Anne Benjaminse

An ACL injury has profound physical and psychosocial consequences for athletes 4,5 as sports participation most often is reduced, as well as for society in terms of high 19 costs for healthcare. Hence prevention of ACL injuries has received considerable attention over the last decades. Implementation of a universal injury prevention program would save $100 per player per season and would reduce the incidence 33 of ACL injury from 3.0% to 1.1% per season. In general, the implementation of ACL injury prevention programs shows conflicting results, and lacks long-term 30 effectiveness with no reduction of ACL injury rates. 1,12,29 This suggests a missing link between improved movement technique in controlled settings and transfer of these learned techniques to the field or during a game.

One reason might be that efficacy is highly dependent on the compliance of athletes and coaches. Coaches are hesitant to implement ACL injury prevention exercises, given the time investment and lack of performance benefits. 9,18,32,36 They may feel it is too much, not of their primary interest, 10,13,15,17,22,35 does not offer a relative advantage over their existing practices, does not align with their 26 needs or is too complex to implement in their setting. Athletes, on the other hand, use arguments like it “takes too long”, “is boring”, “has no performance 25 benefits” or “is too difficult”. Another reason for achieving suboptimal long-term results might be the way preventative training is given. In the current ACL injury prevention programs athletes are mostly encouraged to focus on the quality of their movements in order to improve awareness and knee control during cutting, 14 jumping and landing. However, telling athletes explicitly how to move may be counterproductive and less suitable for the acquisition of the control required for complex motor skills. 21

The implementation of motor learning principles into ACL injury prevention programs has not been previously investigated. This innovative approach is further pursued in this dissertation. Motor skills can be learned with attention directed to the movement itself (e.g. “flex your knees”), which is defined as an internal focus 37 (IF). Whereas with an external focus (EF), attention is directed towards the effect of the movement (e.g. “imagine you are going to sit on a chair”). Employing instructions 37, 37 and feedback with an EF may enhance automatic movement control, very relevant to training or game situations where unexpected and fast movements are required. In addition, improvement in the delivery of instructions and feedback to the athletes may enhance compliance, improve skill retention and its transfer to sport and optimize program efficiency, resulting in enhanced long-term outcomes. The existing programs have yet to be effectively implemented and need to be sustainable over time in different conditions, for example using wide-spread implementation 31 with high compliance rates and long-term retention. It is therefore suggested that researchers focus on improving and universally implementing these programs. The aim of this dissertation was therefore to contribute to the body of knowledge that may help to optimize ACL injury prevention programs through enhanced motor learning.

In Chapter 1 a systematic review is presented. As females have an increased risk 29 for sustaining an ACL injury, sex differences in ACL injury incidence play an important role in ACL-related research. We wanted to gain insight into these differences from a clinical point of view. Seven studies were included for this review to analyze the results of kinematic, kinetic and neuromuscular patterns during sidestep cutting maneuvers. Kinematic and kinetic sex differences were of questionable clinical relevance and quadriceps dominance (defined as an imbalance between knee extensor and flexor strength, recruitment, and coordination) was not found in females. The question arises whether ACL injuries during plant and cutting maneuvers are purely sex related and whether females have to learn to pivot and cut like males in order to reduce injury risk. The results suggest that future research should look beyond the isolated descriptive female/male comparison. It is important to examine the underlying causes and to question whether the biomechanical and neuromuscular 23 differences really reflect the enhanced risk in females. The descriptive studies give important insight, but cause-effect relationships are not well understood. Future research must focus on examining and understanding the causes underlying these differences in incidence and the best way to target this discrepancy. The largest 29 number of ACL injuries still occur in male athletes. If female and male athletes 2 have their own risk factors, different factors need to be considered when developing injury prevention programs tailored towards individual needs.

In Chapter 2 we provide a theoretical framework from our search for optimizing current ACL injury prevention programs. The advantages of an EF compared to an IF especially seem to benefit athletes as skills acquired with an EF are more resilient 3 under psychological 6,11,24 and physiological fatigue, 20,28 while being more durable 34 and robust when fast movements are required. According to the ‘constrained action hypothesis’, a focus on the movement effect (i.e. EF) promotes the utilization 37 of unconscious or automatic processes. Whereas a focus on the movement itself (i.e. IF) results in a more conscious type of control that constrains the motor system and disrupts automatic control processes, as someone is focusing on his or her 37 own body movements. Learning with an EF, e.g. visual instruction or feedback, may enhance automatic movement control. For athletes it is very relevant and beneficial to learn movement skills that become permanent in all situations.

