Thursday, August 19

More science waffle: Blood Lactate

Blood Lactate
Lactate production especially when expressed as a function of power output is the single most important physiological determinant of cycling performance as lactate production is an individual response (McLellan &Jacobs, 1989; Stegman & Kinderman, 1982). Both lactate threshold (1mmol rise in blood lactate above baseline) and onset of blood lactate accumulation (when blood lactate reaches 4mmol) have been shown to be valid predictors of cycling economy in a range of events (Faria et al., 2005). A relationship has been shown to exist between average power output during a 90 minute time trial and the lactate threshold, as calculated by the D-max method (Bentley, 2001). This study also showed a correlation between the average power during a 90 minute time trial and peak aerobic power output. This relationship between the lactate threshold and maximal aerobic power output over longer durations leads us to believe that both peak and sub-maximal measures of blood lactate are possible predictors of cycling performance.

These lactate thresholds becomes ever more important during individual time trials where athletes are working at or just above the lactate threshold (Mujika & Padilla, 2001). In ultra endurance races extended efforts (55-60% of VO2max) are performed at or above the lactate threshold by solo riders (Laursen, 1999). The ability to predict performance over extended durations at this intensity can be of great benefit to a cyclist. The lactate threshold is the level at which a cyclist is able too hold a given intensity for ‘a very long time’ before they start to feel the effects of fatigue. A solo cyclist will aim to remain at this level for as much as possible allowing them to exercise predominantly aerobically utilising both carbohydrate and fat stores to produce ATP. In effect is the steady state that most of a road racing is performed (Muijka & Padilla, 2001). Unlike road racing this effort must be maintained for much longer periods. The greater the percentage of heart rate max or VO2max that the lactate threshold occurs at the more economical a cyclist is said to be. By having a lactate threshold that occurs later a cyclist will be able to produce more mechanical power with less of a metabolic cost. An understanding of the lactate threshold response can allow a cap to be placed on the intensity that they may cycle at in order to complete these events solo while lowering the chance of unnecessary fatigue or glycogen depletion over the duration of the event.

During high intensity ultra endurance the riders are able to recover between bouts of cycling. This results in cyclists racing at or above OBLA (Laursen P., 1999). OBLA is the level at which a cyclist has started to work nearly entirely anaerobically. They are not able to supply enough oxygen to return to aerobic glycolysis and are now producing lactate at a much higher rate. This anaerobic threshold is the level above which a cyclist can only maintain a maximal effort for very short periods of time. A cyclist can only maintain this effort for very short periods of time. If they are able to raise the point at which OBLA occurs it stands to reason that they would be able to prolong the effects of fatigue and hold this intensity for longer. With a cyclist understanding the intensity they are capable of sustaining for these relatively short durations, and its impact on recovery, it is possible to work near maximally to gain the greatest possible result. Unlike solo cyclists a boundary can be placed at a much higher intensity (75-85% VO2max) and result in much higher power outputs and speeds.

For cycling describing the lactate threshold as a heart-rate can lead to problems. As heart rate is a response to an input, a hill or sprint, it lags behind. Consequently measuring the changes in power output at these different blood lactate responses may lead to a much better determinant of cycling performance (Muijka & Padilla, 2001). By giving a cyclist a measure of their lactate threshold expressed in Watts an athlete can pace efforts by this measure. Depending on the demands of an event or session they can attempt to stay as close or far away from their lactate threshold so as to maximise performance.

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