The measurement of the lactate threshold and, in general, the physiological analysis of athletes to establish work profiles is one of the principles of training. Identifying work levels through technologies such as blood lactate analysis or its relationship with thermography can help in training prescription.
From different perspectives, attempts have been made to explain individual behavior during stress tests and training. Following the line of publications on local and global fatigue, one of the most used analyzes is the identification of cardiovascular work thresholds by obtaining blood lactate.
The basic idea of training is to be able to program tasks that improve physical performance. Therefore, it is necessary to evaluate and measure the physiology of the athletes to establish the starting point with data of interest on which to program the exercise.
One of those data is the determination of the lactate thresholds related to the power exerted or the speed during the different training sessions. The identification of the threshold allows to know the functional capacity and the cardiovascular condition profile of the athlete. The greater the functional capacity, the later the rise in lactate levels occurs (Willmore and Costil 1988). Therefore, the goal of training is to shift the curve to the right and down (see Figure 1):
Figure 1. Blue line define the first test previous to training in an incremental exercise while it is measuring blood lactate. Green line represents the improvements in cardiovascular capacity post-training. Extracted from (Willmore and Costil 1988)
The aim of training for athletes who need a high development of cardiovascular capacity is to improve the point at which metabolic acidosis interferes with gas exchange, increasing lactate exponentially and irreversibly. At that moment, we say that we have exceeded the maximum stable state of lactate and fatigue will appear urgently until the athlete's energy pathways are exhausted and they are forced to stop exercising due to accumulated fatigue.
The use of lactate in aerobic exercise has demonstrated its value for exercise control and prescription (Faude et al. 2009). However, the greatest handicap of this type of analysis has been its invasiveness and operability, since it is necessary to puncture the athlete to extract the sample.
For this reason, some authors have investigated the relationship of other variables with the lactate threshold. For example, in an investigation by the group of Fissac, Valenzuela y colaboradores (2018) showed a very high correlation between functional threshold power (FTP) English) produced by cyclists and their lactate threshold. Although it is true that for the most experienced it would be necessary to subtract 5% from the power in the FTP to identify the lactate threshold more correctly (Figure 2):
Relationship between lactate threshold and functional threshold power. Extracted from Valenzuela et al. (2018)
Another tool that has a great relationship with lactate is thermography. Different investigations have wanted to relate the behavior of body temperature with the production of lactate during exercise.
In this sense, our research group published an article in "The Journal of Strength and Conditioning Research" (Gomes-Moreira et al. 2021). where it was evaluated through an incremental protocol, based on a physical evaluation test of Judo where 2 minutes of effort were made and one minute recovered. (see figure 3).
What the researchers found was an increase in temperature 5 minutes after finishing the incremental test, which gradually returned to baseline values in the subsequent 15 minutes. The mean skin temperature was calculated under the Houdas and Ring () considerations: (0.06 x forehead) + (0.12 x subscapular) + (0.08 x posterior-superior arm) + (0.06 x forearm posterior) + (0.125 x medial anterior thigh) + (0.075 x tibial) + (0.075 x gastrocnemius)
During this incremental protocol, the authors also analyzed the blood lactate concentration with the Lactate Pro measurement instrument (Arkray, Japan), before the test, immediately after finishing and at 5, 10 and 15 minutes. These authors found that the mean temperature data of all the regions at 5 minutes after finishing the test had a moderate relationship with the lactate concentration just after finishing the test (see figure 3):
Figure 3. Relationship between lactate post exercise and change in mean temperature measured by thermography. Extracted from Gomes-Moreira 2021
The authors propose a prediction model for calculating lactate through thermographic analysis, where to calculate lactate immediately after exercise, the increase in the average temperature of the regions would have to be multiplied by 2.38 and added by 6.03. Which would explain 44% of the behavior of lactate. (Lactate = 6.03 + 2.38 x Increase in temperature at 5 minutes).
Similar research was conducted by Adamczyk y colaboradores (2014), where anterior and posterior chain temperatures and lactate were assessed prior to completion of an exercise task. one minute in which multi-jumps were performed from full knee flexion, to subsequently assess temperature and blood lactate from the end of the exercise every three minutes until half an hour of recovery.
