Duration of powerful actions

There were significantly greater number of initial accelerations (Z = -3.189, P = 0.001; Fig. 3.3), leading accelerations (Z = -3.115, P = 0.002; Fig. 3.3), and sprints (Z

= -2.325, P = 0.020; Fig. 3.3) performed that lasted for durations between 0.5-1.49 s compared to the number of these activities performed for more than 1.5 s.

109

Figure 3.2. Frequency count of powerful actions performed during (a) the 90 minutes of competitive match-play (n = 13); and (b) the first half (black bars; n = 16) and second half (white bars; n = 13) of competitive match-play. Data are mean ± SD.

* Significantly lower frequency count than initial accelerations (P < 0.050).

# Significantly lower frequency count than leading accelerations (P < 0.001).

& Significantly lower frequency count during second half compared to first half (P ≤ 0.050).

Key: Acc = accelerations.

110 3.3.4 Jump type frequency

There was no difference in the total number of unilateral compared to bilateral jumps performed during the matches (3 ± 3 vs. 3 ± 3; Z = -3.180, P = 0.437). The range of total vertical jumps performed, however, was large (Total vertical jumps: 2 – 18).

3.5 DISCUSSION

We developed a new analysis approach for characterising powerful activity profiles of ESP and are the first to identify differences between the frequencies of powerful actions performed during elite soccer match-play. The video based player tracking notational analysis SSPA coding system (Tables 3.3 and 3.7) developed was shown to be reliable for identifying powerful actions during soccer match-play. The main finding from analysing the individual elite player footage using the New SSPA coding

Figure 3.3. Frequency count of initial accelerations (black bars), leading accelerations (striped bars), and sprints (white bars) performed for durations between 0.5-1.5 s and > 1.5 s during a whole match (n = 13), mean ± SD.

* Significantly greater number of powerful actions with duration between 0.5-1.49 s, than > 1.5 s (P <

0.05).

111 system (Tables 3.3 and 3.7) was that over the course of a competitive game, EPL youth players performed a significantly greater number of initial and leading accelerations compared to both sprints and vertical jumps. Action duration data showed that the majority of initial accelerations, leading accelerations and sprints performed over the course of a game lasted less than 1.5 seconds. Our findings suggest that accelerations of short duration (<1.5 seconds), from different starting speeds that are oriented in the horizontal direction, are the most dominant powerful action in elite youth soccer match-play. This information suggests that soccer-associated maximal power assessment and development protocols could specifically focus on evaluating and enhancing the ability to perform horizontal, rather than vertical, powerful actions.

If a video based player tracking notational analysis coding system is to be used to provide information to inform the prescription of assessment and training strategies for ESP, it is imperative that the coding system has been validated and shown to be reliable for observers using the instrument. Due to the range of acceleration and maximal speed capabilities presented by soccer players in the same team (Mendez-Villanueva et al., 2011, Stolen et al., 2005), and the compromised reliability of automated time motion analysis systems for quantifying soccer specific high intensity actions (Akenhead et al., 2014, Varley, Fairweather and Aughey, 2012), there is currently no gold standard for quantifying the range of powerful actions performed during soccer match-play. Subsequently, the face validity of the SSPA coding system definitions and powerful action descriptions (Table 3.3) in the current study were established after interviewing a series of experts in the applied (one performance analyst and two sports scientists currently working at a Premier League Club) and research (an experienced performance analysis researcher) fields (see The development of the system: Stage 5). This informal validation process was thought to be sufficient to consider the New SSPA coding

112 system (Tables 3.3 and 3.7) as a valid player tracking notational analysis coding system for analysing the powerful action profile of elite soccer match-play. The New SSPA coding system (Tables 3.3 and 3.7) was also shown to have good intra- and inter-observer reliability (see The development of the system: Stages 6 and 7) and was therefore a reliable coding instrument for use within the principle researcher, and others (Tables 3.5 and 3.6). The current study is the first to demonstrate that a novel SSPA coding system (Tables 3.3 and 3.7) is a suitable video based player tracking notational analysis coding system for providing a valid and reliable account of the powerful actions performed during elite soccer match-play.

If certain powerful actions occur more frequently during elite competitive match-play, it can be assumed that the ability to perform this specific task may be important for elite soccer performance and should be prioritised in the training process. EPL youth soccer players undertook 8~fold more horizontal accelerations than sprints (~68 vs. 8, respectively), thus suggesting the ability to frequently perform horizontal accelerations is a more important characteristic of elite youth soccer performance (Figure 2). We also show that elite EPL youth soccer players performed 11~fold more horizontal accelerations (initial and leading accelerations combined) than vertical jumps (~68 vs. 6, respectively) during a competitive match.

