If you're in the U.S. and that's 29 Fahrenheit, then the temperature explains the difference. In a 5k your body never got really hot in those temps.

Indoors we all get very hot, the equivalent would be riding your trainer in a wind tunnel with wind speed of 19 mph or more. That could explain the difference, without the airflow, your heart has to not only pump blood to your muscles for oxygen and energy - it also has to pump some to your skin for added heat exchange.

If you're somewhere else in the world, and you were running in 29 C, then it's something else.

Thank you. I’m in Arkansas and that was fahrenheit.

Nup, just means you are more of a cyclist than a runner.
Maybe if you are short on running training years compared to cycling, you may lift this to match with a serious concentration on running.
I was always a little lower on the bike because I came from a running background, but now that I basically don't run anymore and road cycle a lot I have finally matched my cycling to my running HR and Lactate levels.

Most people are the same, they will test higher in their best sport.
The best at each individual sport will test really high at their sport and surprisingly low at everything else.

Interestingly, I ran on scholarship in College. However, when I got into triathlon, I found I was a better swimmer than biker and better biker than runner. I can definitely push myself harder on the bike. For some reason, I always have a hard time pushing myself on the run (i.e. hitting paces in training that my VDOT would say I'm supposed to hit).

Also, you ALWAYS need more fitness!

The question is, what do you do with this information? Assuming you'll be riding indoors for the next few months, then you just use that indoor heart rate number like normal.

Since you used "lthr", I am assuming you mean this in the Joe Friel sense - which is probably closer to MLSS or to OBLA in the scientific sense.

So, you'll have to think hard about what to do with the heart rate number from the 5k. 5k is a little on the short side to estimate lthr anyway, using heart rate from a 5k that was particularly cold (I am guessing in arkansas most of your running is done in temps warmer than 29) might not be the best idea.

The 5k pace could be useful, but once again, probably not representative of lactate threshold pace in the Joe Friel sense of the word. Though if you can also get yourself a good representative mile time - you can use the 5k and mile times as input into a critical pace calculation and you'd get a good estimate of lactate threshold pace you could use with your zone calculations.

All of the above plus the only true LTHR test is when they draw blood.

Thank you. Despite almost 10 years of triathlon and several more of running, I'm still learning.

I've gone back and forth on using HR, power and pace over the years. Last spring, I transitioned back to HR as a primary measure since I was using Don Fink's IM plan for IM CdA. His zones are based on Max HR which I found work well outdoors for the bike and run and indoors for the run. I could never get my HR to be in the right zone indoors on my bike so I transitioned to power for indoor bike rides (and using power and pace as secondary measures outdoors and on race day).

I'm currently transitioning back to power and pace as my primary measures and HR second. Current plan is to use Daniels T pace (roughly 5k pace + 30 seconds) for tempo intervals and E pace (roughly 5k pace plus 90 seconds) for everything else (currently building mileage to 50 mpw using BarryP template). For riding, tempo intervals at 100% of FTP (using 20 min test * .95% to get FTP) and 120% for VO2max intervals. Those power and pace numbers seem to be generally accepted (e.g. 5k pace + 90 seconds is a good recovery/long/easy pace) and the resulting absolute #s seem right based on my fitness.

I plan to monitor HR to see how it lines up with the zones and maybe play with adjusting the zones to match the paces/power above over time.

It's almost def going to be the 29F cold run conditions compared to the much warmer (72F, likely) conditions you did the Zwift bike FTP test with.

I'll bet if you do that same 5k run at identical intensity on a treadmill in the same room you do your Zwift workouts in, it'll be +7 to +10bpm higher than your 29F outdoor reading.

(I was going to say you've got better bike-specific muscular endurance than running, but sounds like you're a strong runner with collegiate run background.)