Besides mobility (that we’ve covered in our last chapter), another key aspect we’ve identified as playing a role in making the player closer to “injury proof” is making him/her fitter. This is not only achieved by the regular team training, but should be individualized to the players’ characteristics and goals. In other words, the athletic characteristics and their development should be 1) understood 2) assessed 3) planned.

This chapter aims exactly to these steps of Athletic Development, providing a ground based for the reader to understand the concepts that guide this field of knowledge. To ease the reading, this chapter will be divided in 3 parts, following the three components of the athletic development above mentioned .

“Ultimately, individuals who are not strong enough (or not fit enough) to cope with the demands of their sports will eventually break down” Coles, PA (2017)

Strength development has been on a spotlight when it comes to athletic development in collective sports. Currently, the evidence available does confirm this relation, showing that enhancing these capacities through strength training has a positive impact in the performance level of the athlete (McGuigan 2012).

Football, along with other collective sports, involves a range of actions such as walking, sprinting, jumping and changes in direction (Turner 2011). All these actions are correlated with specific physical qualities, such as maximal strength, power, muscle endurance, capacity of changing direction, speed, acceleration/deceleration and the capacity of maintaining these efforts at high intensity. All these qualities have a common ground: muscle strength. 

Specific measurements of power such as the sprinting capacity and the vertical jump height have been demonstrated to have direct correlations with performance in football, having specific profiles depending on the players’ role on the field (Turner 2011), as well as depending on the competitive level. The results of these findings are that players competing at higher levels tend to be stronger athletes, more powerful and more developed on a energy system point of view (Turner, 2014). 

When considering the sprinting capacity (in specific linear sprinting capacity) and performance, the relationship not only seems obvious but it has even correlated as the most preponderant action on goal situations (Faude 2012). This capacity shares, as stated previously, a correlation with the athletic development of the player, such as in the capacity of strength production and capacity of power output. These facts bring the athletic development to the center of the table when in comes to performing more and better (Faude 2012).

Through these lessons, we’ve stated that we believe rehabilitation>injury prevention> performance are all part of the same vector. Taking into account the correlations we’ve introduced to you in this chapter, we haven’t mentioned injury prevention. However, the literature does point out that players that are stronger, faster and capable of maintaining the same capacity in multiple repeated sprint actions (RSA), show lower injury risk (Malone 2018). Although we’ve seen how multidimensional the injury risk is, these findings point us towards a correct approach.

In parallel to the strength development, one other capacity has been mentioned as to be taken into account in the athletic capacity of the player and that may either be a success criteria or a performance limitator: we’re referring to the metabolic conditioning or, in other words, the Energy Systems Development (ESD). When addressing metabolic conditioning, we take into account the endurance capacity, defined as the capacity of sustaining work for a given time or repeating a certain effort maintaining intensity, for as long as possible. This capacity if intimately correlated to Aerobic Fitness.

The positive correlation between aerobic fitness markers and sports performance indicators (such as total distance, time with ball and sprints per game) has been pointed in several studies. Besides this correlation, some studies even point towards a talent differentiation taking these markers as a criteria, showing that athletes with best performance and higher competitive levels tend to present better aerobic fitness indicators. (Wisloff 1998, Gravina 2008, Turner 2014, Bangsbo 2008). The image below shows these findings.

Source: Bangsbo, J, Marcello Iaia, F, Krustrup, P. (2008)

During a 90 min match, an elite player tends to cover 8 to 12 km at a moderate to high intensity. It is also known that besides the total distance covered during a match, the player also has several high intensity actions such as sprints, accelerations/decelerations and jumps that are mostly dependent on an anaerobic system and which require an aerobic system to support the energy repositition (Hoff 2004). In other words, the aerobic system does not only directly impact in allowing to maintain certain efforts on an aerobic regimen, it also aids in the energy restoring process for multiple repetitive high intensity stimulus, suppressed by the anaerobic regimen.

Besides a positive impact on sport performance, aerobic fitness has also been connected to a protective factor towards injury. It has been shown that higher aerobic fitness indexes are correlated to a better capacity of tolerating spikes in the training/competition load (In Elite Gaelic Football) and therefore minimizing injury risk (Malone 2016). This line of thought is corroborated by the known facts that the majority of the injuries tend to occur mainly at the last section of each half of the game (as shown in the figure below), as well as a tendency to an increase of injuries at the end of the season (Ekstrand 2009). Both of these moments of injury are moments in which the fatigue is installing and increasing which, when faced by athletes who are more capable and and “less fatigable” show lower injury risks.

