I saw an interesting study that found in ADHD individuals, aerobic exercise immediately before a test improved concentration to beyond normal levels, while in non-ADHD people it made them perform worse. I will see if I can find it.
https://pdfs.semanticscholar.org/7585/7b94628b425919efde51db...
> The results of this study also show that the GC group has a performance that is 42% better than the GC-EF group. These data seem indicate that after an intense physical exercise, people without ADHD show impairment in attention performance. However, this hypothesis requires further studies, which include the assessment of the duration of this effect.