1. Introduction
Soccer players, on average, perform 609 ± 193 cuts of 0° to 90° to the left or right [
1] during a match, typically in response to an opponent, the ball, or to create space. Similarly, Robinson et al. [
2] found 38.9 ± 13.3 and 36.3 ± 12 directional changes (45–135° movement 4 m/s or faster) per match were performed to the left and right, respectively, by soccer players. Moreover, change of direction (COD) actions (≥50°) which are then followed by a sprint are associated with critical moments, such as goal scoring and assists in soccer [
3]. Consequently, given the frequency of COD actions performed in soccer, and its association with decisive moments (i.e., goal scoring), the ability to change direction rapidly can be considered an important quality to develop.
COD speed is defined as “the ability to accelerate, reverse, or change movement direction, and accelerate again” [
4], and as stated earlier, soccer players frequently perform rapid decelerations, directional changes, and sprints to create space, or to react to an opponent or ball. The determinants of COD speed are multifaceted [
5] and influenced by physical (strength capacity), technical, and speed qualities [
5]. Enhancements in COD speed have been demonstrated as a consequence of COD speed training interventions over 6–12 weeks (i.e., field-based sprint, deceleration, and COD drills) and are particularly effective in soccer players [
6,
7,
8]. Moreover, a recent meta-analysis has confirmed that COD speed and sprint training interventions elicit short-term improvements in COD speed performance in soccer players [
9]. Therefore, COD speed training provides practitioners with a relatively easy to perform field-based method to enhance COD performance in soccer players, requiring minimal equipment.
While CODs are important for successful performance in soccer, directional changes are also key actions associated with non-contact anterior cruciate ligament (ACL) injury [
10,
11,
12]. This occurrence can be attributed to the propensity to generate large multi-planar knee joint loading [
13,
14] during the plant foot contact, which increases ACL load [
15] and can potentially rupture the ACL [
16]. Moreover, greater knee joint loads are also associated with increased risk of developing patellofemoral pain (PFP) [
17]; a common knee injury in soccer [
18]. Despite the recent improvements in sports medicine and strength and conditioning practices in soccer, non-contact ACL injuries are not declining and are still problematic [
19]. ACL injuries can be career threatening, with a plethora of negative economic, psychological, and health consequences [
20,
21]. Despite a high return-to-play rate in soccer following an ACL injury within a year of injury (≥90%), only two-thirds of players play at the same competitive level three years later [
19]. Consequently, knee injury mitigation is of high importance for soccer players.
Non-modifiable ACL injury risk factors include hormones, anatomy, and the environment, but notably biomechanical and neuromuscular control deficits are modifiable risk factors with appropriate training [
20,
21]. These “high-risk” deficits during COD include the following [
14,
22]: wide lateral foot plant distances, greater hip abduction angles, increased internal hip and foot rotation angles, greater knee valgus, reduced knee flexion, and greater lateral trunk flexion over the plant leg. Moreover, with the exception of lateral foot plant distance and limited knee flexion [
14,
22], the “high-risk” COD postures offer no associated performance benefits [
14], and in fact, reducing lateral trunk flexion and encouraging a trunk lean towards the direction of travel could be a faster technique [
14,
23,
24]. COD technique modification training (i.e., coaching cues and feedback to reduce postures associated with increased knee joint loads) can address the abovementioned “high-risk” deficits and reduce potentially hazardous knee joint loading when monitored with three-dimensional (3D) motion analysis [
13,
25,
26]. Therefore, addressing biomechanical and neuromuscular control deficits could be a viable strategy to reduce injury risk in soccer players [
14,
27,
28]; however, no study, to date, has examined the effects of COD technique modification training on cutting movement quality in male youth soccer players.
One strategy to help reduce ACL injury risk in soccer players is evaluating movement quality to identify athletes that display “high-risk” and abnormal mechanics so that individualised training programmes can be created [
20,
28]. 3D motion analysis is the gold standard for evaluating movement mechanics [
20] and has previously been used to monitor changes in COD mechanics [
13,
25,
26]; however, this is a complex, time consuming, and expensive process, which is often restricted to laboratory settings, thus limiting its application in field-based settings. Recently, the cutting movement assessment score (CMAS) qualitative screening tool has been created and validated against 3D motion analysis [
29,
30], with strong relationships (
ρ = 0.633–0.796,
p < 0.001) observed between CMAS and peak knee abduction moments (KAM) (which can load the ACL) and high reliability. As practitioners will implement training interventions to reduce “high-risk” cutting mechanics, it is imperative that the effectiveness of such interventions can be monitored using a valid and reliable screening tool. The CMAS provides practitioners a valid field-based screening tool to identify athletes who generate high knee joint loads and poor movement quality; however, to the best of our knowledge, no study has monitored the effectiveness of training interventions on cutting movement quality using a field-based screening tool.
