MLSS and training in swimmers

A quick thought for the morning, probably to be followed by something more involved later.  I've been trying to find a graph of MLSS (maximal lactate steady state) in different distance athletes, and in my search am running into articles that contain interesting tidbits.

Today's article looks at improving 500m swim times, which are around 2 minutes long, and in cycling this is a highly glycolytic anaerobic capacity interval.  So it's perhaps no surprise that this is figure 2:

Over the course of two months the athletes' times improved and the lactate concentration at MLSS did as well.  Despite this test being at anaerobic capacity, it also shows how much of an aerobic component there is at this exercise level.  Why does it show this?  Let's use Table 1 to guide us.

This looks like a traditional cycling plan to peak for early spring.  Let's note the two directions that MLSS moves in Figure 2.  During the lower intensity training the speed doesn't significantly improve, but the MLSS concentration goes up.  As training involves more time at threshold, the concentration doesn't go up again, but the athlete gets faster and gets to MLSS some seconds earlier.

To explain the December-January increase in MLSS, let's consider the effects of training slightly below and above MLSS, which in cycling we know as tempo and threshold, though due to the imprecision in swimming metrics, this might even be "sweet spot" and "low vo2max".  While these intensities stress the aerobic engine, they also burn a good deal of glucose and glycogen, engaging glycolytic muscles and increasing the rate at which they are capable of burning through their preferred fuel.  The increase in MLSS shows the improvement of the "glycolytic engine".

The January-February improvement in time and not another rise in MLSS could be explained by several things, but my guess is a larger increase of glycolytic enzymes and the building of type IIa/medium twitch fibers from a rest period in the training cycle.  I find myself wanting to see a 500m test performed in November, since low intensity training has different effects on MLSS than we see here.  More on that later, though.

This study encompassed both short and long distance swimmers.  No real distance specialties were mentioned except to group the two into different phenotypes, sprinters and endurance swimmers.

My first thought when I saw this graph: how cool is that?!  There's no explanation for it in the paper since its primary purpose was to investigate an inexpensive way to monitor training progress for swimmers.  I proffer two reasons behind why there should be different rates of improvement: lower intensities (November through January) lead to larger gains in endurance athletes who have greater type I fiber density, while higher intensities lead to larger gains in type II dense athletes.  My second and less likely explanation is that it takes the sprinters more time to see gains from their low intensity training, though you really can't be sure until it's tested.

The other thing I would like to see is Figure 2 separated by sprinters and endurance swimmers.  Hopefully I'll run across something like that soon.