so I normally ride ~78 RPM and at my endurance pace I hover around 120 Heart Rate bpm.
today I rode around 90 RPM and I hover around 130 bpm.
I am just curious, how do the two workouts compare in terms of physical adaptations that come as a result of the sessions?
Short answer is: we don’t know. Seriously, we don’t know.
Trying to piece the adaptation from one riding style vs the other would require large groups of riders double blind assigned to one regime or the other, plus are you interested in this for endurance/ tempo / sweet spot / threshold/ above threshold/ all regions?
I am interested in all of them. I would say especially endurance since that’s where I tend to ride in a slower cadence naturally.
it’s not gonna change much of what I do but I’m just really curious.
seems to me riding with my Heart Rate 10bpm higher for ~10-12 hours a week would work it more? I dunno. just interested / curious.
One thing that we do know is that typically higher cadences tend to rely slightly more on the cardiovascular system (hence the higher HR) than lower cadences which often utilize the musculoskeletal system a bit more.
Obviously, any common cadence will utilize both systems, but there are slight shifts as to how each system is utilized and stressed depending on any particular cadence. The differences are likely rather small, but still exist.
This. As I see it, if you’re doing FTP intervals, varying your cadence is a good way of trying to avoid wearing a rut in your RPE at Threshold.
It’s also a good way to (probably) see that “Threshold HR” isn’t really a thing. If you do 3 × 15 at 60rpm, 75rpm and 90rpm, no matter what order you choose, it’s very unlikely you’ll get the same HR each time.
Maybe we should prioritize maintaining LTHR instead of holding FTP. Can you hold your FTP for the same length of time at 140 rpm as you can hold it at your “comfortable” cadence? Your heart is doing work also, and that should factor into the equation.
Prediction: no
Why would you want to?
Higher rpm = less bio-mechanical efficiency
Its good to get comfortable producing power at different cadences. Make you more well rounded. Also makes intervals on the trainer a little more interesting
I wouldn’t want to. And, I wouldn’t be able to, because I would exceed my threshold HR, which you say isn’t really a thing.
Lower rpm = more wear and tear on the muscles and joints.
A lot of the responses here are thoughtful and insightful 
Research on the impact of cadence work (high and low) on cycling performance is underwhelming, both in terms of quantity and results. Hansen and colleagues conducted a systematic review in 2017 looking at adaptation to training at imposed cadences. They concluded that there is no strong evidence for cadence recommendations. However, it should be noted that making recommendations from this review was always going to be challenging due to the different methods used in the studies included.
I think it’s important to keep in mind that the primary determinant of physiological adaptations in cycling is work done, measured in Watts. When working at high intensities, where achieving the power target is challenging, manipulating cadence could be counterproductive (and very hard).
However, if you are working at relatively lower power targets, there is no harm in varying cadence, and potentially some benefit if it doesn’t distract from the purpose of the workout (e.g recovery).
As @lee82 mentioned, if your heart rate is consistently higher cycling at a higher cadence for a given power, it’s likely an indication that you are less efficient at this cadence. This makes sense given you don’t usually train at this cadence. If you were to continue to cycle at this cadence for the same power output, I would guess that your heart rate would decrease at this power as your efficiency improves.
Here are some reasons that are often cited for training at different cadences (with varying levels of evidence to support their effectiveness):
The question remains about whether these translate into meaningful differences in cycling performance. However, as long as they don’t interfere with the primary intention of the workout, I don’t think there is any harm in experimenting with different cadences.
I’m not trying to be nit-picky here, but I’m wondering if you mean work done, or time spent at certain power levels. Work is a measurement of energy transfer, and we often measure it in kJ, while power is the rate of energy transfer, and we usually measure it in Watts (which by definition, is Joules/second).
I think power makes more sense here. 
You could say “I burned 3,000 kJs during my workout today” which tells us how much work you did in total, or you could say “I did 3x20 @ FTP” which is much more specific about the type of stimulus you applied to your body.
I can’t say that I’ve had very many workouts in which I hit 5000kJ. In fact, a quick sort shows that I have a grand total of two, but only if I’m rounding to the nearest 100kJ, because my two highest ever show at 4981 kJ and 4962 kJ. 
I switched it to 3,000. 
I suspect it would drop as you become more used to staying in that cadence but I doubt it’s particularly important. Just commenting because I have vague memories of Kolie Moore suggesting a higher cadence (~110rpm) during VO2 efforts can have a beneficial effect on heart stroke volume.