Second, designing a performance recumbent requires a very wide gearing range. The mid-drive system has 35 speeds, so there is a good compromise between range and step. Mid-drive allows fully indexed shifting over all 35 combinations of gearing, too. There is no chance of chain jam caused by 'illegal' front and rear combos since the each derailleurs controls a separate chain. And if you add a windshield or a full body, it can be narrower than the windshield for a similar USS bike.
Some designers believe that a solid seat back is best for power transfer.
Some use a breathable mesh back rest with a solid foam saddle. We have found an all mesh design to be the most practical and comfortable for extended riding.