What is the Fatigue Index?

Fatigue is a broad concept that refers to a subjective feeling of tiredness or lack of energy and motivation to perform any activity, whether physically or mentally.

o muscle fatigue, a type of physical fatigue, is due to the feeling that a muscle can no longer work that you have been doing steadily for a while. Muscle fatigue, in addition to the subjective sensation, has a component that can be objectively measured: the power exerted by the muscle.

One of the ways to measure muscle fatigue is the Fatigue Index. Specifically, this index measures fatigue under anaerobic conditions and is widely used in sport as an indicator of athletes’ resistance to intense exercise.

What is anaerobic muscle fatigue?

Like all other cells in the human body, muscle cells use glucose to get the energy they need to do their work. First, glucose is converted into two molecules of pyruvate and two molecules of NADH (nicotinamide adenine dinucleotide). In this reaction, known as glycolysis, two molecules of ATP are produced.

The two pyruvate molecules can continue to produce more ATP in two different ways depending on whether oxygen is available or not.

If oxygen is available, pyruvate molecules follow a pathway known as the Krebs cycle. In the Krebs cycle, the energy obtained from the oxidation of pyruvate is used to synthesize NADH. Subsequently, NADH enters the mitochondrial respiratory chain, also known as the electron transport chain, and this is where oxygen comes into play. This pathway, the aerobic route, produces 36 molecules of ATP in muscle cells, 38 in other cell types. If oxygen is not available, the Krebs cycle and respiratory chain are blocked; NADH that does not enter the respiratory chain is consumed by the oxidation of pyruvate to lactic acid. without additional ATP production. In the anaerobic pathway, only 2 molecules of ATP occur for each molecule of glucose.
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The low performance of the anaerobic route is compensated for by its speed, approximately 100 times faster than the aerobic route. Before sudden intense muscle work, the heart rate and respiratory rate do not supply enough oxygen to the muscle cells. In this situation, the anaerobic pathway is temporarily established until the aerobic pathway is re-established. For example, in sprint-type sprints, the athlete goes from rest to maximum intensity in a very short time and his muscles resort to the anaerobic route to get the energy they need.

The speed of the anaerobic pathway allows the muscle to respond effectively to brusque exercise, something that is undoubtedly very helpful, but as fast as lactic acid is produced and cellular glycogen stores are depleted, the main cause of what is known as anaerobic fatigue. The muscle is exhausted and can no longer perform the work with the same force. The anaerobic route is only effective for short periods of time, between 10 and 30 seconds. By this time, the heart rate and respiratory rate should have increased and should be able to carry enough oxygen for the aerobic pathway to work again.

What does the Fatigue Index indicate?

If you do anaerobic work, rest for a short time, and do the anaerobic work again, muscle fatigue will appear sooner. I mean, the muscle gets tired sooner. There is, therefore, a decrease in anaerobic resistance. This decrease is what the fatigue index measures.

The fatigue index, strictly speaking, measures the rate of decrease in anaerobic power per second. For example, a fatigue index of 10 indicates that power decreases by 10% per second during exercise performance under anaerobic conditions. Therefore, the higher the fatigue index, the lower the resistance to intense work.

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How is it determined? The RAST test

There are several methods to determine the fatigue index and, by convention, it is customary to use the maximum and minimum power differential developed by the athlete3. One of the most used methods is the running test, better known by the acronym RAST, from the English anaerobic sprint test based on running. First, the athlete does a 10-minute warm-up, rests for 5 minutes, and then performs a series of 6 35m sprints with 10 seconds of rest in between. In each sprint time is recorded and the athlete’s speed, acceleration, strength and power are measured:

Speed ​​Divide the distance traveled by the time spent. The average sprint speed is obtained. It is expressed in m/s. Acceleration: Velocity is divided by time. It is expressed in m/stwo. force: the athlete’s weight is multiplied by the acceleration (expressed in newtons, symbol N) power: the force is multiplied by the speed. Power is usually expressed in watts (1 W equals 1 J/s (Joule per second)).

Power is calculated for each sprint, the highest power in the series is taken (Pmax) and subtracted from the lowest (Pmin). The result is divided by the total time of the six sprints (T) and the Fatigue Index is obtained as follows:


Suppose an athlete weighing 80 kg performs all 6 sprints of 35 m. In the first, it records a time of 4.68 s:

speed: 35m / 4.68s = 7.49m/s acceleration: (7.49m/s) / 4.68s = 1.60m/stwo force: 80kg*1.60m/stwo = 128.00N power: 128 N * 7.49 m/s = 958.72 W

The records and calculations are made for the six sprints and we arrive at this data:

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# hour(s) Power (W) 1 4.68 958.72 two 4.80 886.46 3 4.97 827.90 4 5.11 734.32 5 5.53 577.29 6 5.78 509.04

From the table above, we get:

maximum force: 958.72 W Minimum power: 509.04 W total time: 30.87s Fatigue index: ( 958.72 – 509.04 ) / 30.87 = 14.57

In sports, a fatigue index above 10 is considered high and indicates that the athlete needs to work on improving lactate tolerance. Improving this tolerance will increase anaerobic endurance, since the buildup of lactate (lactic acid salt) is the main cause of anaerobic fatigue.

The RAST test is useful for athletes who perform sprints or similar movements. In other sports, other techniques are used to determine the fatigue index; For example, WANT (Wingate 30 Anaerobic Cycle Test) is more specific to cyclists.

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