After harder than normal races, my glutes and eyes get tired. Eyes because I don't blink as much when things are intense. Glutes because I don't TT much on my own so although I'm low much of the time, I'm not working that hard.

+1 on the longer stem thing. The proper fit for back pain may not be a shorter, taller one. I have burst and bulging disks in my back, and I feel super comfortable on the drops.

My back used to get tired but now it doesn't. My frame I got this year puts me in a ~4 cm longer position from my past ones, and this has made a huge difference. (Past = 53.5 TT + 13 cm stem = 66.5 cm. Now 56.5 cm + 12 cm + 2 cm from steeper seat tube angle = 70.5 cm, same bars, same saddle, same post. I'm contemplating putting the 13 back on.)

cdr