> So recruiting more muscle groups really doesn't help. What does is increasing oxygen intake, and this is where recumbent bikes come in. The laid back position opens the thorax and increases effective lung capacity. As well as reducing air resistance, except that that's a very subtle thing that mostly depends on the rules governing the sport in question (fairing on bike and kayak, for example, are variously restricted or banned in most relevant sports).

Recumbent riders generally have much lower peak power and ftp but higher sustained so VO2 differences wouldn't explain it. The main attribution I've seen is aero and marginally less muscle power used for motions that aren't related to pedalling.