1. Introduction
In 2017, we developed and validated the Cutting Movement Assessment Score (CMAS) field-based screening tool to evaluate side-step cutting movement quality related to potential non-contact ACL injury risk (23). Since then, interest has grown in this assessment, and we have expanded and updated the CMAS in light of some of the recent feedback we have had from practitioners and researchers (13, 15). As such, in 2021, we published a review containing methodological procedures, operational guidelines, example images, and training recommendations regarding the best practise and implementation of the CMAS tool (15). In this blog, we will highlight the importance of screening cutting movement quality, and advise practitioners as to how this can be done reliably, and accurately, just simply using the high-speed recording capabilities on your smart phone or tablet. We will also highlight some of the key methodologies and guidelines for CMAS screening, and how to best conduct cutting screening to better inform training practices for ACL injury mitigation
2. The ACL injury problem and mechanism
ACL injuries are a debilitating injury, and typically considered the most devastating lower-limb injury an athlete can experience, with a plethora of negative implications, such as financial (cost of surgery, rehabilitation, loss of earning, salary), extensive rehabilitation periods (6-24 months), and substantially greater risk of developing osteoarthritis. ACL injury risk factors are multifactorial (i.e., anatomical, hormonal, biomechanical, neuromuscular, environmental, biopsychosocial, and neurocognitive), but simplistically ACL injuries occur when a catastrophic mechanical load exceeds the ultimate tensile strength of the ligament (Figure 1) (27), or due to a “fatigue failure” mechanism (27). The fatigue failure mechanism entails accumulated high magnitudes and repetitive cycles of knee joint mechanical loading which, without sufficient rest and recovery, can lead to micro damage and subsequent ACL failure (4) from mechanical loads which previously could be tolerated (Figure 2) (2, 7, 16).
Side-step cutting (involving a lateral foot plant of the main execution lower-limb) is a key mechanism associated with ACL injury inciting events in multidirectional sports. For instance, 60-67% of non-contact ACL injuries in rugby (28), American football (22), and handball (32) occur during side-step cutting, while in soccer (36), badminton (25), and Australian rules football (9), side-step cutting is also associated with non-contact ACL injury. These actions have the propensity to generate potentially large and hazardous knee joint loads (i.e., moments / torques of knee flexion, rotation, abduction and translatory moments) during plant foot contact, typically at extended knee postures, which have the potential to increase ACL loading and strain (10). Additionally, high knee joint loads can also lead to the chronic development of patellofemoral pain and other knee injuries (30). The abovementioned knee joint loads are used as surrogates of ACL injury risk, and are typically used for injury screening purposes and to monitor the effectiveness of training interventions (10). Reducing potentially hazardous knee joint loads is considered the most viable strategy to reduce ACL injury risk (3, 10, 17) and other knee joint health related conditions
60-67% of non-contact ACL injuries in rugby, American football, and handball occur during side-step cutting, while in soccer, badminton, and Australian rules football, side-step cutting is also associated with non-contact ACL injury.
3. So why screen movement quality and neuromuscular control deficits?
Potentially injurious knee joint loads are amplified during side-step cutting, and other high impact tasks such as landing and deceleration, when aberrant “high-risk” movement quality and neuromuscular control deficits are displayed, such as knee valgus, lateral trunk flexion, andextended knee postures etc. are displayed in combination (3, 10, 17).
Movement quality and neuromuscular control deficits are modifiable ACL injury risk factors (Figure 3; (6, 21)), which can be targeted through the use of movement quality screening and ACL injury mitigation programmes (15, 17, 34). We believe practitioners have a duty of care and obligation to provide the best sports science and medicine provisions for their athletes, and while most practitioners will evaluate strength, speed, and power characteristics of their athletes, mitigating injury risk and maximising athlete welfare and availability is also fundamental to this. Consequently, evaluating movement quality should be considered a pivotal component of any sports science and medicine screening protocol, and omitting evaluations of cutting or landing movement quality, in our opinion, is doing your athletes a disservice.
