Your lactate number belongs to the lab, not to you

Threshold values are a product of the testing protocol, which is why two labs can hand the same runner different training zones.

Two athletes on the same squad come back from testing with threshold paces forty seconds a mile apart, and everyone assumes one of them is fitter. Sometimes. Often the difference is that they were tested on different step lengths.

Here is the mechanism, and it is not complicated. A lactate curve is built by running a series of stages and taking a blood sample at the end of each one. Blood lactate lags the muscle. It takes time to appear, and the concentration you measure is the balance between what is being produced and what is being cleared and burned elsewhere. Run three-minute stages and you sample before that balance has settled, so the curve sits low and the threshold looks fast. Run six-minute stages and the same athlete produces a curve shifted left, and suddenly the prescribed pace is slower. Nothing about the runner changed between Tuesday and Thursday.

Then there is the definition problem. Some labs fix a threshold at a round concentration. Others look for the point where the curve bends, which is a judgement made by eye or by a fitted line, and different fitting methods put the bend in different places. Some use finger blood, some use an earlobe, and the two do not always agree. Add the fact that what the athlete ate the day before shifts the whole curve, and you have a number with more inputs than the athlete's fitness.

I am not saying testing is worthless. I am saying the number measures a system, and the system includes the protocol. Treated as a repeated measure, same lab, same stages, same time of year, same breakfast, it is genuinely useful, because the noise is roughly constant and the change is real. Treated as an absolute value you can carry to a new coach in another country, it is close to meaningless.

The damage is practical. Athletes chase a pace that was never theirs, hold it because a spreadsheet says the session is a threshold session, and turn a controlled effort into a hard one week after week. The tell is simple and old fashioned. If your threshold reps feel like racing, the number is wrong, whatever the lab charged for it.

Trust the trend. Distrust the decimal place.