Source: Ekstrand, J., Hagglund, M., Walden, M. (2009)

Summing up the above presented ideas, the athletic development of collective sports players may enhance their performance by two means: 1) reducing risk of injury, creating athletes who are more resilient and 2) by creating athletes that are more capable and more developed on an athletic point of view, which allows them to maintain competitive levels of activity for longer, recover faster and stand out in individuals situations that may turn to be critical for the game.

The physical capacities training is therefore to be complementary to the field training and shouldn’t be neglected. The aim of this supporting work should always to fulfill the athletes’ specific gaps and trying to optimize the athlete in his/her whole, from strength training to energy system development.

Looking at the current paradigm of training methodology in a sports modality such as football, we have to consider that the field training isn’t enough to fulfil the above mentioned gaps, specially in the capacities that are needed in sporadic and critical moments. Here is where complementary training steps in. In order to give a concrete example of this assumption, let’s look at the short field games, a methodology with widespread use in football aiming for the development of tactical competences: in these exercises, only 22% of the players reach average requirements, regarding high intensity sprints. This shows that this model is insufficient to prepare athletes for some of the common actions in a match, like the high intensity sprint when facing critical opportunities during the match. (Nassis, 2019)

Source: Nassis, G.P. et al (2019)

This does not mean that this type of exercise is a bad kind of exercise! In fact, it may have very productive results when it comes to developing the athletes’ individual actions in a technical/tactical point of view, as well as the specificity of the movements in his role on the field. Knowing that the requirements change between different roles (and taking into account that these changes are considerable), we know that the training should be as specific and individualized as possible; this is the only way to prepare the athlete to the fullest, not only for his playing style and characteristics of his role, but also towards the game model planned by the coach. In the table below (retrieved from Casamichana 2019) we can see these different requirements, between different roles.

Comparing distance ran throughout the match, for each playing position. Source of the table: Casamichana, D. et al (2019)

Throughout this chapter, we will see the WHY and the HOW these attributes should be developed through the competitive season, allowing for the combination of the periodization and methodology of the training field to be complemented by the physical training.

We’ve now identified the need and value of both the strength and energy systems development, so the next step will be to develop some theoretical key concepts of both these worlds. After this, we’ll dive into the application of the concepts, such as the assessment of the players and finally into the application into the daily routine of football practice.

Understanding Muscular Strength – theoretic key concepts 

Strength Conditioning factors and manifestation forms

The development of Muscle Strength is crucial in the athletic development of any collective sport athlete. Looking at the specific actions, we see that part of them are about the ability of the individual projecting his own body mass (actions of sprint, jumps, etc.), others are about dealing with the confrontation of an opponent (contact with another player) as well as exerting a force on an external object (as shooting a ball, throwing, etc.). All these are depending on the strength profile of the player and the evidence has been showing us the same: best performing athletes (as seen in the actions stated) are also achieving higher results in several performance tests (Counter Movement Jump -CMJ-, Squat Jumps -SJ-, Change-of-direction -COD- tests or Repeated Sprint Ability -RSA- tests), as well as in maximum strength tests. (Arnsaon 2004, Rampinin 2007, Turner 2011, Turner 2014).

The development of muscular strength is of big importance and is achieved through two main factors: Muscular factors (Morphological) and Neural factors (Suchomel 2018). In muscular factors, the muscle architecture is one focus of the training, trying to potentiate the structural characteristics of the muscle in order to optimize its action in the different tasks. In neural factors, we can distinguish between central factors and peripheral factors, with the latter ones being where training has the most influence, through optimization of the inter and intra-muscular recruitment coordination (Suchomel 2018).

Muscle architecture encompasses adaptations like muscle cross-sectional area (CSA), pennation angle and fibre length. These are the main features that are linked with different capacities of muscular force production, for example, greater CSA, pennation angle and shorter muscle fibers are associated with greater capacity of muscular force production, but on the other hand, smaller pennation angle and longer muscle fibre are better suited for high velocity contractions, with smaller force production capacity.

