Running injuries: https://pubmed.ncbi.nlm.nih.gov/1439399/

The studies were sport specific, so I'm not sure where to find specifics on other activities too but I'm sure there's data out there for other sports.

> that says the lack of weight-bearing means cyclists don't build bone strength and are more prone to fractures

This is true, especially as you get older, and riding in harder gears don't really help either. There is more torque being applied, but its still a smooth, continuous force there's no impact traveling through to stimulate bone density. You need high magnitude, short duration forces (3x+ body weight) to stimulate that.

By far the most effective way is weightlifting/strength training, which IMO everyone should do, at least 1x or 2x/week. Even runners will benefit from strength training, it's necessary for injury prevention.

> Doesn't that also make it less efficient? Running seems to provide more exercise/hour. Again, maybe higher gears would solve that problem.

Running will get you a higher total energy expenditure, yes. A 45 minute run will be more strain than a 45 minute casual bike ride.

The big difference is total weekly volume. With running, because its such high impact, you have a limit on the weekly volume you can hit before you get an injury or start overtraining. About 5 to 10 hours/week for recreational runners.

A cyclist can sustain 15-20+ hours/week of training volume before you run into the same overtraining or overuse risks.

Cycling is technically less time-efficient on an hour by hour basis, but it does allow for significantly more total weekly volume and absolute calorie burn without overtraining stress or injury.

Gearing doesn't really change the formula at all. Gearing changes which system is being used cardio vs. muscular fatigue. If you shift into a harder gear, but your cadence drops proportionally, your power output remains identical. But mashing (a lower cadence) a harder gear changes from using your slow-twitch muscle fibers to fast-twitch fibers. You'll fatigue faster, and burn out well before you hit the time needed for good cardiovascular stimulus. Gearing isn't used to make the ride harder or easier, it's used to maintain your cadence in a specific power band (e.g., 85rpm @ 200W) like the transmission in a car.