Novel methods of instruction in ACL injury prevention programs, a systematic review

Accepted Manuscript

Novel methods of instruction in ACL injury prevention programs, a systematic review

Anne Benjaminse, Wouter Welling, Bert Otten, Alli Gokeler

PII: S1466-853X(14)00040-6

DOI: 10.1016/j.ptsp.2014.06.003

Reference: YPTSP 613

To appear in: Physical Therapy in Sport

Received Date: 26 December 2013

Revised Date: 4 May 2014

Accepted Date: 11 June 2014

Please cite this article as: Benjaminse, A., Welling, W., Otten, B., Gokeler, A., Novel methods ofinstruction in ACL injury prevention programs, a systematic review, Physical Therapy in Sport (2014),doi: 10.1016/j.ptsp.2014.06.003.

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http://dx.doi.org/10.1016/j.ptsp.2014.06.003

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1) Clinicians are encouraged to use an EF to reduce the risk of ACL injury.

2) Performance-enhancing benefits when using an EF should be addressed to

coaches.

3) ACL injury prevention programs could potentially be improved when using

an EF

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Novel methods of instruction in ACL injury prevention programs, a

systematic review

Anne Benjaminse

University of Groningen, University Medical Center Groningen, Center for Human

Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands

School of Sport Studies, Hanze University Groningen, Zernikeplein 17, 9747 AS,

Groningen, The Netherlands

[email protected]

Wouter Welling



[email protected]

Bert Otten



[email protected]

Alli Gokeler



[email protected]

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Corresponding author:

Anne Benjaminse



[email protected]

tel +31-50-3639148

fax +31-50-3633150

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Novel methods of instruction in ACL injury prevention programs, a systematic 1

review 2

3

ABSTRACT 4

Background: Anterior cruciate ligament (ACL) injury prevention programs have 5

been successful in the short term. Motor learning strategies with an internal 6

focus (IF) to body movements have traditionally been utilized, but may be less 7

suitable than an external focus (EF) for the acquisition and control of complex 8

motor skills required for sport. 9

Objective: To investigate the available literature and provide an overview of the 10

effect of IF and EF instructions on jump landing technique. 11

Methods: Systematic searches were conducted in PubMed (1966 to May 12

2014), CINAHL (1981 to May 2014) and PsycInfo (1989 to May 2014). A 13

priori defined inclusion criteria were: (i) full text; (ii) published in 14

English, German or Dutch; (iii) healthy adult subjects (mean age ≥18 15

years); (iv) jump and landing performance tested and (v) study used 16

comparison between an EF and IF. Performance (jump height and 17

distance) and technique (kinematics and kinetics) were the primary 18

outcome variables of interest. 19

Results: Nine papers were included. Significant better motor performance and 20

movement technique was found with an EF compared to an IF. 21

Conclusions: Considering the beneficial results in the included studies when 22

utilizing an EF, it is suggested to implement these strategies into ACL injury 23

prevention programs. 24

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Keywords: ACL injury prevention, external focus, jumping performance, motor 25

learning 26

INTRODUCTION 27

Anterior cruciate ligament (ACL) injury prevention training has been shown to 28

the risk of injury (Gagnier, Morgenstern, & Chess, 2013; Postma & West, 2013; 29

Stojanovic & Ostojic, 2012; Yoo, Lim, Ha et al., 2010). These programs entail a 30

combination of interventions like strengthening, stretching, agility, balance and 31

feedback training to enhance proper technique (Alentorn-Geli, Myer, Silvers et 32

al., 2009b). Motor learning strategies with an internal focus (IF) to body 33

movements (e.g. ‘flex your knee’) have traditionally been utilized in ACL injury 34

prevention programs (Irmischer, Harris, Pfeiffer et al., 2004; Myklebust, 35

Engebretsen, Braekken et al., 2003). Although these programs have been 36

successful, there is an ongoing discussion on how to improve long terms 37

results and reduce the ACL injury incidence (Sadoghi, von Keudell, 38

Vavken, 2012; Stojanovic & Ostojic, 2012; Sugimoto, Myer, McKeon et 39

al., 2012). An ACL injury increases the risk of subsequent ACL injuries as well 40

as early onset of osteoarthritis (Oiestad, Holm, Aune et al., 2010; Oiestad, Holm, 41

