eurodonkey wrote:
Most of the year, I live within 200m of Wimbledon Common (London, UK), the spiritual/ancestral home of XC running, and run on grass/dirt/golf-courses every day.
For about 3 weeks every year (as now), I am on vacation in Croatia, where the only runnable surfaces close to the coast are man-made tarmac or concrete.
I have learned over the last 20 years that I have to halve my mileage when on a hard surface. 70->35, no kidding.
Roads are important for race-simulation (tempo, kilometre reps etc), and are great for hill reps where you have no impact but a predictable (and maybe well-lit) surface to sprint on. Otherwise, hard to find a runner in favour of hard surfaces.
Same for me. I'm a heavy, tall runner prone to shin splints and every time I run on the roads I feel it the day after. I agree that hill sprints (8-10s) are great on roads. If you run all out, you need to do this on a road. However, even for longer hill sprints, 20-30 sec and longer I already switch to softer surface as these are more for building strength than pure power.
For k repeats and tempos, yes it's impossible to run them on most of the trails we have access too. Not everyone has the perfect flat dirt roads of Boulder, Kenya or the Clermont clay loop for example. Once you get to a certain speed, say 5:20/mile and faster, it is just not safe to do this on many forest trails.
But switching to asphalt? That would be even more impact than an easy run on asphalt, which already significantly damages the lower legs muscles and joints. I'd say do the tempos and k reps on the track, not road. Salazar even does 400s on the track often in 50-50 fashion, where he uses the grass for the straights, puts a cone in the curve and his runners only run the curve on the track.
I'd say soft surface (grass, dirt trails, forest) > track > asphalt > concrete, but the difference between asphalt and concrete doesn't really matter as both are terrible.
And yes, some (many?) runners can run asphalt with no problem, do same mileage and don't feel any fatigue in their legs. For them, it doesn't matter where they run. Then there are some, that twist ankles a lot - I've seen them, even on the easiest trails possible something happens to them. But for the majority imo, all training should be done on trails/soft surface and only the faster stuff on track, with the one exception being marathon runners where the advantages just outweigh the risks.