I do believe doing all-treadmill for prolonged time can lead to slower results, mainly due to less training of the road impact. I’ve had this experience myself.
The reasons I think why I got slower when doing ALL treadmill for months:
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Reduced impact training from the soft treadmill vs the road. When you’re racing fast outdoors, you gotta have as much road impact readiness as possible, particular at racing speeds. TMs are almost invariably softer than road on impact, so even if you’re doing fast work on the TM, it’s not quite the same. I do find though that you don’t need a ton of outdoor miles to get the needed road impact adaptation, and the reduced TM impact can actually be helpful if used correctly to reduce strain and stress.
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Lack of true downhill training where impact really counts. Even treadmills that go downhill don’t go downhill enough to get your legs fully ready for all the downhills you’ll hit on the road.
I’ve found that if I’m fully road impact trained, since the TM is softer, I can run faster than I normally would had I just been training on the exact same TM.
I also found for myself that just running faster or increasing the incline on the TM doesn’t solve all the road impact issues - I still have to run on the road a bunch to acclimate, even if I’ve been killing myself on the TM. Luckily, it doesn’t seem to take much road impact training for shorter distance races (I haven’t tried it for the marathon, though.)
Note however that I’ve found that for me, where at my age I get arthritis-related bone pain when I run too many miles, the TM can be super helpful and I mix it in whenever possible to reduce joint impact so I dont get deep bone bruising that can take months to heal. I’ll go to mostly if not all road near race day, but I’ll go back to significant TM miles afterwards so I can let any bone soreness heal up.
I think that in the real world the faster we go there is an acceleration and deceleration phase in each stride, whereas the treadmill is going at a fixed speed relative to your body. It is why true “sprinting” on a treadmill is weird, because running fast is effectively connected successive bounds.
Also the treadmill does not have variation in terrain, so your head and eye positions don’t change (head position changes have massive impact on real running before your head motion changes your balance), plus you don’t really have downhill running (even if you have a treadmill that goes downhill, you won’t get 5-10 percent downhills like you may get in real life.
I hear you, but honestly that’s not what my body seems to lack when going from months of TM to road. It’s not like my legs aren’t accelerating fast enough or having problems varying pace. If anything, they’re doing an extremely good job of that.
The clear missing factor I’ve noticed is that tendon-strength needed. After that first real-pace run on the roads from months of TM, my leg tendons are sore - it’s not the muscles, it’s the fibrous tendons. If I subsequently remove the impact by water running or even elliptical, I can go super hard on those same legs, but it’s hard to even run slowly due that tendon pain on those legs. At least for me, that’s the only real limiter I’ve noticed when going back to road.
I’ve had no weird proprioception or other issues at all. Might be different for other folks, but I’ve been surprised how I had pretty much zero other issues on the transition - only the (big) one of road impact on tendons.
I actually really like doing speedwork on a TM since it feels less injurious to my arthritic ankles, but again, you can have too much of a good thing, and as race day approaches I move it all to the road.