Originally Posted by bruce19
I've been told that cyclists have two kinds of "energy" that we use. The first is stored in muscles and the second in body fat. When the energy in muscles is depleted, the only way to re-energize is with rest. When the body fat energy is depleted it can be renewed by eating and drinking. Mashing gears uses muscle energy and spinning uses body fat energy. Therefore, if you spin rather than mash gears, one can ride longer by continuing to stoke the body fat stores.
Yeah, kinda sort of. There's actually a continuous range between burning 100% fat to burning 100% glycogen (carbs) and it's related to your max muscle-strength and LT-lactate threshold. LT is actually a muscular-force point, not heart-rate, but it can be correlated if you keep certain things like speed & cadence constant. The LT point is where your muscles go from aerobic glycolysis go anaerobic fermentation for energy. The differences in efficiency is extreme, aerobic generates 38-ATP while anaerobic makes only 2-ATP. The LT point is usually 50-75% of max-exertion 1-rep force.
Here's what happens when you're riding, if you're cruising along at 10mph, pushing on the pedals with something like just 20-lbs of force, you're gonna be far away from LT and burning mostly fats. You also won't be feeling any strain on your muscles or knees at all.
Let's say you increase your speed to 20mph by pushing 90% of your maximum force at 80rpms. You're over your LT, your muscles will ache and get sore and you won't be able to keep it up for long. You'll also be burning 100% glycogen as well and will bonk in about 2-hours.
Then you shift down two or three gears at the same speed, 20mph, but spin 100rpms instead. Working through the physics equation of POWER = (force*distance)/time, you'll find that you can generate the exact same power as before to go 20mph, but you'll only need to be pushing on the pedals at 68% of your maximum strength instead. This amount of pedal-force is well below your max and slightly below your LT, so the muscles will be much more efficient and burn a mix of fats & carbs. Probably 30% fat & 70% carbs depending upon your fitness.
So overall, using two to three gears lower and spinning at 100rpms at the same 20mph speed will lower the stress on your knees by 25%, increase your muscle's efficiency by pushing them at a lower-percentage of their max, and you'll be able to ride that same speed for much longer. Endurance will also increase because you're burning more fat and you'll be able to ride 3 hours before bonking instead of just two.
Note that riding at higher RPMs at the same speed tends to use more oxygen. But that's not of a big concern because both of those are below your VO2-max. It's the muscle's LT that you're really worried about and keeping them as far below LT as possible for any given speed will give you much more efficiency and lower muscle-fatigue, allowing you to go further at that speed.