The aim of this study, therefore, was to determine the effects of a six-week COD speed and technique modification training intervention on cutting performance and movement quality in male youth soccer players, using the field-based CMAS screening tool and timing gates to assess COD quality and performance, respectively. Since the introduction of the elite performance player plan (EPPP), injury rates in adolescents have increased three-fold [
31], and because youth players are striving for professional contracts, injury mitigation is paramount [
31]. If athletes can reduce “high-risk” deficits (i.e., reduce the CMAS score) and improve performance (i.e., completion time and COD deficit), the COD speed and technique intervention can be considered successful. However, if athletes can reduce “high-risk” deficits while maintaining cutting performance, it can still be viewed as a positive effect. It was hypothesised that a COD speed and technique modification intervention would result in faster cutting performance and improved cutting movement quality in comparison to a CG.
4. Discussion
The aim of the present study was to determine the effects of a six-week COD speed and technique modification training intervention on cutting performance and movement quality in male youth soccer players. The primary findings were that six-weeks COD speed and technique modification training performed during the competition phase, in addition to normal skills and strength training, produced meaningful improvements in cutting performance times, CODDs, and cutting movement quality (i.e., lower CMAS) in male youth soccer players (
Table 3 and
Table 4,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7 and
Figure 8), thus supporting the study hypotheses. The observed improvements in performance times and CODDs for the IG were, on average, two-times greater than the CG, who also continued their normal field-based warm-ups (
Table 3 and
Table 4). However, the CG demonstrated no meaningful or significant improvements in CMASs, in contrast to the IG who demonstrated meaningful improvements in cutting movement quality.
As cutting actions are frequently performed manoeuvres in soccer [
1,
2] and linked to decisive moments (i.e., assists and goal scoring) [
3], the ability to cut rapidly can be considered an important quality to develop. The results of the present study substantiate previous research that found COD speed training interventions improved COD speed completion times in male youth soccer players [
6,
7,
8], with the present study finding COD speed and technique modification training resulted in meaningful improvements in cutting completion time (
p < 0.001,
g = 1.63–1.90, ~9–11%), and these changes were greater than the CG (
Table 3 and
Table 4,
Figure 3 and
Figure 4). Moreover, CODD has been recently developed and suggested to provide a more isolated measure of COD ability [
34,
35,
36]. Previous studies, which have shown improvements in COD speed tasks, have only assessed completion times [
6,
7,
8], and these tasks are mainly comprised of linear running and thus, biased towards athletes with superior acceleration and linear speed capabilities [
34,
35]. Conversely, to the best of our knowledge, the present study is the first to monitor changes in CODD in response to a training intervention in youth soccer players. Critically, substantial and meaningful improvements in CODD were demonstrated by the IG (
p ≤ 0.012,
g = −1.63 to −2.43, ~40–52%), which were approximately two times greater than the CG (
Table 3 and
Table 4,
Figure 5 and
Figure 6). These findings highlight the effectiveness of field-based COD speed and technique modification training in youth soccer athletes, which can be achieved in-season with two, twenty-minute sessions per week. This form of training can be simply and easily integrated into the warm-ups of field-based tactical/technical sessions in soccer, highlighting the applicability and feasibility of COD speed and technique modification training.
Cutting is a key action associated with non-contact ACL injuries in soccer [
10,
11,
12] due to the propensity to generate large multi-planar joint loads that can strain and potentially rupture the ACL [
16]. COD technique modification training has been shown to be an effective modality for addressing “high-risk” postures (lateral trunk flexion, lateral foot plant distance, knee flexion angle, internal foot rotation angles) and reducing potentially hazardous knee joint loading when assessed using 3D motion analysis [
13,
25,
26]. Although the present study used a qualitative screening tool (CMAS) to monitor changes in cutting movement quality, the CMAS has been recently validated against the gold standard of 3D motion analysis [
29,
30] which presents as a more practical, less expensive screening tool to implement in applied sporting environments. Nevertheless, the results of this study confirm that COD speed and technique modification training (with feedback and externally directed verbal cues from a coach) resulted in meaningfully lower CMASs post-intervention, which were greater than the SDD (
Table 3 and
Table 4,
Figure 7 and
Figure 8) (
p ≤ 0.025,
g = −0.85 to 1.46, −23% to −34% vs. +6%–19%), while the CG remained unchanged.
Similar to Stroube et al. [
53], which assessed task-specific changes in the tuck jump assessment (i.e., changes in the frequencies of deficits demonstrated) following a neuromuscular training intervention with task-specific feedback, the IG in the present study demonstrated improved cutting movement quality and reductions in frequencies in “high-risk” deficits including lateral trunk flexion, extended knee postures, knee valgus, and improved PFC braking strategies (
Table S2 and Figure S1). These findings are noteworthy because the aforementioned “high-risk” postures are associated with increased knee joint loading [
14,
22] which increases ACL [
15,
54] and PFP injury risk [
55]. Furthermore, these postures are also characteristics of non-contact ACL injury [
10,
11,
56]. As such, six-weeks COD speed training and technique modification with externally focused coaching cues and feedback from a coach is an effective training modality for reducing “high-risk” biomechanical and neuromuscular control deficits, and overall, improving movement quality in male youth soccer players. It is worth noting, however, that CMAS deficits are still demonstrated by youth soccer players (
Table S2 and Figure S1) and thus, they should continue performing mitigation training interventions to address these deficits [
20,
28].
