Overtraining - lactate throws a curve at overtraining researchers

Training is a balance of work and recovery. When the balance is perfect, peaks in performance can be reached. If the balance shifts too far toward the recovery side, an athlete fails to provide his/her body with enough training stimulus to achieve the best-possible performances. If the balance shifts too much toward work, overtraining occurs, and the athlete's performances decline, in spite of the prodigious quantities of work he/she has done.
Unfortunately, finding the right balance is seldom easy, and many athletes have had their training plans wrecked by overtraining (or even undertraining, ie, overrecovering). To help serious athletes with this problem, exercise scientists have attempted to uncover the definitive physiological variables which indicate that an athlete is entering the overtrained state. Resting heart rate, hormone levels, lactate production, psychological profiles, and muscle enzymes have all been looked at closely, and this pursuit of the 'markers of overtraining' is of more than esoteric interest. In theory, once the key variables have been identified, athletes can be monitored closely, and perturbations of key variables toward their 'negative domains' can act as warning signals, telling athletes that their training should be reduced.
For years, scientists have recognised that in overtrained endurance athletes, there is often a 'right shift of the lactate curve'. The lactate curve is simply a plot of the relationship between exercise intensity and blood-lactate levels, with intensity on the 'x' axis and lactate on the 'y'. A 'right shift' of this curve basically means that the amount of lactate in the blood at a given, high-quality intensity of exercise is diminished. This might seem to be a good indicator of overtraining, then, except for one thing: when non-overtrained endurance athletes improve their overall fitness and performances, the lactate curve is often driven to the right, too!

What they did in Montreal
In an attempt to differentiate beneficial changes in blood-lactate dynamics from the alterations associated with overtraining, Luc Leger and his colleagues at the Department of Kinesiology at the University of Montreal decided to divide the blood-lactate levels of hard-training athletes by their ratings of perceived exertion (RPE) during intense exercise, and they also monitored lactate levels as a percentage of peak blood-lactate concentrations ('Blood lactate response to overtraining in male endurance athletes,' Eur J Appl Physiol, Vol. 84(1-2), pp. 107-114, January-February 2001). You can see what the researchers were thinking about: in an athlete who has gotten fitter, blood lactate will have diminished at a particular speed, the effort should feel easier, and thus RPE will be lower. However, in an athlete who has overtrained, blood lactate will be diminished but the pace should feel significantly harder, causing RPE to climb. Thus the ratio of lactate/RPE should be significantly lower in the overtrained athlete, compared with the fitter individual, and the two types of 'right drive' can be teased apart.
In this very interesting study, 10 experienced endurance athletes increased their usual amount of training by 100% within a four-week time period. An incremental test and a time trial were performed before and after this period of extremely strenuous training, as well as after two weeks of recovery. When both physiological and psychological criteria were utilised, seven athletes were found to be overtrained after the four-week barrage of workouts.
As expected, there was a right shift of the lactate curve, for both overtrained and non-overtrained athletes. As mentioned, this meant there was less blood lactate around at high-quality speeds. However, RPE did not change significantly, and thus the lactate/RPE ratio did not permit differentiation between well-trained and overtrained athletes (the lactate/RPE ratio decreased similarly through the six-week period for all athletes). Thus, lactate/RPE seems to provide little information about overtraining.
The key factor which was different between the groups was peak blood-lactate concentration, which was greater in the non-overtrained athletes. Therefore, Leger and colleagues propose retaining peak lactate as a key marker of overtraining for endurance athletes. As training progresses, peak lactate levels should actually increase, because athletes should become better able to attain very high power outputs during exercise. If peak lactate begins to fall during training, it is a key sign that muscles are beginning to lose their ability to generate power - and perhaps that the overtrained state is very close at hand.