Engebretsen et al., 2012, Paterno, Rauh, Schmitt et al., 2014). This high 42

prevalence of adverse long-term outcomes and the suboptimal long term 43

outcomes support the need for optimizing current ACL injury prevention 44

strategies. 45

The long-term outcomes of ACL injury prevention programs could 46

potentially be improved when instructions with an external focus (EF) are 47

incorporated instead or in combination with an IF (Benjaminse et al., 2014). An 48

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EF directed to the outcome of movements has been shown to enhance skill 49

acquisition more efficiently and increase the potential to transfer to sport with its 50

complex motor skills (Wulf, 2012). This type of motor learning is more resistant 51

to the realistic effects that happen during a training or game, like fatigue and 52

psychological and physiological stress. Furthermore, retention and transfer is 53

facilitated as learning with an EF seems to be more durable (Allen & Reber, 54

1980) and more robust (Turner & Fischler, 1993), especially when a fast 55

response is required. More durable means better retention in terms of better task 56

performance over time. Robustness is the resilience against fatigue (Masters, 57

Poolton, & Maxwell, 2008a; Masters, Poolton, & Maxwell, 2008b; Poolton, 58

Masters, & Maxwell, 2007a) and stress (Hardy, Mullen, & Jones, 1996; Hardy, 59

Mullen, & Martin, 2001; Masters, Poolton, Maxwell et al., 2008; Ong, Bowcock, & 60

Hodges, 2010). 61

Evidence has also emerged that an external focus of attention has a 62

positive effect on the actual performance and learning strategies of different 63

motor skills, like vertical jumping (Wulf & Dufek, 2009; Wulf, Dufek, Lozano et 64

al., 2010; Wulf, Zachry, Granados et al., 2007), long jump (Makaruk, Porter, 65

Czaplicki et al., 2012; Porter, Anton, & Wu, 2012; Porter, Ostrowski, Nolan et al., 66

2010; Wu, Porter, & Brown, 2012), balance (Laufer, Rotem-Lehrer, Ronen et al., 67

2007), kicking a soccer ball (Wulf, McConnel, Gartner et al., 2002), throwing a 68

basketball (Al-Abood, Bennett, Hernandez et al., 2002) and golf putting (Poolton, 69

Maxwell, Masters et al., 2006; Shafizadeh, McMorris, & Sproule, 2011; Wulf & 70

Su, 2007). 71

More relevant for ACL injury prevention, it has been shown that adoption 72

of an EF results in more efficient EMG activity compared to adopting an IF 73

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(Hollmann & Hettinger, 2000; Vance, Wulf, Tollner et al., 2004; Zachry, Wulf, 74

Mercer et al., 2005), greater knee flexion (Makaruk et al., 2012; Onate, 75

Guskiewicz, Marshall et al., 2005), more CoM displacement (Wulf et al., 2007) 76

and reduced vertical ground reaction force (GRF) (McNair, Prapavessis, & 77

Callender, 2000; Onate, Guskiewicz, & Sullivan, 2001; Wu et al., 2012). 78

Cumulatively these factors are related to reduction of the risk of ACL injury 79

during jumping and landing (Alentorn-Geli, Myer, Silvers et al., 2009a). 80

In summary, adopting an EF seems to be more effective in motor learning 81

because it directs the athlete’s attention away from their own movements and 82

shifts it towards the outcome of movements (Wulf, 2012). A continued IF may be 83

detrimental to motor learning as conscious control of movements decreases 84

automaticity in the motor control system (Wulf, Chiviacowsky, Schiller et al., 85

