Training Load and Physical Fitness
The benefits of physical activity and chronic physical exercise on health are well known, with numerous musculoskeletal, cardiovascular, metabolic, and cerebral adaptations. Paradoxically, acute exercise induces significant physiological stress: increased acidity or mechanical constraints on the muscular and skeletal systems that are objectively detrimental to the body. However, it is necessary to consider the body’s ability to adapt to this physiological stress to limit its impact on its own functioning. For example, an increase in mechanical constraints on skeletal muscle promotes hypertrophy to limit the stress applied during subsequent physical exercise.
Consequently, the goal of sports training is to repeat these physiological stresses in order to stimulate the beneficial adaptations associated with exercise (e.g., an athlete seeking to increase their muscle mass will perform strength training exercises applying mechanical constraints to the muscles to stimulate muscle hypertrophy). Nevertheless, these exercises induce damage and fatigue to the human body, which are reversible with rest. Thus, it is necessary to control the dose of exercise, or training load, and that of recovery with the aim of improving health and/or athletic performance.
Specifically, a training load that is too high relative to recovery leads to a transient overload, while a load that is too low relative to recovery will not allow for progression. Furthermore, if the overload is maintained over time, it can lead to the appearance of an overtraining syndrome, characterized by decreased performance, persistent fatigue, and an increased risk of injury.
In this context, it becomes necessary to control the training load when the volume and/or intensity of exercise increases, in order to avoid overload, or even an overtraining syndrome, while maintaining progression. For this purpose, sports preparation has numerous tools to evaluate external load (weekly mileage, weight lifted, etc.), internal physiological load (heart rate variability, blood tests), and perceived load (perceived fatigue, mood, etc.). When interrelated, these parameters allow for the assessment of the quantity of external constraints on the body (external load), their impact on the body’s physiological functioning (internal load), and on the psychological level (perceived load), and thus to adapt it according to the goals and recovery capacity of each individual.
Phase Angle, a New Indicator to Evaluate Training Load and Athletes’ Fitness Status
Muscle damage associated with acute exercise is responsible for the transient secretion of pro-inflammatory molecules whose role is to initiate muscle repair processes1. However, in a context of prolonged overload, or even overtraining syndrome, the repetition of exercises coupled with a deficit in recovery promotes the onset of low-grade inflammation, concomitant with a decrease in performance and a significant feeling of fatigue. In a recreational or high-level athlete, this low-grade inflammation can therefore be a sign of training overload, if it is associated with a drop in performance and/or persistent fatigue.
In bioelectrical impedance analysis (BIA), the phase angle is an easily measurable bioelectrical parameter that is related to the health status of individuals, as well as the level of systemic inflammation2. Furthermore, in athletes, the phase angle has been shown to be associated with the strength and power produced by skeletal muscles3, as well as sprint performance and the capacity to repeat this type of effort4. In these studies, a high phase angle was associated with high performance levels, while the less efficient athletes presented the lowest phase angles. These various data have led a group of researchers specializing in sports science and BIA to consider that the phase angle could be used as a marker of training load5.
In this context, an increase in the phase angle throughout the preparation for an event or during a sports preparation cycle would be associated with an improvement in performance, as previously observed in elite swimmers6. Conversely, a decrease in the phase angle indicates an increase in systemic inflammation and could therefore be associated with training overload. In this case, it is interesting to monitor and/or quantify other parameters associated with the training load to confirm or refute this hypothesis based on their evolution. For example, an increase in external load associated with an increase in perceived fatigue and a decrease in heart rate variability in the medium term, a marker of nervous system fatigue7, are representative of training overload. However, it is legitimate to ask at what point we can consider a decrease in the phase angle to be significant and potentially reflective of training overload.
The normal physiological variation of the phase angle between two measurements is 0.3°, so it is possible to consider that a difference greater than this figure is significant and may reflect physiological modifications. In the context of detecting an overload, a punctual drop may be correlated with a transient overload caused by a competition or a particularly intense and/or long exercise, but also by an illness or another event causing fatigue. If the phase angle value returns to a value close to its normal value after adequate recovery, this is solely related to a transient overload; however, if it remains at a relatively low value, this may be a reflection of a much more significant overload, potentially leading to an overtraining syndrome over time.
Conclusion
The principle of applying chronic physiological stress in a controlled manner is the basis of sports adaptations associated with health and performance. However, if the applied physiological load is too great and/or recovery is insufficient, this can favor the onset of training overload, or even overtraining syndrome. These situations are notably characterized by the appearance of basal inflammation, a physiological event with which the phase angle is associated. Furthermore, this parameter is also related to muscular and sprint performance, which has led several researchers to propose this parameter for monitoring training load and potential overload.
Bibliography
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- Barrea L, Muscogiuri G, Pugliese G, Laudisio D, Alteriis G de, Graziadio C, et al. Phase Angle as an Easy Diagnostic Tool of Meta-Inflammation for the Nutritionist. Nutrients [Internet]. 2021 May [cited 2024 Oct 3];13(5). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8145306/
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- Martins PC, Teixeira AS, Guglielmo LGA, Francisco JS, Silva DAS, Nakamura FY, et al. Phase Angle Is Related to 10 m and 30 m Sprint Time and Repeated-Sprint Ability in Young Male Soccer Players. Int J Environ Res Public Health. 2021 Apr 21;18(9):4405.
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