anaerobic metabolism of glucose produces not just "lactate" per se but lactic acid, which dissociates at blood pH of 7.4 into hydrogen ion and lactate ion. The hydrogen ion (the "acid" part) is buffered by bicarbonate ion in the blood to produce CO2 (which is breathed out by the lungs) and water. So the step-up in breathing effort at VT1 is stimulated by rising production of CO2 as acid-buffering is added to metabolic production from the on-going aerobic respiration of fat and glucose. So yes above VT1 you do have rising lactate ion levels but blood pH remains normal and breathing is being stimulated by CO2 and not (yet) by acid itself. This may sound like semantic hair-splitting but it is central to the understanding of what VT1 and VT2 "mean" physiologically. I'm going to to go out on a limb and suggest that your ability to tolerate long periods well above VT1 could be due to your ability to breathe out large amounts of CO2 without getting tired ...and perhaps your ability to regenerate bicarbonate buffers on the fly (kidneys are so smart!) so that you stay in the comfortable place above VT1 (yes with higher lactate levels) and still stave off acidosis. Lactate ion itself does not seem to be deleterious to muscle function; "fatigue" seems to due to be acidification of the body fluids or perhaps to other by-products of anaerobic contraction that are less obvious and less easy to measure (always a problem in physiology.) Middle distance runners (800 m) have cheated by surreptitiously ingesting large quantities of baking soda just before a race on the idea that temporarily increasing their bicarbonate stores will allow them to tolerate more lactic acid production before they become acidotic. Don't know if it works but it is agin' the rules. (Because the extra bicarbonate is rapidly excreted in the urine, the effect, if any, lasts only a few minutes.)

My understanding:
Less than VT1: normal (very low) lactate level, normal pH, RQ <= 1 (RQ is the respiratory quotient, the number of molecules of CO2 excreted per mole of O2 consumed. It's ~ 0.8 at rest, reflecting the major contribution of fat to basal and light-exercise states.)
->Respiration in this range is being stimulated by CO2 production from aerobic metabolism.

Above VT1 but less than VT2; rising lactate level, falling bicarb, normal pH, RQ > 1
->Respiration in this range is being stimulated by CO2 production from buffering acid, in addition to aerobic production of CO2 as above.

Above VT2: high lactate, low bicarb, low pH (acidosis), low CO2 in blood and breath as it is being excreted faster than produced in response to acidosis.
->Respiration in this range is being stimulated by acidosis and is in excess of CO2 production from either source.

Lactate still has fuel value and after exercise it is recycled for use in aerobic metabolism -- this is the oxygen debt. This process, and several other recovery tasks (including regeneration of bicarbonate), require metabolic effort and would show up as increased breathing for some time after the exercise stops. If you are spending time at a higher percentage of VO2max than your riding companions you will, all else being equal, have a larger amount of lactate to deal with. It would be interesting to test this, especially since real-time blood lactate levels are now practical to use in exercise physiology studies, sadly not yet feasible to include on a FitBit.

Finally, ...prolonged, exhausting exercise activates the inflammatory response which increases CO2 production from catabolism of protein and fat for some days afterward. That's what you feel when you are listening to your body telling you to take it easy and rest some.

Unfortunately none of this explains why the hardworking stoker gets a lower HR on the back seat than she does in other sports. Could it just be that if you can't see where you are going -- if you can't see the rabbit you're chasing, it's harder psychologically to flog yourself to the speed that higher power output would propel you to?

conspiratemus1

To answer your question definitively you (and your stoker, since the original post was ostensibly about her, but she's been silent here) would need to measure your VO2 (not just guess at it with heart rate measurements) at various measured workloads using different types of machines -- treadmill vs. stationary bicycle vs. stationary rower vs. elliptical -- and see if your HR response to those known workloads is reproducibly different. Then you would know how if your HR profile was really not THAT different from riding or how much of your musculature you were really using. I don't know if this has been studied. Bicycle ergometers are usually used in exercise physiology research because it's easier to hook up the various measurement apparati (gasbags, particularly) and quantifying the actual work being done is easier than on a treadmill where the runner's body oscillates up and down with each step and leads to some unmeasured energy losses. You can measure VO2 on a treadmill, sure, but it's harder to compute exactly how much external work is being done, to compare the two (if that's one of the necessaries of the particular study -- it isn't always.)

The message I think, though, is that HR probably doesn't matter much in training for us ordinary folks. It's just an easily measured (and, more important, uploadable to social media) number that has some predictive value in terms of how close to VO2 max you are, but also has some error because it is not the only determinant of VO2. So forget about it. If your assessment of your breathing effort puts you above VT1, you are getting a cardiovascular training effect. It has the additional beauty of making you more "organically" aware of your body. Caution that not everyone shows a definite slope change of RQ as lactic acid starts to leak out of the muscles, but if you are breathing hard and can't recite Shakespearean soliloquies, your probably there. Think of a comet: it's easy to tell the head from the surrounding sky but much harder to tell the head from the tail, and the tail from the background sky. But somewhere in there, there is definitely a comet. (I borrowed that analogy from a lecture on impending heart attacks.)

If you just can't make yourself stop looking at your HR, you could keep track of what HR you typically show when you reach VT1 for that particular excercise. If your own response is consistent, then HR above that can tell you how deeply into the zone between VT1 and VT2 you are. It could also warn you if you unintentionally slipped back below VT1 from loss of concentration, and remind you to think about your breathing and maybe goose the pace a bit. Don't compare your HR to anyone else's, and don't compare it to what it does in other sports -- then you are looking more at noise than at signal and you'll likely find there's no "there" there (to quote the FBI. But I think it was originally said about San Jose.)

A good reason for a HR monitor would be if you were in a medically supervised exercise rehab program for which the exercise prescription advised you not to exceed a certain heart rate. If you are a for-real competitive athlete, keeping good relations with your coach may require you to keep track of HR. It does pick up talk-is-cheap people who aren't motivated enough to work hard at all and advise them to stop wasting their money on coaching and personal trainers. And yes, I suppose it's possible that lower HRs in a particular activity do indeed simply mean that one is not putting out as much effort in that activity as in some other. But that's not the first conclusion I'd jump to, even about myself.