There are some clues in their body language. they appear to be straining. And the pedal movement looks a little jerky, as if the load is changing dynamically

There are some clues in their body language. they appear to be straining. And the pedal movement looks a little jerky, as if the load is changing dynamically in relation to the effective flywheel/inertia of the system. Spinning a higher speeds with less torque is supposedly less tiring. So they might want to gear this down a bit and include a larger flywheel/inertia. Very impressive device tho. Maybe a hybrid approach with a solar panel for charging on the beach...