Whether we can screen and predict ACL injury is a topical and contentious issue; we are by no means claiming that screening movement quality will definitely predict ACL injuries nor mitigate all non-contact ACL injuries. However, by screening movement quality (as shown in figure 3), this allows practitioners to create an “injury risk profile” of our athletes (i.e., does the athlete display good movement quality and good movement strategy with minimal neuromuscular control deficits?). With this information, practitioners can then devise and implement individualised ACL injury mitigation programmes to address the specific deficits highlighted through the movement quality screening, which could potentially be more effective than generalised injury mitigation training. This process should modify the movement quality (and strategies) and reduce potentially hazardous knee joint loads and thus decrease the relative risk of injury (15). Finally, the potential success of a training intervention should be evaluated by reevaluating movement quality. We suggest that practitioners continually monitor movement quality throughout their cycles of training.
By screening movement quality, this allows practitioners to create an “injury risk profile” of our athletes (i.e., does the athlete display good movement quality and good movement strategy with minimal neuromuscular control deficits?). With this information, practitioners can then devise and implement individualised ACL injury mitigation programmes to address the specific deficits highlighted through the movement quality screening.
Figure 3. Injury screening process
It is central that practitioners use screening tools which are valid and reliable. 3D motion analysis, in this regard, is considered the gold standard for evaluating movement quality, strategy, and neuromuscular control deficits; however, its cost, complexity, time to test, and the required laboratory facilities generally restricts its implementation in the field, particularly when large mass screening of athletes is required. This has led to the implementation of movement quality assessments of landing or slow-velocity tasks related to high-risk deficits linked to potential ACL injury. Qualitative screening tools such as the Landing error scoring system (LESS) (33, 35), Tuck Jump Assessment (TJA) (19, 29, 31), and qualitative analysis of single leg loading screening tool (QASLS) (1, 18) have been developed, and can simply be administered in the field, just requiring video-recordings of the movements and qualitative screening of the athlete’s trial against a screening criteria. It is worth noting that the aforementioned tasks are generally vertically orientated tasks, which typically evaluate landing mechanics. Thus, they may lack specificity to directional changes, with evidence indicating the examination of landing mechanics will not identify athletes who are poor at cutting and vice versa (8, 24, 26). This is where the CMAS comes in.
4. Implementing the CMAS in the field
To overcome the previous limitations, and the need for a side-step cutting screening tool, we have developed the CMAS, which is a qualitative screening tool based on the technical determinants of peak knee abduction moments (Figure 4 & 5). For an excellent overview regarding the rationale and determinants of knee joint loads, please read the systematic review by Donelon et al. (10). The CMAS is a valid and reliable screening tool for evaluating side-step cutting movement quality, using only 2-3 cameras and free video analysis software (13, 15, 23). We have had a lot of success implementing this in the field, consulting with sports teams in Greater Manchester, and generating reports related to cutting movement quality. In addition, we have also monitored the effectiveness of training interventions using the CMAS, observing favourable changes in cutting movement quality post intervention (12). Thomas Dos’Santos is also working with several colleagues at the Football Science Institute who have implemented the CMAS in elite youth soccer players and semi-professional soccer players, so please keep an eye out for this work! Moreover, Chris Thomas has implemented the tool successfully in his role at Aspire Sports Academy, screening over 100 footballers as part of their testing battery to inform training interventions.
4.1 CMAS methodology
The CMAS is a 9-item screening tool; these items relate to penultimate foot contact (PFC) braking strategy, and trunk, hip, knee, and foot postures and motions during the final “plant” step. Practitioners may decide to also include an extra item related to foot contact position at initial contact and we expand on our rationale in our recent review. The tool is presented in Figure 5 above along with strengths and advantages in Figure 6 below.
The CMAS involves filming athletes simply (ideally ≥100 Hz) during a side-step cutting task (30-90°) using cameras placed approximately at hip height in the sagittal plane, frontal plane, and if possible, with an additional camera 20-45° relative to the cut. The CMAS is not intended to be used for cross-over cutting actions or sharper CODs >90° such as pivot tasks. We have some preliminary evidence, in preparation, that 2 cameras could be sufficient in screening movement quality, as long as one of the cameras is a sagittal plane camera. A CMAS screening tool methodology checklist with recommendations is provided in Figure 7. However, for best practise, 3 cameras are recommended following the placements outlined in Figure 8 to best limit parallax error. From our experience, following the above recommendations, we have been able to collect CMAS video footage for 6 trials (3 trials per limb) in groups of athletes as large 12 in less than 15 minutes, highlighting the simplicity and application for large mass screening in contrast to 3D motion analysis which can take 2-3 hours of data collection and analysis per subject.
