Sorry OUZO, didn't mean any offence.
You realy can sustain 140 rpms for a few minutes or longer, but you'll need to adjust your gears accordingly or use rollers or a turbo with little resistance. The idea behind high cadence training is that you are overloading your neuromuscular system and forcing it to adapt. You will see new riders that can't pedal above 80 rpm, but with a few weeks training they can maintain 90 rpm. They have addapted their neuromuscluar systems. Perhaps you should give it a try and see what effect it has on your max cadence of 150 rpm.
Sprinters are always pushing their max cadence, hence why they can reach 300 rpm. While endurance riders who ride arround at 90 to 100 rpm with a few sprints can only reach 200 rpm. This is not really associated with fitness because even when low gears/no resistance is used, endurance and sprint riders still reach the same values. Its just that they can't coordinate their body fast enough to pedal quicker.
The values I use are values I have seen in a sport science lab with national and international level riders. Normaly I would give ranges, but I gave single values as examples I have seen.
Just like 'if you want to race faster, train at higher speeds', 'if you want to pedal faster, pedal faster in training'.
Some training sessions that you can use at a high cadence (140 would be suitable if your aiming to pedal like Armstrong, chose something more appropriate if you cant pedal too quickly)...
(1) 2+ mins at a continuous cadence followed by rest (i.e. intervals of cadence)
(2) ramps up to a high cadence (e.g. 140) and down to a low cadence (e.g. 90), repeated. Use short and long ramps in different sessions.
Ouzo, how do you manage when you sprint? Do you just change gear or do you pedal quicker (i.e. explosivly increase pedaling)?