It depends on whether your wrist is stiff from non-use or from wrong use. Assuming the latter, I should recommend studies in wrist octaves, but you must watch your wrist while playing and rest at the slightest indication of its stiffening.
[6. ACTION OF THE ARM]
When Tremolo Proves Unduly Fatiguing
I cannot play tremolo in the left hand for any length of time without great fatigue. I have tried changing the position of the hand from high to low, the sidewise motion, and the quiet hand. What is the correct method, and may the difficulty be overcome by slow practice?
The tremolo cannot be practised slowly, nor with a stiff or quiet hand. The action must be distributed over the hand, wrist, underarm and, if necessary, the elbow. The shoulder forms the pivot whence a vibratory motion must proceed and engage all the points on the road to the fingers. The division of labour cannot be done consciously, but should better proceed from a feeling as if the whole arm was subjected to an electric current while engaged in playing a tremolo.
Play Chords With a Loose Arm
Should octave chords be played with rigid arms, the wrists and fingers thereby increasing the tone volume, or should the arms be loose? My teachers differ in their methods; so I turn to you for advice.
With few exceptions, dictated by certain characterizations, chords should always be played with a loose arm. Let the arm pull the hand above the keys and then let both fall heavily upon them, preparing the fingers for their appropriate notes while still in the air and not, as many do, after falling down. This mode of touch produces greater tone-volume, is least fatiguing, and will have no bad after-effects.
[7. STRETCHING]
Fatiguing the Hand by Stretching