I had a personal trainer for a month a few years back and he pointed this out. I was running with a heart rate monitor (belt around chest type). He pointed out that my heart rate fell 10 bpm when I switched from watching readouts on the treadmill to watching whatever was on the many TVs. Same speed and incline, but if I focused on something other than those numbers, I relaxed and it became easier.

It's interesting to see a real test of this. The clever bit in here was asking the cyclists to count red circles on the screen occasionally, ensuring the TV had a large share of their conscious attention.