Thankyou RolandG for your thoughtful and intelligent response.
In reference to the above quote: Yes, mammals stretch, but if faced with sudden danger no mammal stretches and does warm-ups before fleeing. If they did they would be dead. So evolution favours mammals (and humans are mammals) who have the ability to go from rest to full exertion without a warm-up.
In reference to your whales-can-hold-their-breath-for-a-long-time analogy: How can I refute such brilliance? But I'll try. If humans had taken to the water early in their development then breath-holding for lengthy periods would be a survival advantage for humans too. But humans stayed on land so what is a survival advantage for whales is not an advantage for humans.
Again, thanks for your comments. Your insights are always welcome.