2010). Athletes who consciously intervene in the control of their movements, i.e. 86

using an IF, seem to constrain the motor system and degrade the natural 87

movement (Hardy et al., 2001; Masters, Poolton, Maxwell, et al., 2008). There is 88

evidence available in terms of the positive effects of EF in regards to motor 89

learning (Wulf, 2013). Less is however known about the effects of exercises that 90

adopt an EF in ACL injury prevention training programs. The purpose of this 91

systematic review was therefore to investigate the available literature and 92

provide a comprehensive overview of the effect of IF and EF on jump landing 93

performance and technique in healthy subjects, in order to optimize 94

current ACL injury prevention programs and decrease ACL injury risk. 95

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METHODS 97

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Literature search 98

A systematic literature search was conducted with the databases 99

PubMed (1966 to May 2014), CINAHL (1981 to May 2014) and PsycInfo 100

(1989 to May 2014) (Table 1). The results of the three searches were taken 101

together and duplicates were removed. Furthermore, reference lists were 102

screened to find additional studies. All titles and abstracts were analyzed by the 103

second author (W.W.). Full texts of potential relevant studies were analyzed for 104

final inclusion by two authors independently (A.B. and W.W.). Studies were 105

included based on the following inclusion criteria: (i) full text; (ii) published in 106

English, German or Dutch; (iii) healthy adult subjects (mean age ≥18 years); 107

(iv) jump and landing performance tested and (v) study used comparison 108

between an EF and IF (Figure 1). 109

Data extraction 110

The following study characteristics were extracted from each included study: the 111

subject characteristics, jump task, the internal- and external instruction used, 112

study design, outcome measures of interest (jump performance (jump height 113

and distance) and/or jump technique (kinematics and kinetics)) and the 114

key findings. Cohen’s effect size (ES) statistics (Cohen’s d) were calculated to 115

determine the magnitude of observed significant performance differences with 116

d=0.2–0.5, d=0.5–0.8 and d≥0.8 representing a small, moderate and large 117

effect, respectively (Cohen, 1988). 118

Methodological quality 119

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A modified version of the Cochrane Group on Screening and Diagnostic Test 120

Methodology (Deville, Buntinx, Bouter et al., 2002) list was used independently 121

by the first two authors (A.B. and W.W.) to appraise the included studies and 122

assess any risk of bias. Questions pertaining level of evidence, use of control 123

group, similarity at baseline, selection criteria and demographic information of 124

subjects and statistical analysis were added. The maximal score of the modified 125

list that could be reached was 16. 126

127

RESULTS 128

Methodological quality and study characteristics 129

The combined search yielded a result of 1651 studies in PubMed, 130

589 in PsycInfo and 360 studies in CINAHL. After excluding 2565 studies 131

based on screening the titles and abstracts for the predefined in- and 132

exclusion criteria, nine studies were included in this systematic review 133

(Bredin, Dickson & Warburton, 2013; Makaruk et al., 2012; McNair et al., 2000; 134

Porter et al., 2012; Porter, Ostrowski, et al., 2010; Wu et al., 2012; Wulf & 135

Dufek, 2009; Wulf, Dufek, et al., 2010; Wulf et al., 2007). Reasons for exclusion 136

and flow diagram of the search strategy are shown in Figure 1. The 137

methodological quality of the included studies is shown in Table 2. The mean 138

score on the modified scoring list was 11.6 (range 9.5-14). The number of 139

included subjects per study ranged from 8 to 120. Examined tasks were the 140

maximal vertical jump (Bredin et al., 2013; Wulf & Dufek, 2009; Wulf, Dufek, et 141

al., 2010; Wulf et al., 2007), standing long jump (Makaruk et al., 2012; Porter et 142

al., 2012; Porter, Ostrowski, et al., 2010; Wu et al., 2012), the 143

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countermovement jump (Makaruk et al., 2012) and double legged drop jump 144

(Makaruk et al., 2012; McNair et al., 2000). None of the included studies was 145

performed blindly. Some studies did not report selection criteria in detail (Wu et 146

al., 2012; Wulf & Dufek, 2009; Wulf, Dufek, et al., 2010; Wulf et al., 2007), full 147

demographic information (Wulf & Dufek, 2009) or did not use reference jumps 148

(Porter, Ostrowski, et al., 2010; Wu et al., 2012; Wulf & Dufek, 2009; Wulf, 149

Dufek, et al., 2010; Wulf et al., 2007) or a control group (Porter, Ostrowski, et 150

al., 2010; Wu et al., 2012; Wulf & Dufek, 2009; