By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.
View post (Tensing Up)
View thread
Maybe its just because you are trying to play faster than you are comfortable with. Try working up to the speed and warm up before. That way you should find it less straining on your arm.