All over the place I have seen runners, particularly from the US, go out on these 6-8km (4-5 miles) "tempos" at ~15km - half marathon pace. I fail to understand why this is an efficient workout physiologically.
What are the purpose of these tempos? People say "its to filter lactate more effectively". The majority of lactate is filtered by the lactate shuttle (lactate from glycolysis in type IIb fibres ------> mitochondria in type 1 fibres where it is converted back to pyruvate and undergoes oxidative phosphorylation). Your body adapts to this by increasing the concentration of lactate dehydrogenase enzymes, lactate transporters on cell membranes, lactate transporters on the mitochondrial membrane etc blah blah. This in turn will allow you to run at a sustained faster pace for longer periods as you can filter more lactate.
Cool. Makes sense.
But aren't there better ways of getting these adaptions? Lets say a runner has a "threshold" is 3.30/km or ~5.39 per mile. You go do a 4 mile run at this pace. Workout complete. OR you could do 7 x mile on the track building up to paces FASTER than the tempo. For example, starting at 5.39/mile and working down to 5.20 with 1-1:30 rest or so. In this workout, you are getting much more mileage done, producing and filtering much more lactate, working at a more specific pace (if 10k or down runner) AND working at a higher cardiac output for a longer period (intervals are also the best way to increase left ventricular stroke volume and therefore increase VO2 max). It also isn't more tiring (from experience of myself and others).
Essentially all I am saying is threshold track workouts >>> tempos
Please enlighten me if you disagree, I actually want to hear other peoples opinions on my logic hence the post..