Technique

[kikosp2001]

Can somebody please tell me why when I use strong fingers I get pains if I play octaves,and how to avoid that?

[general disarray]

What do you mean by "strong fingers?"  Do you mean a firmly arched hand with a rigid and unyielding hand position?  Sort of claw-like?

If so, the pain could be caused from this tension.  With any repeated action, such as octaves and chords, you have to find that millesecond after attacking the keys where you instantly let the tension dissolve, just before you move on to the next attack.  You can experiment with this in your octave playing by practicing very slowly, an octave at a time.  Prepare your hand, play the octave, and don't play the next one until you feel yourself letting go of the tension needed to play the previous octave.  Then move on to the next octave using the same technique.

Of course, in rapid successions of octaves you don't have time to release that tension between each attack.  Here, you should imitate Martha Argerich.  In videos, you can see how she channels tension (think "energy" more than "tension") into the octaves of the strong beats and almost allows a rebound of  that energy to carry her through the octaves on the weak beats.  Sort of like rhythmic bursts of energy that she rides like a wave.

Doing this, you grab that precious space of tension-release that helps you avoid pain or, even worse, injury. 

[tengstrand]

That's very well put GD, I usually tell students that the "energy-bouncing" is like knocking three times on a door, you just put the energy into the first knock and the two following comes as a rebound.

I might want to add something I found very useful, and that is that when you play octaves, still feel like you really are playing with the thumb and fifth...that the joints in the fingers are still kind of active, much like if it was single-note playing. I have found that to increase speed and above all, take away tension in the hands. This will be a little trickier, but maybe not impossible, if the hands are small.

[mcgillcomposer]

You need a firm hand position (do not let your fingers collapse, especially at the outer most joints of the fingers). It simply requires a single impulse, which sets off a chain reaction of momentum. With this energy, you basically 'shake out' the octave. The initial impulse is extremely important...that's where you get the energy that will carry you through the passage.