A central goal of modern elite sports is to individualize the management of exercise load and training.(6) While every athlete initially experiences subjective exhaustion in the context of load and recovery in terms of an internal load, also various body systems, such as muscular integrity, metabolism, or immune system, are brought out of homeostasis and re-regulate during recovery. This leaves molecular traces in the blood which can be used as objective markers for the differential quantification of these processes. Many studies point to a rather unstable interindividual regulation of these markers by high standard deviations, hindering many studies from yielding clear results in the application of various molecules as biomarkers.(9) The present study elucidated that various blood biomarkers exhibit differential response patterns after acute bouts of endurance exercise. For most of the markers studied, a distinction can be made between individuals who show a stronger response, while others show a weaker response to a similar endurance exercise program, (e.g., IL-10, CK). In addition, our data revealed that individuals also differ in terms of basal and post-exercise levels of some markers (e.g., IL-8, cortisol). These differences in baseline and response behavior reveal the necessity of a tailored treatment of each physiological system from which the biomarker originates. The present study further provided evidence that the response pattern for IL-10, IL-6, IL-8 and CK exhibited a high degree of stability within the investigated individuals, implicating a high intra-individual reliability for the given markers. Our results further show that, depending on the candidate marker, exercise and recovery cycles can be predicted with reasonable precision for specific markers. The most accurate predictions are obtained for cortisol, the cytokines IL-8 and IL-1RA as well as CK. Given the baseline level, subjective and objective recovery as well as re-regulation can be best predicted for cortisol and IL-8 compared to the other markers. For CK, a good prediction of recovery of maximum strength and subjective feeling of exhaustion can be made. For IL-1RA, especially its re-regulation can be predicted when knowing the baseline level. When merging the results of all conducted analyses, CK, IL-8, IL-10 as well as cortisol appear to be the best performing biomarkers in the context of the present study. In the next paragraphs, we will discuss these possible candidates and their suitability in more detail.
Cortisol as a biomarker for exercise control?
Cortisol is a glucocorticoid hormone secreted by the adrenal cortex in response to physical, psychological, or physiological stressors.(10, 11) Exercise is one such stressor that has been shown to significantly alter the circulating amounts of cortisol in the human body.(12, 13) This is due to the fact that exercise causes the activation of the hypothalamus resulting in the release of corticotropin-releasing hormone (CRH), which then stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH) followed by the release of cortisol from the adrenal cortex.(14) The present results revealed two groups differing regarding their pre- and post-exercise level, by re-regulation to baseline as well as with regard to their VO2max, whereby Group 1 exhibited lower pre and post exercise levels, a shorter recovery period and a higher VO2max. In this regard, a study by Lucertini et al. showed that, in healthy elderly men, higher cardiorespiratory fitness levels are associated with a lower diurnal cortisol output and with minor effects on the cortisol response to acute mental stress.(15) Among the subjects who responded to mental stress, the amplitude of cortisol response and the steepness of recovery decline displayed an increasing trend in the high fit subjects. These as well as our data pin to the notion that higher fitness might lead to reduced cortisol levels, a reduced responsiveness of the hypothalamic–pituitary–adrenal axis (HPA), as well as to a shorter recovery period. It is possible that the cause of the influence of training status on cortisol levels is that while acute exercise stimulates the HPA axis, regular training induces an adaptation of HPA axis activity to repeated exercise. Thus, particularly intense physical training might lead to adaptive changes in basal HPA function, including a phase shift and increased pituitary ACTH secretion, but also blunting of the adrenal cortisol response. The fact that cortisol levels seem to reflect exercise-induced changes is interesting in terms of its potential use as a biomarker.(11)
It is noteworthy that cortisol is one of the tested markers for which subjective and objective recovery as well as re-regulation could best predicted only by knowing its baseline level. Thus, cortisol as a molecule reflecting the (exercise-induced metabolic) stress-level appears to be very promising for predicting recovery cycles. However, the GAQ for cortisol is 0.76 (Fig. 2) reflecting only a moderate stability over the two days and some interchanges for assigning an individual to the same group on both testing days. Hence, it must be stated that cortisol has a clear circadian rhythm with flatter and wavier diurnal cortisol curves depending on many behavioral and health factors. This characteristic must be considered as a confounding factor that makes cortisol difficult to use as a biomarker without control measurements outside of sports(16), including, for example, sleep-wake cycles, diet and the daytime.
IL-8 as a biomarker for exercise control?
IL-8 is an important chemotactic factor involved in neutrophil granulocyte recruitment and activation. It can be secreted by many structural and immune cells, including macrophages, bronchial epithelial cells, and muscle cells, which also classifies it as a myokine. IL-8 increases in response to the intensity and duration of exercise, and the increase seems to be somewhat flatter but more persistent than for IL-6.(17) This is because IL-8 is probably released mainly in muscle to act paracrine and autocrine as an angiogenic factor in human microvascular endothelial cells.(18) As an inflammatory mediator, it seems to be quite clearly assigned to the dimension of the exercise-induced immune response. Since it is affected in many disease states, such as asthma, its stability as a biomarker is to be expected especially in healthy individuals.(19) Our results point to two groups that differ in terms of their initial level as well as their level after exhausting exercise. Thus, there are individuals with a significantly higher basal level of IL-8. The assignment to the groups is quite stable. Different basal levels of IL-8 could be due to present, even small, sources of inflammation, such as periodontitis, or could be due to genetic factors, such as polymorphisms in genes related to IL-8 and CXCR2.(20) However, these variables were not assessed in this study, and we can only speculate on it.
