Assessing the Sprint Force-Velocity Profile in International Football Players with Cerebral Palsy: Validity, Reliability and Sport Class’ Profiles

Cerebral palsy (CP) football is a 7-a-side football modality played by people with CP or acquired brain injury with a minimum impairment of hypertonia, athetosis or ataxia (International Federation of Cerebral Palsy Football Classification Rulebook). It is played on a 70 x 50 m pitch during two halves of 30-min, and the rest of rules and adaptations are established by the International Federation of Cerebral Palsy Football (IFCPF). Eligible players for CP football are classified into three sport classes according to the severity of their impairment (FT1: severe profile, FT2: moderate profile, and FT3: mild profile).

As in conventional football, CP football is an intermittent sport with high anaerobic metabolism requirements alternated with recovery actions performed at low intensities (Yanci et al., 2016). A high performance in CP football is characterized by short high-intensity actions such as accelerations, linear sprints or jumps, among others, which are frequently the decisive actions in a match (Faude et al., 2012). In the last years, there has been an increase of research about the physical performance characteristics in CP football (Daniel et al., 2020; Peña-González et al., 2020; Reina et al., 2020; Tufekcioglu 2020). The linear sprint (Pastor et al., 2019), the vertical jump (Reina et al., 2018), change of direction (Reina et al., 2016) and dribbling (Reina et al., 2017), are the most commonly assessed abilities to evaluate physical performance in CP football. According to Yanci et al. (2019), CP football players cover an average of 186 m per sprint, while the number of medium and high-intensity accelerations is on average 66 and 4.5, respectively, in those players who play a minimum of 40-min. In addition, some studies have shown the importance of sprint and acceleration performance in CP football players’ evaluation, for example, to discriminate between national and international football players (Peña-González et al., 2021). Under this perspective, sprint performance seems to be a discriminant factor to succeed in CP football and it has become a crucial ability to improve by coaches and conditioning trainers in their training programs.

Although the traditional assessment of sprint ability has been carried out by recording the time taken to run a given distance (i.e., 30-m sprint) (Jiménez-Reyes et al., 2020), there is an increasing interest in evaluating the players’ force-velocity (Fv) profile to obtain more specific data about players’ capability to produce horizontal force during the sprint (Marcote-Pequeño et al., 2019; Samozino et al., 2016). This assessment is based on the inverse and linear relationship between force and velocity (Samozino et al., 2016). The sprint Fv (SFv) profile assessment allows coaches and conditioning trainers to obtain information about the specific components of sprint performance, being able to implement a more specific and individualized training program to improve sprint performance (Morin and Samozino, 2016). The specific characteristics that influence sprint performance are the theoretical maximal capability of a player to produce horizontal force (F₀) and velocity (V₀), the associated maximal power output (P_max), the slope of the Fv relationship (Fv_slope), the theoretical maximal effectiveness of horizontal force application (RF_max), the players’ capability to maintain the inevitable decrease in horizontal force production when the velocity increases (D_RF) and the maximal speed reached (V_max) (Morin and Samozino, 2016). With this information, coaches and conditioning trainers could analyse the players’ specific components of the SFv profiles. It could be useful to determine the players’ strengths and weaknesses in sprint performance and to individualize their training programs to be more effective and efficient in the conditioning of players (Morin and Samozino, 2016).

The SFv profile has been reported and characterized with regular football players in several studies (Jiménez-Reyes et al., 2020; Marcote-Pequeño et al., 2019; Mendiguchia et al., 2014). This literature from regular football has reported the correlation between the variables of the SFv profile and other physical performance determinants in football (jumping and sprinting abilities) (Marcote-Pequeño et al., 2019). Moreover, recent research has studied the usefulness of the SFv profile assessment to detect the possible decrease in players’ physical performance at the end of the season (Jiménez-Reyes et al., 2020) and to control the “return-to-play” after a hamstring injury (Mendiguchia et al., 2014). However, the SFv profile has been not characterized in CP football, in which the players’ disability has an impact mainly on their force production (Cans, 2000). As the main tests used to evaluate physical performance in CP football assess the players’ ability to produce high levels of force in a reduced time (force-velocity relationship), the possible correlations between the SFv profile and the rest of physical performance tests could help coaches use a unique test to measure their players’ physical performance efficiently. Under this perspective, the assessment of the SFv profile may shed some light on CP football players’ sprint characteristics and may help coaches and conditioning trainers detect the specific features (i.e., initial horizontal force or power application in the acceleration phase or the maintenance of the force/power application at high velocities) to improve throughout the training process. Thus, the aims of the present study were to assess and describe the SFv profile in international football players with CP, to report the reliability of this test in this population and to show the relationship between the players’ SFv profile and their physical performance in the linear sprint, the vertical jump, change of direction ability, dribbling ability and specific endurance.

The present study aimed to describe the SFv profile in international CP football players and to show its validity by comparing it to the players’ performance in linear sprint, vertical jump, change of direction, dribbling and specific endurance tests. To the authors’ knowledge, this is the first study that examines the SFv profile in CP football players, and the main findings revealed high positive correlations between SFv variables (F₀, V₀, P_max, RF_max, and V_max) and players’ performance in the 30-m sprint, CMJ, MAT, dribbling and Yo-Yo IR1 tests. In addition, RF_max seemed to be the main predictor variable from the SFv profile of players’ physical performance in the linear sprint, change of direction, dribbling and endurance tests. This study also showed good to excellent reliability for all the variables from the SFv profile (ICCs from 0.77 to 0.99) with low standard error in the measurement (SEMs from 0.86 to 0.89%). These reliability values are similar to those found in previous literature for other physical performance tests in CP football as the modified agility test (ICC=0.82-0.91; SEM=5.84-5.75%) and the dribbling test (ICC=0.85-0.92; SEM=6.30-4.66%) (Peña-González et al., 2020; Reina et al., 2017). These results reveal that the SFv profile is a reliable tool that may be used with CP football players.