Intra-muscular recruitment coordination relies on three different mechanisms: motor unit (MU) recruitment, firing frequency and motor unit synchronization. MU recruitment is based on the Henneman’s Size Principle, which dictates that smaller MUs are recruited first and that, with an increase in intensity, the MU recruitment follows a sequenced and progressive activation based on MU’s size. The firing frequency is also related to force production properties, where greater force production are associated with higher firing frequency. MU synchronization, on the other hand, is more related with force capacities within a time frame, improving the rate of force development. Inter-muscular coordination dictates how different muscles work together and involves mechanisms like agonist and antagonist coordination (inhibition of the antagonist muscle to facilitate the action the agonist muscle) and synergist activation (where different muscles work together to increase global force production).

Besides the factors that influence the production of strength, other key concept is related to the various forms of manifestation of strength. Maintaining a simple approach, Maximal Strength (1) is the basal capacity of muscle function and is the most used indicator used to characterize, on an absolute scale or relative to the bodyweight, the ability to produce force.

Following from the maximal strength, we can characterise Explosive Strength (2), where we differentiate the Rate Force Development (RFD), with a relation between time/force (Bottom figure), and the Power, a relation between force/speed (Top figure).

• In RFD, we talk about the capacity of producing force in relation to time, which means, at which rate can we produce force? It is a very interesting indicator of the strength profile given that most sports actions are limited by the time.Interpreting this means that, the higher the RFD of a given athlete, the higher is the capacity of the player to produce force in the different actions of the game.
• In Power, the relation is given by the strength/force and speed; it can be calculated by both these variables and characterised by the strength-velocity curve. This curve as a inversely proportional profile where decreases in the velocity of the actions involve higher levels of strength (and vice-versa). In this way, the development of any of these variables will result in an increase in power and the result being a right shift in the strength-velocity curve. Knowing the importance of developing power in sports performance, this is also an interesting indicator in characterizing the athletes profile.

Force/Velocity curve and Force/Time curve. Source: Science for Sport

When considering Explosive strength, we also have to consider the Reactive Strength (3) which is the capacity of the muscle to respond to a stretch-shortening cycle (ssc); an application of this type of strength is the right extreme of the strength-velocity curve and an example could be, for instance, when considering plyometrics (as in the opposite of the RFD, in which we are located at a left extreme of the curve). Lastly, the Resistance Strength (4), defined as the ability to maintain a the capacity of muscle work during a period of time.

Taking these characteristics of the muscle strength and it’s behaviour, we can already conclude that in order to adapt the training, we must know the athlete and their strengths and weaknesses: we do this by creating the athletic profile. For this, we’ll need to create an assessment method that is specific (taking into account the necessities of the sport and the athlete on their own), reliable and reproducible.

Understanding Energy Systems – theoretic key concepts

Energy production pathways and performance markers

The energy systems are the foundation when it comes to understand the production of energy in order to produce work, whether this work is at a muscular level or at any other type of bodily tissue/organ. Our organism has the ability of metabolizing the macronutrients we ingest with the ultimate goal of producing energy to our biological needs and this capacity is fine tuned depending on the requirements we are regularly exposed to.

In a broad perspective, we can distinguish between two systems that look towards producing energy at a muscular level:

1. an Aerobic System, which utilizes macronutrients to produce energy in the presence of oxygen and is characterized by a bigger capacity but a lower rate (in other words, not the most efficient on a time/work point of view);
2. an Anaerobic System, characterized by the production of energy without the presence of oxygen and considered a reduced capacity system, with high rates of energy production. In this system, we can point out two independent means. First, a Phosphocreatine Pathway (PC) also called allactic way, which occurs in the metabolization of phosphocreatine and consequent production of energy without producing lactate – it is the main source of immediate energy but also the one that depletes the quickest, lasting 8 to 10s. Second, the Glycolytic Pathway, or lactic pathway, resulting from the metabolization of glycose in the absence of oxygen and therefore having a high rate of energy production, a capacity that lasts 15 to 60s but also that produces lactate.

In the image below we see a graphic scheme of the expected timing of each of these pathways, as well as their implication in the overall performance.

Source: WJEC CBAC “Energy Systems and their application to training principles”, available at AS Physical Education platform 

Many markers can be taken into account as aerobic fitness markers. The gold-standard is VO2max, representing the maximum consumption of oxygen. This marker is correlated with the maximum capacity of the aerobic system in using oxygen in the energy production pathway. Nevertheless, the evidence doesn’t appear to show a better levels of performance connected to higher values of VO2max (Sapp 2017, Jones 2006, Bangsbo 2015 and Tonesson 2013).