The present results also show that the basal levels of IL-8 are very well suited for predicting the re-regulation, the subjective exhaustion as well as the recovery of muscle function 24 hours after exercise. This is particularly significant for practical application as a biomarker in sports, as it allows effective planning of a regeneration cycle and thus the start of the next training session in a reasonable and adjusted period. The use in a competition or a match can also be planned based on an objective parameter against the background of sufficient recovery. Physiologically, we would interpret the role of IL-8 here as being directly involved in the recovery process as an inflammatory parameter, particularly in the phase in which inflammatory tissue repair takes place, which then transitions into a reparative remodeling process.
Creatine kinase as a biomarker for exercise control?
CK is an enzyme that is found in higher concentrations in muscle cells and only slightly concentrated in plasma. The appearance of CK in the blood has been generally considered to be an indirect marker of muscle damage, particularly for diagnosis of medical conditions such as myocardial infarction, muscular dystrophy, and cerebral diseases.(21) After muscular exercise, plasma CK levels increase, indicating a loss of integrity of the sarcolemma. CK is one of the few blood parameters that is more appropriately used in competitive sports.(4) Here, CK is used to diagnose muscular recovery with test strips and point of care analysis. However, there is controversy in the literature concerning its validity in reflecting muscle damage as a consequence of level and intensity of physical exercise. Non-modifiable factors, for example, ethnicity, age, and gender, can also affect enzyme tissue activity and subsequent CK serum levels.(21) The present results revealed two groups that differ with respect to their CK level prior to exercise, the post exercise level, the initial exercise response as well as to the recovery period. Group 2 showed a higher baseline and post-exercise blood concentration, a steeper increase and decrease in CK concentration. Thus, individuals in this group react much more sensitively and intensively to the exercise stressor and that their recovery times are also longer. The GAQ for CK was 0.86, which means that the respective CK response seems to be a characteristic response for the individual under strenuous endurance exercise. This is supported by data revealing that there are high responders regarding CK, who develop very high values after exercise, and low responders, with only a very flat response what seems to be genetically based and related to the stability of the muscle architecture.(22)
Our data further showed that there is a clear relationship to muscular recovery and to the subjective sensation of exertion as both can be predicted when knowing the baseline level prior to exercise. This is consistent with the literature, which has also already linked muscle pain to the CK response.(23) Consequently, when using CK as a biomarker, it is critical to determine the athlete’s baseline level and gather knowledge about the athlete's specific group assignment to ensure a proper individualized evaluation.
Il-10 as biomarker for exercise control?
IL-10 is expressed by cells of many leukocyte subpopulations including macrophages, natural killer cells and T cells. It is a rather immunoregulatory and anti-inflammatory cytokine that has a strong down-regulatory effect on the secretion of proinflammatory cytokines such as IL-1, IL-1β, TNF-α. The magnitude of the increase in the concentration of IL-10 seems to be mainly dependent on the release of IL-6 and the exercise duration. It is not yet fully understood what triggers the increase in IL-6 during exercise. There are indications of a progressive depletion of glycogen stores as well as leaky gut phenomena during prolonged exercise, which initially induce a pro-inflammatory and then an anti-inflammatory counter-reaction.(24) The degree to which group differences are due to genetic factors around IL-6 polymorphisms remain rather speculative.(25)
IL-10 analysis resulted in two clusters of 35 and 14 subjects, respectively whereby group 2 revealed a higher pre- and post-exercise level as well as a steeper increase speaking to the notion that that there are IL-10- high- and low responders to exercise. The GAQ was very high with 0.96 reflecting that none of the participants changed groups over the two testing days. However, IL-10 does not perform as well as the other markers in predicting recovery. IL-10 triggers changes in macrophage phenotype that promote muscle growth and regeneration. Due to its secondary release to an inflammatory stimulus, IL-10 might be less directly related to recovery than the other markers.(26)
Should we use a panel of different biomarkers?
Despite the remarkable stability and predictive value of single biomarkers, it would be worth considering a combination of biomarkers in the form of a panel for the diagnosis of exercise response and recovery processes. The risk in the use of individual biomarkers is always a certain susceptibility to error as well as the limited informative value for individual physiological systems.(27) A panel of markers identified here as particularly stable and predictive would add particular value in their use as biomarkers to diagnose exercise recovery cycles. A combined analysis of cortisol, IL-10, IL-6, IL-8 and CK can be very helpful in giving information about the homeostasis of different systems. While interleukins are markers related to inflammation, cortisol is a stress marker related to metabolism and CK is a marker related to muscle damage. Accordingly, by using this marker panel a clearer and more holistic picture can be obtained of athletic stress regarding different physiological systems. The differentiated regulation of the markers can then provide valuable information on the recovery process and possibly be used for differentiated and personalized recovery management.(28)
Perspectives
Overall, the present data show that some molecular blood markers for the diagnosis of athletes’ stress and recovery cycles exhibit a high degree of intraindividual stability and, therefore, are possible biomarkers. This is especially remarkable because studies often must deal with high standard deviations undermining successful analyses of group means. Based on the present analyses, IL-10, IL-6, IL-8 and CK seem to be suitable for the intraindividual diagnosis of exercise recovery cycles. In addition to their stable regulation and thus high reliability, they also show a relation to subjective stress perception and functional muscular fatigue. It should be noted that for all markers there are either groups of stronger and weaker responders or individuals with different basal levels. Such a finding must be included in the use of the markers and determined by pre-analytics before the markers are used as biomarkers. At the same time, we would recommend analyzing the markers as a panel because they address different systems, such as metabolism, muscular integrity, and exercise-induced immune response. Accordingly, future studies should focus further factors influencing these markers and the practical use in athletic training.