Figure 7. CMAS methodology recommendation checklist
4.2 Screening the Video footage
Video footage should ideally be viewed using video analysis software, and the athlete’s cutting trial is screened using the CMAS screening tool. Software which enables videos to be played at various speeds, frame-by-frame, can open multiple video windows, create photo-sequences, and also contains drawing tools is also advantageous.
The majority (7-items) of the CMAS criteria follow a dichotomous scale (yes or no), with lateral leg plant distance and frontal/transverse plane trunk positioning involving a description classification, with 3 and 4 possible descriptors, respectively (Figure 5). Athletes are awarded a score if they display any of the items of the CMAS criteria, and greater scores are typically indicative of suboptimal technique and greater peak knee abduction moments. From our experience, it takes 2-3 minutes to screen one trial.
Due to the subjective and qualitative nature of screening, it is integral that you follow the operational definitions highlighted in our recent article that we produced (15). Some key considerations are as follows:
- Testing standardisation is key!
- Ensure staff are familiar and trained using the CMAS.
- Use the operational guidelines we provide in our recent review (15) and some example images are provided in Figure 9 below.
- Departments should perform their own in-house training and collect pilot video footage where they discuss and agree on “low-risk” and “high-risk” movement patterns.
- These should be consensually agreed by all coaches, practitioners, and potential raters within department, team, or institution.
- This is to ensure that high levels of intra- and inter-rater reliability are obtained, and more importantly so that the player obtains accurate, consistent, and reliable evaluations.
5. CMAS interpretation
A higher score is generally indicative of sub-optimal technique and potentially greater non-contact ACL injury risk when using the CMAS, but we are not stating that they will definitely sustain an ACL injury. As such, practitioners can consider using the scores of ≥7 (High), 4-6 (Moderate), and ≤3 (Low) to grade an athlete’s movement quality. We have had recent success using a traffic light system to evaluate and visualise results back to coaches and athletes.
A higher score is generally indicative of sub-optimal technique and potentially greater non-contact ACL injury risk when using the CMAS, but we are not stating that they will definitely sustain an ACL injury.
- Practitioners should note that sub-optimal and unsafe technique may still be displayed during trials with low CMASs; therefore, it is advised to focus on the CMAS criteria where athletes scored deficits. For example, an athlete with a low CMAS may still display “high-risk” cutting deficits, such as knee valgus, lateral trunk flexion, limited knee flexion, or hip internal rotation. Therefore, in this scenario, the athlete would still warrant specific injury risk mitigation training and conditioning.
- Athletes with low scores could progress to more complex screening tasks such as unplanned / externally directed attention cuts.
- Practitioners are recommended to use the CMAS to assist in identifying “high-risk” cutting deficits in their players which can be used to inform and help develop individualised training programmes to modify the specific deficits and mitigate potential ACL injury risk.
- Readers are encouraged to read specific reviews regarding training strategies to mitigate non-contact ACL injury (5, 11, 14, 20, 34) and look out for future blogs on this!
6. Summary and practical applications
- Side-step cutting is a skill and we have a duty of care to evaluate movement quality.
- We need to bring biomechanical / screening assessments to the field.
- In addition to screening landing mechanics, we should evaluate side-step cutting movement quality in multidirectional sports players (depending on the sport’s needs’ analysis).
- The CMAS offers practitioners a cost-effective and easily applicable field-based method to identify athletes who generate high knee joint loads and poor movement quality during side-step cutting. This can be simply done using the high-speed recording capabilities on smart devices (i.e., tablets and phones).
- The CMAS can be used to identify specific deficits that can be targeted through biomechanically and neuromuscular informed training interventions to mitigate potential ACL injury risk.
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