The evidence that endurance training increasing myoglobin is shaky at best. Several studies have found that elite endurance athletes actually have lower myoglobin levels than couch potatoes. Other studies have found the opposite; either way, it is irrelevant, because all the studies report variances of less than a factor of 2. In our case, we need to increase our myoglobin by a factor of at least 20 times, so endurance training, even if it did increase myoglobin by a factor of 2, would be hardly noticeable for O2 storage (even if it helps for O2 diffusion during exercise).
Concerning the question about whether myoglobin in one muscle can help provide O2 to the rest of the body, it is a question that I have wrestled with for a long time, and the answer is complicated.
Myoglobin has a high affinity for O2, so O2 will always flow from the blood to the myoglobin; the myoglobin will never give O2 back to the blood. However, given that myoglobin stores O2, it can provide a muscle with fuel. All muscles are constantly consuming ATP, whether they are being contracted or not.
So, the muscle must constantly burn fuel, either from ATP/CP, from myoglobin O2, or from blood O2. The more myoglobin the muscle has, the more of a 'local' O2 supply it has. Under certain specialized conditions, this 'local' O2 supply in the myoglobin can reduce the rate at which the muscle draws O2 out of the blood, indirectly 'extending' the apnea by 'conserving' blood O2.
However, the conditions for this to happen are very specific. Normally every molecule of O2 stolen from the myoglobin is immediately replenished by the blood. O2 stolen from myglobin will not be immediately replenished from the blood if either 1) the blood is very deoxygenated, 2) blood flow is inhibited, or 3) intensity of contraction is extreme.
General, myoglobin desaturates when the O2 demand exceeds the O2 supply. If the O2 demand never exceeds the O2 supply, then the myoglobin will never desaturate, and it may as well not be there.
Now, performing the calculations on these cases is very complicated and I'm not confident I have done it properly, so I still can't answer the question for sure. I think at best the answer is 'maybe'.
The emperor penguin stores almost all its O2 in myoglobin. We also know that its deep dives last around 10-12 minutes, yet it can make shallow dives (i.e. closer to static apnea), for 22+ minutes. Perhaps this gives us a clue that myoglobin helps even in non-moving situations?
Eric Fattah
BC, Canada