Is there any reason why we one should dorsiflex.. while doing leg curl and leg extension? eventhough the working muscle don't even cross that join?
I can't think of one* - doing so could be a distraction. But exercising the tibialis anterior via dosrsiflexion
is something that should be (but rarely is) done
*(I'll assume by dorsiflex that you mean actively flexing and pointing the foot. If you mean holding the foot in a flexed position, I doubt if it makes any difference. Perhaps the kinesiologists out there have better info)