For me, mental growth often comes when practicing with lower-ranked aikidoists. I learn to break down the components of a technique so that I can teach it to them part by part and as a whole, and in so doing, I refine my own technique. It’s all in the details.
Physical growth often comes when practicing with higher-ranked aikidoists. They keep me honest and show me where my technique needs improvement by not moving when I tense up when trying to move them, blending with me when I relax and flow.
Physical and mental growth are parts of a whole, and happen when I pay attention in practicing, regardless of my partner’s rank. I find that I notice one part over the other at times.