It has been suggested a “performance-injury trade-off” could exist when modifying the COD technique [
14,
24,
27], whereby addressing “high-risk” postures could be detrimental to performance. While Dempsey et al. [
13] reported reductions in knee joint loads due to changes in foot plant distance and trunk position, the authors failed to consider the implications of such changes on cutting performance (i.e., completion time, ground contact time, and exit velocity). Jones et al. [
25] found changes in pivoting technique resulted in lower KAMs and faster completion times in netball players. However, these studies have used 3D motion analysis to monitor changes in COD technique and more importantly, these studies have not contained a CG; therefore, the results should be interpreted with caution. To the best of our knowledge, the present study is the first to consider the effect of COD speed and technique modification training on both performance (completion times, COD deficit) and injury-risk (CMAS) in comparison to a CG, using a field-based screening tool which was performed in a real-world setting. Notably, COD speed and technique modification training with feedback and external verbal coaching cues was effective in improving cutting performance and improving cutting movement quality (
Table 3 and
Table 4), and these were significantly and meaningfully greater than the CG and SDD (
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7 and
Figure 8). Collectively, these findings highlight that COD speed and technique modification in youth soccer players is an effective training modality for enhancing performance and addressing movement deficits associated with increased knee joint loading and potential injury-risk. This finding is noteworthy because, since the introduction of the EPPP, injury rates in youth-soccer have increased three-fold [
31] and as such, reducing biomechanical and neuromuscular risk factors in youth-soccer is considered highly important, particularly as these players are striving for professional contracts.
The COD speed and training intervention focused on modifying biomechanical deficits associated with increased injury-risk [
14,
22,
27] and promoting techniques required for faster performance [
14,
22,
23,
24]. For example, the programme focused on several aspects: a wide foot-plant is required for medio-lateral propulsive impulse generation and subsequent exit velocity during cutting [
22,
24]; faster performance and lower knee joint loading has been associated with increased PFC braking forces [
41]; and trunk lean towards the direction of travel and reduced lateral trunk flexion is associated with faster performance and reduced knee joint loads [
14,
22,
23]. Moreover, knee valgus is also a hazardous “high-risk” posture [
56] with no associated performance benefits [
14,
23,
24] and thus, was a further desired technical change in response to the intervention. Consequently, the IG training programme consisted of COD drills and externally focused verbal coaching cues (“slam on the brakes early”, “push the ground away” and “lean/face towards the direction of travel”) to promote safer mechanics [
42], promote faster performance [
43], and facilitate better retention [
42]. These cues were used in order to evoke technical changes to encourage PFC braking and trunk lean towards the direction of travel, which are techniques associated with faster performance and reduced knee joint loads, while medio-lateral propulsive impulse was also emphasised to promote faster exit velocities. Interestingly, post-intervention, the IG demonstrated lower CMASs (
Table 3,
Figure 7 and
Figure 8), which can be attributed to reduced incidences of CMAS deficits, such as lateral trunk flexion, initial and excessive valgus, hip internal rotation, and lack of PFC braking (
Table S2 and Figure S1), while a wide lateral foot plant remained unchanged. Although these were qualitative evaluations only, it is speculated that the IG demonstrated safer cutting mechanics, and due to the strong relationship observed between CMAS and peak KAM [
29], theoretically, the IG may demonstrate lower knee joint loading, which subsequently reduces non-contact ACL [
15,
54,
56] and PFP injury risk [
17,
55]. Further research is required to determine the effect of COD technique modification on knee joint loading and performance using 3D motion analysis to further substantiate this claim.
Limitations
It should be noted that the present study only investigated a 70° side-step cutting task from a short approach distance (5 m), and thus, is only reflective of low-entry velocity side-step cutting ability. As the biomechanical demands of COD are angle- and velocity-dependent [
27,
36], and other COD actions are also performed, such as crossover cuts, split-steps and pivots, further research is needed to determine the effects of COD speed and technique modification training on CODs from different angles and approach distances, while also investigating different types of COD actions. Additionally, it is worth noting that male youth soccer players were only investigated, as such caution is advised regarding the generalisation of these results to athletes from different athletic populations. Further insight is required into the effects of COD speed and technique modification training in different athletic populations where cutting is a highly prevalent action for performance and also, non-contact ACL injury, such as rugby, American football, and handball. Moreover, it should be noted that there were five dropouts for the IG (
Figure 1) due to match-related injuries or illness. However, the present study still achieved a priori sample size statistical power recommendation and dropouts reflect the environment of performing an in-season training intervention in a “real-world” professional soccer setting. Finally, while improved cutting movement quality was demonstrated following the COD speed and technique modification training, it is unknown whether if and how long the improved motor skill performance can be maintained/ retained for, and is, therefore, a recommended area of future research.