Subsequently, powerful efforts oriented in the horizontal-forward and mediolateral directions can be considered more important during elite youth soccer match-play than efforts oriented the vertical direction. Considering it has been previously documented that unilateral jump capabilities in different directions are independent qualities that should be assessed and developed separately (Hewit, Cronin and Hume, 2012, Meylan et al., 2010), it appears that future soccer specific power assessment and development protocols should prioritise horizontal-forward and mediolateral power production. However, the range in vertical jump frequency in the

113 current study (2-18) may also suggest that this capacity is more important for some positions than others, or perhaps dependent on the type of game and opposition.

To optimise muscular power assessment and development programmes for soccer players, knowledge of the detail of these actions is important. The majority of horizontal accelerations and sprints were performed for durations of less than 1.5 s (Figure 3). This information suggests that the ability to initiate short duration explosive horizontal efforts, is an important characteristic of elite youth soccer match play. These findings are in agreement with Cometti and Colleagues (2001) and, as acceleration performance has been associated with horizontal-forward jump performance (Dobbs et al., 2015, Maulder and Cronin, 2005, Meylan et al., 2009b), these findings may support the inclusion of this assessment in elite soccer talent identification protocols.

Our study is the first to categorise accelerations as initial or leading, based on the inertia that the player is required to overcome before initiating the acceleration.

As there was no difference in the number of initial and leading accelerations completed by EPL youth soccer players (Figure 2), our results show that both of these actions are of equal importance and should be developed during soccer training interventions. Previous literature documenting the biomechanical detail of acceleration from a static start illustrated that as the acceleration progresses, ground contact times and positive horizontal impulse (impulse = force x time) decreased [For review please see: (Wild et al., 2011)]. Indeed, Salo and colleagues (2008) found propulsive impulse to decrease from 93.5 Ns, to 49.1 Ns, between steps one and four of a maximum acceleration action from a static start. The maximal power capabilities required to accelerate are therefore dependent on the initial inertia of the player. Initial accelerations performed from a walking or static start may require the production of greater positive horizontal impulse and relative ground reaction forces to overcome the inertia. In contrast, leading accelerations are initiated when the

114 player is already moving and therefore may require less relative ground reaction force and propulsive impulse to overcome the moving inertia. Our data therefore suggests that elite soccer match-play requires the ability to produce horizontal impulse equally from both static and moving inertias and therefore, training interventions with elite soccer players should aim to develop acceleration capabilities from both static and moving starts. Moreover, soccer specific assessments of maximal power should evaluate the ability to produce horizontal impulse from both static and moving inertias, using both acyclic (i.e. countermovement jumps) and cyclic unilateral assessments (i.e. drop jumps or multiple jumps).

Analysing the effect of time on the frequency of powerful actions may also provide useful information for the prescription of soccer conditioning programmes.

Elite EPL youth soccer players performed fewer powerful actions in the second half in comparison to the first half. More specifically, significantly fewer initial accelerations, and a tendency for fewer leading accelerations, were performed in the second half, thus suggesting that powerful horizontal actions were reduced as a function of time. Due to the complexity of factors that could influence the reduction in running performance during soccer match-play (Paul, Bradley and Nassis, 2015), the significance of these findings is not yet known from a practical or theoretical perspective. Nevertheless, our data may suggest that soccer practitioners could aim to apply specific conditioning interventions to help players perform initial accelerations more frequently during the second half of competitive matches.

We must acknowledge that there were a number of limitations in our study.

Due to the time consuming procedures associated with tracking and recording an individual player with a camera for 90 minutes, the sample size was relatively small, and only included players from one EPL club. The powerful activity profile reported may therefore have been biased by the tactical and/or training strategies employed by this club. The protocol for establishing the face validity of our coding system also

115 only included individuals connected with only one club and could also have been affected by bias towards certain club philosophies. However, as we are the first to document the powerful activity profile of an ESP population (twelve of the players were members of their respective youth national teams with three players having also previously represented their senior national team) in sufficient detail to inform detailed assessment and training intervention protocols, we have provided novel information for the applied practitioner and a platform for future research in this area.

Conclusion

Our study demonstrated that a novel video based player tracking notational analysis coding system (novel SSPA coding system; Tables 3.3 and 3.7) has sufficient face validity and can be used to provide a reliable account of the powerful actions performed during elite soccer match-play. While we show that EPL U18 and U21 elite soccer match-play requires the performance of powerful actions oriented in multiple directions, vertical jump and sprint actions were performed less frequently than both initial and leading horizontal accelerations. Our results show that initial and leading horizontal accelerations of short duration (< 1.5 s) are the most frequently performed powerful actions during elite youth soccer match-play. This suggests that the assessment and development of acceleration capabilities from different starting speeds should be prioritised in soccer talent identification and development programmes. More specifically, our results may imply that the ability to generate maximal power during actions oriented in the horizontal-forward and mediolateral directions could be considered paramount to successful soccer performance. Our data could be used to inform the detail and specificity of novel soccer-associated maximal power assessment protocols.

116