CP football, as regular football, is an intermittent team sport in which players’ high physical performance is determined by their capability to perform short and high-intensity activities such as sprinting, jumping and changing direction, among others (Stølen et al., 2005). Table 1 shows the SFv profile and other physical performance variables (mean and standard deviation) of international CP football players according to their sport class and for the total sample. As it was expected, these data suggest that CP footballers with lower impairment (i.e., FT3) were able to obtain higher values in the SFv profile variables, linear sprint, vertical jump, change of direction, dribbling and specific endurance tests than players with greater impairment (i.e., FT1). In spite of not including a statistical analysis to show differences between sport classes, main standardized differences can be observed between FT1 and FT2 (Table 2) for all variables and these differences are reduced between FT2 and FT3. Similarly, Peña-González et al. (2020) observed that FT1 players presented statistically lower performance in change of direction and dribbling tests than FT2 and FT3 players, without statistical differences between these last two sport classes. However, such an interpretation from this study should be taken with caution and it is necessary for future research to show whether these differences are significant between sport classes or not.

The Pearson’s correlation analysis revealed a positive and significant relationship between the players’ sport class and their SFv profile, except for the FV_slope and D_RF variables. Players with a lower impairment profile (i.e., FT3) seemed to be able to produce higher theoretical maximal horizontal force (F₀), reach higher theoretical maximal velocity (V₀) and higher theoretical maximal power output during the acceleration phase (P_max), obtain a higher maximal ratio of horizontal force (RF_max) and higher maximal velocity during the sprint (V_max) than their peers with higher impairment (i.e., FT1). All these variables from the SFv profile also correlated to the rest of physical performance tests (time during the 30-m sprint, the CMJ, the MAT, the dribbling test and the Yo-Yo IR1). This suggests that players who are able to produce higher levels of F₀, V₀, P_max, RF_max and V_max are also able to reach higher values of sprint, vertical jump, change of direction, dribbling and endurance performance than players with lower values in the SFv variables.

More interestingly, players’ performance in the linear sprint, change of direction, dribbling and specific endurance tests may be explained by their effectiveness when applying horizontal force during the acceleration phase. RF_max is the direct measurement of the proportion of the total force production which is directed in the forward direction of motion at the beginning of the sprint (Morin and Samozino, 2016). This RF_max was the included variable by the stepwise regression analysis to explain players’ performance in the 30-m sprint, MAT, dribbling and Yo-Yo IR1 tests. This indicates the importance of the horizontal force application in the acceleration phase in these physical tests as well as in CP football, in which players perform an average of 1.06 moderate (1.0– 2.78m·s^-1) and 0.07 high (>2.7 m·s^-1) accelerations, and between 0.9 and 0.13 high intensity changes of direction per minute in a match (Yanci et al., 2018). In the same way, Jimenez-Reyes et al. (2020) observed that RF_max was one of the SFv profile variables that professional football players (without CP) improved during a season, and these changes correlated to 10-m sprint performance. Our results are also in line with Morin and Samozino (2016) who presented an example of two players with similar SFv profiles and identical time in a linear sprint and horizontal power output production, in which RF_max and the decrease in this ratio of force application in the horizontal vector (D_RF) are two crucial variables for coaches and conditioning trainers, as they provide information about the specific and individual needs of each player (Morin and Samozino, 2016). However, in the present study D_RF did not correlate, and it was not a predictor of any physical performance test, suggesting that the initial application of force in the horizontal vector is more important than the decrease in this ratio of force for performance in these tests and performance in a match, given the short distances covered by CP footballers (an average of 12.0-m at between 13.0 and 18.0km·h^-1, and 3.27-m at >18.0km·h^-1) (Yanci et al., 2018).

The SFv profile seems to be an efficient (easy to carry out and simple for athletes) test to assess international CP football players as it is related to players’ physical performance and it provides information about the mechanical characteristics of players in the sprint, showing the players’ strengths and weaknesses and allowing coaches and conditioning trainers to individualize the training programs. Nevertheless, this study presents some limitations that should be considered for the interpretation of the results and for future research. The SFv profile has been shown according to the CP football players’ sport class, but the low sample size in some classes and the asymmetry in the sample size between sport classes did not allow statistical comparison between groups. In future research, it would be interesting to study the differences in the SFv profile between international CP football players according to their sport class, their impairment profile and their field position.

Sprint performance is one of the most important features in CP football, and players’ performance in the linear sprint is commonly assessed by coaches and physical trainers to have information about their players’ physical performance. Thus, the SFv profile can be used to test CP football players as it is valid, reliable, easy to carry out and time efficient. The SFv profile provides meaningful information about players’ sprint characteristics (e.g., maximal force and power applied to move forward, the ability to maintain this force and power production at high velocities). With this information, it is possible to individualize the players’ training program to impact their specific needs, allowing individual responses to training, maximizing improvements and reducing players’ weaknesses in sprint performance.

This study describes the Sprint Force-velocity (SFv) profile according to the CP football sport classes. The analysis revealed high reliability for all the SFv profile variables and Pearson’s correlation coefficients showed a significant relationship between the SFv profile, the CP football players’ sport class and their physical performance in vertical jump, change of direction, dribbling and intermittent endurance tests. In addition, players’ maximal ability to produce force in the horizontal vector (RF_max) was the main variable from the SFv profile that predicted players’ performance in other physical performance tests. These findings could be interesting for CP football practitioners, coaches and physical trainers as they will allow them to assess the strengths and weaknesses in sprint performance and individualize the training stimulus to the optimization of sprint performance.