Besides this marker, there are others in which the evidence appears to have a better correlation with performance, being also more sensitive to training. For this reasoning, we present the following:

• Lactate threshold, from which we not only retrieve the % of work reached but also the time to exhaustion at the threshold. It is represented by the first increase in the values of lactate comparing to rest levels  (2 mmol/L), for a certain work intensity. It is a clear marker of aerobic capacity and, for trained athletes, it is expected to be reached at intensities of around 75% to 90% VO2max (joyner 2008, Hoff 2004, Lundby 2015).
• Critical power (or critical velocity) is also another marker that should be taken into account, and it correlates to the maximum intensity to which a steady-state of VO2 (and other metabolic markers as lactate levels) is maintained. Beyond this point, the metabolic stability at the muscle level is compromised and a decrease in the stored levels of phosphocreatine occurs, as well as a progressive accumulation of the metabolites resulting from the anabolic process (Lundby 2015). It represents, therefore, a maximum work intensity that is maintainable for long periods of time, without the induction of fatigue by the accumulation of the metabolites or a depletion of the stored subtracts. This marker constitutes the superior limit of the heavy intensity domain, located therefore well above lactate threshold that limits the superior limit of the moderate work intensity domain (see image below).

Source: Polchowicz, M. P., et al (2018)

Consequently, athletes presenting a bigger capacity of work in the moderate domain (higher lactate thresholds) or a bigger capacity of work at a heavy domain (elevating the critical power) are athletes that are more developed at an aerobic point of view, being more resistant to fatigue. Being sensitive to the training process, these two markers and capacities are themselves, trainable. (Jones 2000).

Lastly, the efficiency is another marker, commonly used to characterise aerobic fitness and based on the question: how much work is the athlete capable of producing for a given consumption of oxygen? This is a marker that is also able to differentiate the energetic performance of an athlete  (Joyner 2008, Hoff 2004) but since its complexity, it won’t be explored in this chapter.

Given the “stop-and-go” characteristic of football, it is easily understood the dependency of both aerobic and anaerobic systems. During a 90 min match, an elite player runs 8 to 12 Km at an intensity near their lactate threshold (Hoff 2004) and with an average heart rate (HR) of 85%  (reaching a maximum of 98%) (Bangsbo 2008). Given the relationship between the HR and the oxygen consumption, we can claim that this consumption rounds an average of 70%. In this way, the football source power is estimated to be 98% from aerobic origin, with the remaining 2% due to the anaerobic system (Hoff 2004).

Regarding the adaptations to training, there are three big levels of the adaptation: 1) cardiovascular; 2) neuromuscular, 3) metabolic (Bangsbo 2015, Stone 2009). These adaptations are dependent of several factors in terms of the training planning (as the intensity, frequency and duration) as well as the initial level of fitness of the athlete, being the combination of all these factors the reason for the principles of individuality and specificity that training requires.

For lower intensities, in a moderate domain, the adaptations tend to occur mainly at a central component; this means that the adaptations are mainly at a cardiovascular level. Among these we find the increase in the Cardiac Output, an increase in the systolic volume, and increase in the cardiac contractility and an increase in the end diastolic volume.

The training in higher intensities and heavier domains tends to produce adaptations at the periphery, both at a muscular and at the metabolic level. Among these adaptations we can identify some as the increase in the muscle capillarization, an increase in the enzymatic activity, and increase in mitochondrial volume and density, increase in the levels of myoglobin and a shift towards a utilization of fatty acids as energy source (Stone 2009). All these adaptations will produce positive effects in the measurements of VO2max, Work Efficiency, Lactate threshold and Critical Power.

This concludes our theory overview on muscle strength and energy systems and, with that, the end of the first part of this chapter. We hope that, at this point, the reader as fully understood the importance of both these areas of the athletic development, as well as their role in improving the performance and preventing injury. After understanding the concepts and in order to be able to know where we’ll want to intervene, we have to know who the athlete in front of us is. And that means assessing, which brings us to the second part, where we’ll dive into the assessment of both these capacities in the football athlete.

Authors: André Mendes e Lucas Brink Carvalho

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