Mike,
While going from 3x3x50 to 3x3x80 is a large increase in volume for that one session I feel that without knowing how the rest of the week is manipulated volume wise it could work.
You're right but I much prefer to keep the training loads more evenly distributed over the micro rather than overloading one day and then having to back off on other days. There are rare circumstances that I'll use this format but I really don't like to do it as it seems that adaptation tends to be better over the long haul if you don't put athletes too deep in to a hole.
Keep in mind over the course of the last few mesocycles they have been building volume in say accel, Max V, and Perhaps int. tempo and now one of those sessions may be dropped or the volume may be dropped within one of those session which would allow the increase in volume in the Short Speed Endurance sesssion.
I actually don't like to increase volumes of those values too much over time. While there obviously might be some increase in session volume (but likely not micro volume due to factors you mentioned) once speed / special endurance is introduced, I actually don't like to increase volumes all that much since the very nature of the short-to-long training progression inevitably increases intensity throughout the course of the year anyhow so trying to increase both concurrently doesn't seem to work that well (at least in my training setups).
What exactly do you mean your athletes cant get away with increasing volume and intensity? Do you notice a drop off in performance, are they unable to complete the session…?..?
They just don't adapt too well. They tend to feel like crap in the 2-3 days following.
ELITETRACK Founder