Define Fatigue Index

The fatigue index is a measure of anaerobic capacity, or endurance. It is the rate at which power declines in each individual athlete; in other words, it signifies the rate at which you tire when sprinting. The running-based anaerobic sprint test -- or RAST -- for fatigue index integrates statistics from a series of six sprints, using your maximum power -- the statistics from your best sprint -- your minimum power -- the statistics from your slowest sprint -- and the average of your sprints in order to arrive at a figure given in watts per second. The higher the fatigue index, the lower your ability to maintain power over a series of sprints. According to the website Peak Performance, a high fatigue number can help a coach identify athletes who need to concentrate on improving lactate tolerance.

The RAST is a sports-specific anaerobic test developed by the University of Wolverhampton, United Kingdom, to measure fatigue index simply and economically. In addition, it also allows coaches to appraise an athlete's peak power, average power and minimum power. For cyclists, a similar test called the WANT -- or Wingate Anaerobic 30-cycle Test -- can be used, but it is more complicated to administer and requires a computer and a cycle ergometer.

The athlete performs six 35-meter sprints -- each representing maximal effort -- with exactly 10 seconds allowed between each sprint for turnaround. Two timers with stopwatches are used, one for the sprint and one for the turnaround. Using the athlete's weight, the distance run and the time, you can calculate the athlete's power. The fastest sprint time is used to calculate maximal power, while the slowest time is used for the minimum power. The fatigue index is found by subtracting the minimal power from the maximal, then dividing the sum by six: the number of sprints in the test. The average -- or mean -- power output shows the athlete's ability to maintain power over time. It is found by adding the power output for each of the six sprints and dividing by six. Peak Performance notes that the higher the mean power score, the better the athlete's ability to maintain anaerobic performance.

During intense exercise, muscle and blood lactate can rise to very high levels. Lactate accumulation causes an increased concentration of hydrogen ions and corresponding acidosis, a primary factor in muscle fatigue. Athletes with high fatigue index numbers should train to improve lactate tolerance in order to promote quicker recoveries from explosive bursts of speed and power. Lactate tolerance training usually starts midway through the pre-season, after an aerobic base has been built with continuous or interval training. Drills involving repetitions of sprints and shuttle runs produce high levels of lactic acid; as the body's tolerance to lactate grows, so does its capacity for efficient removal.