RPE and its relationship with the risk of injury in footballers
Overtime, the competitive distance among elite football teams has shortened, so the focus is currently on those aspects that can tip the scale to one side or the other.
Glycogen plays a key role in sports performance.1 During exercise, especially when performed at high intensity (> 80 % of maximum oxygen consumption), the glycolytic metabolism takes on a special relevance, becoming the main energy source (together with phosphocreatine) in sports such as football, where actions like sprints are repeated. Furthermore, glycogen has been shown to be important in numerous processes of muscle contraction.2 For example, the availability of glycogen has been shown to be related to the ability of the sarcoplasmic reticulum to release calcium. Thus, an intense effort of one hour (specifically, a cross-country ski race) has been shown to reduce glycogen levels in the arm muscles by 69 %, which meant a 15 % reduction in the ability of the sarcoplasmic reticulum to release calcium.3 Given the importance of intracellular calcium in muscle contraction, this data shows how lowering glycogen levels can contribute to fatigue.
In this context, and in order to correctly plan nutritional strategies before, during and after football matches, it is important to know what the effects are on glycogen stores. A study carried out in 2006 by Jens Bangsbo and Peter Krustrup analysed muscle glycogen levels in football players (Danish 4th division) before and after a match using muscle biopsies of the vastus lateralis.4 The results showed that glycogen levels decreased by 42 % during the match, with 47 % of muscle fibres being completely empty after the match. Moreover, sprint ability decreased 2.8 % in the second half of the match.4
In a recent study led by the Spanish researchers Dr Íñigo San Millán and Dr Julio Calleja, the changes in muscle glycogen stores during a football match have been indirectly analysed.5 To do this, a system called MuscleSound has been used, which estimates glycogen levels non-invasively (that is, without biopsies) using high-frequency ultrasound. The glycogen levels measured with this system have been shown to present a high correlation (r = 0.93-0.94) with the levels measured by means of muscle biopsy in athletes, being able to determine changes with an exercise session (r = 0.81 for the changes in the glycogen levels with an intense exercise session).6 Therefore, the authors used this system to assess football players from the Major League Soccer in the United States before and after a match .5 The results showed that, although the players consumed carbohydrates before (40 grams after warm-up) and during the match (65 grams during the break), muscle glycogen was reduced by an average of 20 % (with reductions ranging from 6 % up to 45 %). In short, the greatest reductions were seen in forwards and midfielders compared to defenders, while the goalkeeper suffered almost no decrease in his stores (6 %).
These results highlight the importance of an appropriate carbohydrate intake before, during and after football matches. Glycogen stores must be recharged before starting, for which it is important to increase carbohydrate consumption from at least the day before and try to minimize their decrease during the game as well as promote recovery after it. These results also show the great heterogeneity in glycogen depletion among athletes, which would support the individualization of nutritional strategies depending on factors such as position or playing style (for example, higher carbohydrate intake in midfielders or forwards who perform greater number of efforts at high intensity).
In summary, glycogen stores play a key role in performance in sports such as football, and their levels are markedly decreased (20-40 % on average) during matches. This decrease can have important consequences for performance, especially in the second half of matches, therefore, it is necessary to develop strategies to ensure a correct carbohydrate intake before, during and after them.
Pedro L. Valenzuela
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