Tullio Campagnolo's derailleur completely revolutionized the modern bicycle, giving us up to 22 gears to play with in an effort to find that ideal combination of cadence and resistance to maximize our power output over a range of terrain. But when it comes to training, does training with specific cycling cadences have their benefit?
In the early days of cycling, all road bicycles were single speeds or, at best, required completely taking out and putting the rear wheel back in the other direction with a different rear sprocket. Contrast that with the rise of the rear derailleur, and also the ever-increasing number of gears on the rear wheel. In the mid-1980s, my first high-end racing bike had a 6-speed freewheel for 12 total gears. And now Campy has found a way to jam 11 gears into the rear cassette.
So nowadays and especially with STI/Ergopower/DoubleTap, there's really no excuse for being in the wrong gear, but just what does being in the right gear entail? The right gear for cyclists typically means the highest possible power output in whatever terrain from our finite pool of energy, and this does not always mean the biggest or the smallest gear, but generally something in between.
The Force-Velocity Relationship
Mainly, this "right gear" comes down to our muscles' force-velocity relationship. Namely, the amount of force that our muscles can exert is greatest with an isometric contraction (where the muscle doesn't change length—think of pushing against a rock wall with your arms). As your muscles contract faster, the amount of absolute force that the muscle can generate decreases exponentially.
Now remembering that power output is force times speed of movement (i.e., an isometric contraction, where you have maximal force but zero speed and hence zero power output), what you get is an inverted-U shaped relationship between cadence and power output. So getting the highest power output does not always mean slamming it into the 53x11, because if you can't get the gear turned over at a decent cadence, you won't be generating much power. Similarly, we've all experienced "spinning" out a gear where your cadence may be greater than 100 rpm but your power output is very low. End result, at any power output, you need to pick the right gear at the right cadence in order to maximize power output.
Un-Optimizing Your Training
The above is all well and good while racing, but what the force-velocity and power-cadence relationship means is that, even at a constant power output, we can vary the stress on our body by varying our gearing and therefore cadence. A simple example is doing intervals over the same course in either a 53x12, 17, or 21 gear. Each gear choice will place more or less stress on your muscles, which can be seen in the torque profile of your power meter.
So this raises the question—is it beneficial to do intervals at possibly a lower cadence/bigger gear than normal to enhance muscular adaptations? Or if your goal is a flat time trial, does it make sense to train with hill intervals, where the cadence might be lower and the muscular stress higher? Or if you're targeting a hill climb championships, do you need to find hills for training or can you make do with flat intervals? Just how much specificity does your body require?