Actually, since both the hemoglobin-O2 and myoglobin-O2 dissociation curves are functions of temperature, pH, and possible 2,3-DPG, it is possible, under near-impossible circumstances, for O2 to travel from myoglobin to blood.
If, for example, the muscle were extremely acidic, both the blood and myoglobin (in the muscle) would be less sticky to oxygen, and then if some totally alkaline, cold blood suddenly flowed into the muscle, the myoglobin could theoretically, for a tiny instant, give up its O2... since the myoglobin would still be very acidic/warm (less affinity for O2), and the blood would be very alkaline/cold (high O2 affinity), although this condition would only persist for a fraction of a second, before the acidity & temperature equalize.
Eric Fattah
BC, Canada