Optimization of the current prevention programs could be achieved by assisting athletes in finding optimal individual movement patterns by controlling forces belonging to complex motor skills. This is fundamentally different compared to teaching a ’one size fits all’ movement pattern. The effect of immediate visual feedback of the athlete’s own movement technique is an area which is still relatively unexplored. Theoretically this can have better long lasting effects and therefore aid in achieving long-term results. In order to fill the gap between the short-term positive results and the actual reduction of ACL injury rates, individualized feedback might enhance movement patterns better than instructing females ‘to move like males’ based on observed sex differences in descriptive studies. 23

Based on this theoretical framework, we continued by providing practical examples in Chapter 3. Adoption of instructions that induce an EF has important implications for ACL injury prevention as it potentially optimizes retention and transfer to the field. In addition, motor learning with an EF is effective for the establishment of safe 7 movement technique with optimal performance (e.g. jump height). Therefore, the development of novel feedback techniques with an EF component for ACL injury prevention seems promising. For example, this can be accomplished by using real-time, video feedback or dyad training with observation of oneself or a model, while adding positive, EF verbal feedback. Providing athletes individual visual feedback on movement technique allows them to view how they personally perform the movement task and actively problem solve by evaluating their trials and thus find individual ways to achieve optimal movement technique. Within this chapter we have proposed ideas for training staff to use in an attempt to advance ACL injury prevention.

Future research should focus on which, if any combinations of the presented novel techniques in Chapter 3 creates the least dependency on training staff as well as yields good retention and transfer to the field. For ACL injury prevention, it is imperative to reduce knee joint loading strategies during sport-specific tasks such as the complex maneuver of unanticipated sidestep cutting. Therefore, the effect 16 of novel feedback techniques on unanticipated sidestep cutting has been examined in Chapter 4. With this experiment we wanted to better understand the often observed phenomenon of sex differences in movement strategies and investigate how this can be addressed with proper interventions. The purpose of this study was therefore to investigate the effects of an IF (by a verbal stimulus) and an EF (by a visual stimulus) on peak knee joint loading during sidestep cutting over time in 90 female and male recreational basketball athletes. One session with feedback for the visual and verbal group was conducted, with two retention sessions (one week and one month after the first session) with no feedback for either of the groups. The largest effect was seen in the males receiving visual feedback, tending to reduce the frontal plane moments, while increasing the vertical ground reaction force (vGRF) and external knee flexion moment over time. Overall, females tended to keep their knee flexion moments low, potentially placing them at greater risk for injury. Receiving visual feedback works advantageous in potentially reducing injury risk in males considering the increase in knee flexion moment and/or decrease in knee valgus moment, while performance (running speed) was not compromised. Learning new strategies can be hampered in females if they use their muscles less effectively. Perhaps, female athletes need more time to adopt a safe landing strategy. Moreover, it is plausible that females prefer different learning strategies (e.g. combination of visual and verbal feedback). The results of this study suggest that providing individual visual feedback to males may be one method of modifying sidestep movements, with promising results in terms of retention.

To further optimize novel feedback techniques, the last research project was conducted. The effect of attentional focus during a double legged drop vertical jump (DVJ) is investigated in Chapter 5. Self-controlled feedback along with providing expert videos was added to the protocol. The DVJ was evaluated with the real time Landing Error Scoring System (LESS). This practical tool gives an 27 indication of injury risk to evaluate and optimize the DVJ landing technique. Forty recreational athletes performed DVJ’s in five sessions: a pretest (baseline measurement), two training blocks (TRN and TR2), a posttest directly after TRN and TR2 and a retention test one week later. General instructions were provided before the pretest, group specific instructions were offered before TRN and TR2 started and after every 5 trials in TRN and TR2. The IF group received instruction regarding attention to the body, the EF group received instruction directed to the movement effect and the video (VI) group viewed an expert video. In addition, subjects in the IF, EF and VI groups were free to ask for feedback after every DVJ in TRN and TR2, which included their real time LESS score. Subjects were aware that a lower LESS score implied a better landing strategy. This study demonstrated beneficial effects of VI instruction in female and male athletes. Females also benefitted from verbal EF instructions. EF and VI instructions led to significant improvement in landing technique (while jump height was maintained) and remained present one week later. The fact that retention was achieved after a short training session has great potential to improve the effectiveness of current ACL injury prevention programs.

In summary, this dissertation contributed to the understanding of motor learning during complex tasks and an innovative approach in ACL injury prevention is presented. It is advised to consider sex specific EF feedback strategies when incorporating ACL injury prevention exercises. Professionals are advised to give instructions and feedback focusing on the result of the movement (EF) instead of specific body parts (IF) to 1) improve performance of the athlete (e.g. jump distance or jump height) and 2) decrease the risk of ACL injury. When training staff realize that injury prevention and performance enhancement go hand in hand, injury prevention can be viewed as ’part of the game’. 8,22