The limit for athletes is normally cardiovascular, commonly oxygen - VO2max the the measurement there. For less fit people it can be the cardio side, their heart just isn't up to it so their muscles fail and they lie on the ground twitching. Oxygen-deprived people pant and gasp.
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).
You can also reverse that and exercise at high altitude... less oxygen for everyone!