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The question "which is better" isn't quite right - it's like asking what's better: a barbell or dumbbells. It depends on your goals and circumstances. Over 13 years of working from home, I've tried both and figured out that ideally you'd want to switch between them.
There's definitely more instability outside - uneven ground, wind, elevation changes. It makes your stabilizer muscles work harder, and yeah, you do engage more muscles overall. But here's the thing: that instability is a double-edged sword. Your knees and joints take more impact stress, especially on hard asphalt. A treadmill really cuts down stress on the joints by like 40 percent, I'd say - that's objectively good for recovery or if you already have issues. Plus the treadmill lets you control pace and incline, which matters for interval training.
Right now I'm mixing it up: on workdays I use the treadmill - it's quick, doesn't depend on weather, can work out before work. On weekends I get outside when I've got time and the weather's decent. If your climate lets you run year-round - sure, outside would be more varied. But realistically, most people will actually stick with it more on a treadmill because it doesn't depend on rain and frost.
Do you actually have a goal - just staying in shape or training for something specific?
Because these two options tackle different things, and there's no universal answer! When you run outside, you're working against natural conditions - wind, terrain, uneven surfaces - all of that engages stabilizer muscles that don't work as hard on a treadmill. Plus there's the psychological factor - new routes, scenery, fresh air. That's a serious plus for your health, and not just physical health.
But your colleague from the clinic is right about the joints! If you already have back or knee problems, a treadmill with good shock absorption is honestly a lifesaver. You can control the load there, adjust the incline, and there are no harsh impacts from pavement. Plus the treadmill wins on practicality - 20 minutes in any weather, no need to go anywhere.
My advice - mix it up! Three times outside when the weather's decent and you've got time, and when you're in a rush or it's pouring outside - hit the treadmill. Both types of running are good, just for different situations. The main thing is to run regularly instead of overthinking which way is "more correct"!
The main thing is to choose something you'll do regularly, because any running is better than a perfect plan you don't follow through on.
I've noticed that in summer people love running in the park, but in fall and winter a treadmill saves the day - you don't have to freeze and you can throw on a series. If your joints are bothering you, a treadmill is definitely softer, but running outside engages your stabilizer muscles more naturally, which is also important.
I think the main difference is that when you're running outside, you're constantly adapting to the terrain, wind, and uneven surfaces - that puts way more strain on your stabilizer muscles.
On a treadmill, the running mechanics are simpler, but you can focus on intensity and control your load, which is helpful if you're recovering from an injury or keeping a close eye on your heart rate.
Ideally, you'd alternate depending on what you're trying to achieve at any given time.
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