Aska,
You could use the Mann-Whitney-Wilcoxon (MWW) within a day to assess whether the trained or untrained had different reactivity. As each subject is assessed on each day, it would be inappropriate to simply lump all of the observations into one MWW analysis (violates the independence assumption).
As far as interpretation is concerned, this Wiki is reasonable
https://en.wikipedia.org/wiki/Mann%E2%80%93Whitney_U_test
and you can also get a lot from the SAS documentation.
I generally use the two-sided t-approximation for the test for the MWW.
I've not used GLIMMIX in years, so I can't comment on it's appropriateness.
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