What the authors obtained were results contrary to those of (Gomes-Moreira et al. 2021), since there was a relationship between the drop in temperature and the increase in lactate (see figure 4):
Figure 4. Comparison of the blood lactate level evolution at rest, immediately after the exercise and during the 30 minutes recovery period. Extraído de (Gomes-Moreira et al. 2021)
Although it is true that the nature of the task and the physiological response are very different due to the stimulus, since in the first case it is an incremental test where cardiovascular fatigue is produced, and in the second case fatigue is the product of tension mechanics of a task that lasts one minute. As we have seen in a previous post, the systemic regulators of thermoregulation try to control the internal temperature in an acute way, generating vasoconstriction and lowering the temperature of the skin.
Finally, one of the most important investigations in the field of human physiology and thermography was carried out by Akimov y colaboradores (2011). It analyzed 20 young athletes who practiced different sports during an incremental cycle ergometer test, which started from 60 W riding for 5 minutes and increased every 2 minutes by another 60 W until fatigue, the lactate threshold was established at 4 mmol/ l and the temperature was measured with a thermographic camera, establishing the forehead as the region of interest.
The mean of the test lasted 14 minutes and 35 seconds, which corresponded to 285W and a maximum oxygen consumption of 62.2 ± 10.4 ml/(min kg). The forehead temperature before the test was 32.2 ± 0.55°C.
The authors divided the athletes into two groups due to the difference in skin temperature response relative to lactate. Group 1 was composed of athletes from cyclical modalities such as athletics, cycling or cross-country skiing with higher aerobic performance. These athletes, when they reached the lactate threshold, changed the temperature trend, recovering part of the temperature lost at the beginning of the test, showing a total decrease of only 2% with respect to the beginning. While in group 2, which includes all mixed-modality athletes who had a lower aerobic performance, they showed that when they reached the lactate threshold established at 4 mmol/L, forehead temperature decreased sharply, up to 7% with relative to the initial values (Figure 5).
The authors point out that these differences correspond to the different strategies that groups have to deal with physical stress through the thermoregulation system. Although some reasons for them remain unclear.
Figure 5. The dynamics of the skin temperature and the concentration of lactate in the tow groups of subjects during the test run on the bicycle ergometer. Solid curve without markers, the average forehead temperature: GROUP 1. Dotted line without markers, the average forehead temperature: GROUP 2. Solid curve with square markers, the average blood lactate concentration: GROUP 1. Dotted line with triangular markers, the average blood lactate concentration: GROUP 2.
From the ThermoHuman department we hypothesize that the different profiles of athletes, their adaptation to the type of test and the state of the nervous system have an influence on the temperature responses to exercise. In this case, group 1 more oriented to cardiovascular exercises have greater tolerance to fatigue and allow the thermoregulation system to deal with greater stress by increasing body temperature due to the phenomena described in the local and global fatigue post. While less trained subjects fail to thermoregulate and vasoconstriction and fatigue will appear earlier to a greater extent.
Thermography has a direct relationship with the physiology of exercise, as shown by the relationship between the behavior of the temperature of the forehead and lactate during the test and the increase in temperature 5 minutes after finishing the exercise and lactate accumulated during it.
As pointed out by the research group of Sillero et al. (Gomes-Moreira et al. 2021), 44% of the behavior of lactate during exercise can be explained by the increase in body temperature post-exercise.
In addition to being a predictor of accumulated lactate, it allows athletes to be differentiated between those with better physical condition and those with lower performance.
Finally, the thermoregulatory systems governed by the nervous system have a great influence on physiological behavior, so an in-depth study of the relationship between the nervous system and temperature is necessary.
References
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ThermoHuman has had the support of the Funds of the European Union and the Community of Madrid through the Operational Programme on Youth Employment. Likewise, ThermoHuman within the framework of the Export Initiation Program of ICEX NEXT, had the support of ICEX and the co-financing of the European Regional Development Fund (ERDF).
Thermohuman has received funding from the Centre for the Development of Industrial Technology (CDTI), in participation with the European Regional Development Fund (ERDF), for the R+D activities involved in creating a new tool, based on thermography, for the prediction and prevention of rheumatoid arthritis. See project detail.
