Personally I would say that a SS makes a person fitter for two reasons:
– You pedal harder up hills thereby developing leg strength and promoting adaptation to slow twitch muscles
– You spin faster on the flats and therefore a person's avg HR rate will be higher for a given speed. Lance Armstrong avg cadence was around 10 – 15 rpm higher than other riders as pedalling a higher cadence vs slower cadence is proven to be more energy efficient.
Of course the points made above are generally true for "average riders". Pros will always use gears to mash up hills and have a £1000 worth of power meter attached to the bike to ensure they are completing the correct training workout dictated by their coach.
It is possible to train within HR zones on SS. Flat road circuit on the fixie with appropiate gearing for base training. HR zone used so that the level of exertion doesn't reach the lactate threshold zone.
Whilst intervals can be accomplished on a turbo trainer with variable resistance. I had a road bike with gears and was never really comfortable doing intervals on the road due to traffic and other hazards. So I use a SS MTB on a trainer.
The type of riding I enjoy is long distance stuff and 24hr solos. Whereby a lack of gears is a mental advantage as it is one less thing to think of and one less thing to break.
My regime is specific to using the SS to the extent, for example, of practising steep hill repeats without hitting my lactate threshold which results in pedalling up the same hill VERY slowly many times. This way I can train the leg muscles not to tire as much as if I were to blast up a hill. So my "granny gear" is simply pedalling slower though the level of exertion will still be more than if I were using gears and therefore more tiring except I try to overcome this by performing such hill repeats.
It is a tradeoff in the end, but one that I enjoy 🙂