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If you go to a 40 sec work period, 8-10 would be enough. Might need to drop the resistance a bit.
If you keep it at 20 seconds you could probably get to about 12 intervals. Keep the rest period at or higher than the work period. If you get to 12 then increase the resistance but keep the RPMs steady.How many intervals would you increase to? I was thinking of doing that, and increasing the resistance dial. I'd scale back the number of intervals if I did longer sprints and shorter rest. Maybe work up to 40-second sprints and 20 seconds rest for sets. That sounds... brutal.
If you go to a 40 sec work period, 8-10 would be enough. Might need to drop the resistance a bit.