Playing successive octaves can tire the arm/wrist. This is especially true if an octave reachis nearing your 'span of the hand' limit. 2 things: Even with the extended hand, it is important to feel 'loose' /supple in the wrist/arm/shoulder and even the neck, somewhat.Practice at a speed that allows you to feel the movement this way. Otherwise, you are drilling in the wrong approach. Play somewhat quietly, mp, to reduce the possibility of tension. AND make sure that on the successive octaves that the fingering is alternatedbetween the 4th (good for flats) and 5th finger as much as possible. This last sentence is very important if you are not already doing it.Let us know how it goes.