There’s several factors at play here. An athletic heart is not only more efficient down to the conductance of the cardiac tissue, but it also has a larger stroke volume. With those 120 bpm each beat could be pumping 40cc of blood while in the other heart each beat might only be able to pump 30cc. This is because an athletic heart is able to more fully contract to squeeze out all of the internal volume. Think of the conductance of the heart as a snowy hillside. The first time you take a sled down the hill the snow hasn’t been compacted to make a path. The more often you take your sled down that path, the more compacted it gets and the faster you and your sled will go down the hill the next time. Plus I haven’t even mentioned blood pressure decreasing in an athlete due angiogenesis and dilation of already present veins and arteries.

So to summarize, it’s not just bpm that need to be accounted for here. You also have to consider:

• conductance lowering the cardiac impulse threshold
• stroke volume
• blood pressure
• levels of contraction

Energy use increases with bpm, change in pressure (systolic - diastolic) and the stroke volume (amount of blood pumped per beat).

Note that there is also an inverse relationship between stroke volume and bpm because the faster the heart beats, the less time for blood to return to the heart for the next beat.

That said, heart “strength” is more about reserve capacity (ie ability to ramp up when necessary) than energy efficiency. It’s like comparing a Ferrari to a Corolla: at 100 mph the former can still increase its power whereas the latter is getting near its limit.

So if the Ferrari has a “car attack” and suddenly loses 50% of its max speed then it can still keep up on the highway, the Corolla maybe not. That’s more important than which one is more energy efficient.