Is cycling or running better for fitness?

Neither is better. Both build the same kind of fitness, and the best one is whichever you'll keep doing. If you want a tiebreaker: running gives you more in less time, and cycling is easier on your body so you can do more of it.

Where running wins

• Time. You get a harder workout per minute because your whole body weight is moving. A 30 minute run is a decent session, and it takes a pair of shoes and a door.
• Bones. The impact of running strengthens bone. Cycling does very little for it.
• Simplicity. No bike to maintain, no helmet, no traffic.

Where cycling wins

• Joints. There's no pounding, so it's much friendlier if you're heavier, coming back from an injury or prone to shin splints.
• Volume. You can ride for two hours with less wear on your body, which lets you build a bigger aerobic base.
• Recovery. An easy spin the day after a hard workout is very doable. An easy run can still leave you sore.
• Leg strength. Hills on a bike load the quads in a way that running doesn't, though not as much as lifting does.

Which one should you pick

My take is to choose based on your situation, not on which is better in theory:

1. Short on time: run.
2. Have knee, hip or shin trouble from running: ride.
3. Want to train for a specific event: do the thing the event is.
4. Like both: do both. Cross-training is a good way to build fitness while giving your running muscles a break.

League of Fitness takes both. Runs and rides sync from Apple Health and are scored by heart rate zones, so a hard run and a steady ride each count toward your daily match, and mixing them as described above doesn't leave you with half a record.

The fitness carries over partly but not fully. Cyclists who start running are often limited by their legs not being used to the impact, and runners who start cycling find their quads give out before their lungs do. Give yourself a few weeks to adapt.

What about calories

People ask this one a lot, and the difference is smaller than you'd expect. It depends on your speed, weight and effort, so I wouldn't pick one on that basis.

If you have pain that keeps coming back in either sport, see a doctor or physical therapist instead of training through it.