In general, cyclists get their heart rates up much better than triathletes on the bike.
Longtime triathlete and full-time lifeguard Mark Montgomery got me thinking about this, and I asked him to write a bit of a historical prologue:
"There is an apparent difference between cycling and running heart rates," Monty writes. "Do you notice when you go out for an hour run at 85% of your max, it is just a good hard run, but when you try to ride at 85% you can last for about 10 minutes? Back in the old days we all just figured that that was the way it was, so what are you going to do?
"But Gary Hooker _ yes, the aero-bike-maker fellow _ did a lot of research on this kind of stuff, and as I pondered it, I found something very interesting. When Kenny Souza would ride and run in a race, his rate wouldn't vary but a couple beats in each discipline. Since he was the best in the world at the time at these two sports, I figured that he was doing something that the rest of us weren't. It turns out that it isn't the heart that is the limiting factor in cycling. After more tests on world class cyclists-turned-triathletes, it turns out that they also have comparable heart rates in both disciplines.
"Gary figured out that it was just a function of strength-to-weight ratio. Most of us at the time were swimmers and runners, so we had never developed the necessary strength to push our hearts on the bike like we could running and swimming. So we were able to alter our training to include more weights and long, hard hill climbing, and eventually we were able to get those cycling heart rates closer to the running ones. The difference wasn't carved in stone like we thought."
Monty is, from a layman's view, right on in everything he says, with my picky exception regarding the causality. I don't think strength-to-weight ratio is the precise rationale. But it is entirely true that in general, cyclists can get their heart rates up during cycling much better than triathletes can when they ride the bike.
In fact I looked at a dozen or so studies _ this has been studied up the wazoo _ and in each case pure cyclists had fairly comparable heart rates riding the ergometer and running on a treadmill, whereas runners did fine on the treadmill but couldn't get their hearts up while on the bike. (Realize that in each case the athlete was able to achieve a higher heart rate while performing work in his given specialty.)
Road cycling is one of those odd sports, like speed skating and Nordic ski racing, in which aerobic capacity is obviously much needed, yet muscular power is almost as important. It may or may not be strength-to-weight, as Monty said, but cycling-specific power _ at any weight _ appears to be necessary.
But it isn't power at the top end that matters. One very smart physiologist with whom I correspond reminded me that Chris Boardman set a world hour record without being able to generate over 1,000 watts in a sprint and was unable to squat even a moderate amount of weight. But he could generate 440 watts for an entire hour. That sort of power is not necessarily gained through bigger muscles, but mostly through physiological changes at the cellular level.
Look at it this way. Swimming is an endurance activity just like running, yet swimmers are able to pull much more water than a runner-turned-triathlete. Yes, cycling and running are much more similar to each other than either are to swimming. The point is, cyclists develop their muscles in a sport-specific way that allows them to generate much more power cycling than a runner turned-triathlete is able to.
There are several things you can do. Monty mentioned the most obvious, and that is to increase leg strength. Yes, lower-body weights is one way to go. I rather like his other option, which is hill riding. We are blessed in my area (north-inland San Diego County) because of our close proximity to many, and long, and steep inclines. You can pick your poison here: 20% grades or 4,000 foot grades. About the only thing San Diego doesn't have, where I ride, is a lack of grades.
Why is hill riding important? Because it forces an athlete to ride at a very high effort level for a sustained period of time. Let's face it, if you're toodling around town on the flats with your friends, your pulse isn't going to get up that high, and neither is your sustained power output. But what happens when you're climbing a 7% grade for several miles?
What I hope to impart is that it isn't set in stone that your heart rates must be 10 or 15 beats lower while on the bike vs. running. If you can't get your heart rate up while cycling it's simply because you're a better runner than a cyclist. The idea is not to attempt to raise your heart rate for the heck of it, but to raise the level of your cycling ability so that your well-trained cardiovascular system can get off the bench and into the game.
One way to achieve this is certainly to lift lower-body weights, as has been noted above. But I prefer more sport-specific ways, because that is why cyclists are able to generate cycling power that triathletes can't hope to match.
I'd prefer to perform power workouts while on the bike. Cyclists often motorpace for this, but there are logistical impediments to riding behind a powered scooter. One might resort to trainer workouts, such as the one I describe in another article on Oxygen Consumption Drift (q.v.). Another way is to ride a lot of hills, and specific hill workouts can be found on cycling training sites throughout the Web.
There are two additional things I'll mention, both of which are controversial, and for which I have no scientific evidence to offer. They don't even rise to the level of hypotheses, and it might be best to style them "notions." Both center on the assumption that the better triathletes tend toward endurance physiology, i.e., they are less muscled, have predominantly slow-twitch muscle fibers, and they're geared toward endurance. Furthermore, they train themselves in a way that favors endurance physiology.
The art of competitive cycling rewards those who contain a curious admixture of abilities. Imagine a 10-mile footrace that occurs mostly at a steady jog, but with an occasional all-out sprint up a hill or along a quarter mile stretch. Imagine that only those who keep up during these ballistic efforts are allowed to continue the race. It is apparent that power is crucial to top-level cycling.
In light of this, it is perhaps easier to imagine why a cyclist's leg power is so well-developed that when he rides a bicycle he can generate the same heart rates and oxygen consumption levels as when he runs.
What can be done for the poor run-trained slow-twitch triathlete? In addition to doing everything he can to increase his cycling power, there are some "work-arounds." One of these, in my view, is for a triathlete to ride with a steeper seat angle in conjunction with everything else we note in our articles on tri-bike fit.
In my view, this not only preserves a wider and more biomechanically efficient hip angle while riding in the aero position, it also allows him to spread out the work of the pedal stroke to other groups making up the hip musculature.
In other words, your peak power during the pedal stroke may be lower than a cyclist's, but your power application throughout the pedal stroke may be the same. Triathletes may never develop the huge vastus medialis muscles obvious on just about every pro cyclist, but his hamstrings may be able to take up much of the slack, and riding with a steeper seat angle make this easier.
Cadence also is critical to a triathlete's power, and this is not just an issue for triathletes. It's ironic that cyclists understand cadence and work at this to a degree triathletes don't.
In both examples above, triathletes will find themselves better able to work up to their endurance capacity, and to ride with higher heart rates without experiencing muscular exhaustion. Through a better use of cadence and "levers" they can trade in peak power _ which they have a hard time generating in any case _ for a more constant application of power, and it's application for more cycles over a given period of time.
In our discussion of the slow component of VO2 [see link above] one potential aggravator is the recruitment of fast-twitch muscle fibers when slow-twitch fibers are more appropriate for the work. I suspect that this is exacerbated by the application of higher peak power in place of sustained power, i.e., gear mashing instead of spinning.
This might lead to race phenomena like leg cramps _ even during the run segment _ that a triathlete might blame on electrolyte imbalance when he ought to look no further than his cycling training and technique.