One of the great problems that training has today is to measure the effect that a stimulus has on the body. That is, how much fatigue we have accumulated. The body responds to stimuli by adapting to them, and this is how we gain strength. But, how do we know what stimulus we have given to the body? If we could measure it, we could get to know what is the best stimulus to achieve our goals. The velocity of execution allows us to know precisely what is the stimulus that we have given to the body.


What is fatigue?

The involuntary and inevitable reduction of the force applied after an effort. (Enoka & Stuart, 1992). The velocity of execution under the same load decreases as we train, we are no longer able to apply so much force. The loss of velocity, It is a good expression of fatigue that occurs in a workout. (Gonzalez-Badillo, Sanchez-Medina, Pareja-Blanco, & Rod riguez-Rosell, 2017)

Then, if we measure the velocity of execution of the fastest repetition (which is where we apply more force), and the slowest (which is the least force applied), knowing that we have done all the repetitions at the maximum velocity , we can have a reference value of what stimulus we have given to the body.

For example, in a series of 8 repetitions of bench press, if my first repetition was at 0.65 m/s and my last repetition at 0.4 m/s, making a simple rule of three, I know I have lost 39 % of the execution velocity, also known as 39% of inter-series fatigue.

The science behind the measurement of fatigue

Today it is demonstrated that this loss of velocity is related to changes in some indicators of fatigue in our body, to be more specific: lactate and amium.

In a classic paper of the velocity of execution, it was shown that the average of the loss of velocity of the three series that were made had a very high correlation (for the squat r=0.97, and for the bench r=0, 95) with the post-exercise lactate peak (Sanchez-Medina & Gonzalez-Badillo, 2011).

What does this mean? We know that lactate is an indicator of fatigue, because it is shown that it INCREASES according to fatigue. It is not the cause of fatigue, but we know that as intensity increases, lactate increases in our body. So if it has been shown that the higher the velocity loss, the higher the post-exercise lactate peak, means that the % velocity lost is a very good way to measure the real fatigue that the body has.


What variables do we measure to quantify fatigue?

Inter-fatigue: % of fatigue that we have accumulated in an exercise. If the fastest repetition is 0.4 m/s and the slowest 0.3 m/s, we will have lost 25% velocity.

Inter-Serial Fatigue: Average of all percentages of intra or inter serial fatigue. If I have accumulated 15%, 20% and 25% in the three series that I have made, it opens accumulated a 20% loss of velocity (fatigue).


Interesting data

In the study that we mentioned before, they also realized that lactate and ammonium levels did not go off until more than half of the repetitions could be done. Which means, that probably, if we work with those volumes, the recovery will be faster. This does not mean that it is the optimal, but it is a point to take into account to include work that does not produce much fatigue to our body.


  • The first repetition, or the fastest of the series, is very important, since it indicates to us what intensity we are working with, it will also serve as a reference to know how much we have fatigued, when we lose velocity due to fatigue.
  • Loss of velocity is a great way to quantify fatigue, since it has a very high correlation with post-exercise lactate peak, a physiological indicator of fatigue.


Through the Speed4lifts encoder we can modify and establish a velocity loss as a target. Instantly we can know when we reach that velocity. So we could stop the training, modify the load or the number of repetitions to achieve the perfect stimulus for that training.


Gonzalez-Badillo, J., Sanchez-Medina, L., Pareja-Blanco, F., & Rodríguez-Rosell, D. (2017). La velocidad de ejecución como referencia para la programación, control y evaluación del entrenamiento de fuerza.
Hirvonen, J, Nummela, A, Rusko, H, Rehunen, S, Härkönen, M, (1992) Fatigue and changes of ATP, creatine phosphate, and lactate during the 400-m sprint. Canadian journal of  sport sciences 17 (2) 141-4
Sánchez-Medina, Luis, González-Badillo, J. J. (2011) Velocity loss as an indicator of  neuromuscular fatigue during resistance training Medicine & Science in Sports &  ExerciseFebruary issue 1725-34
Enoka, R M, Stuart, D G (1992) Neuriobiology of muscle fatigue Journal of applied physiology  72 (5) 1631-48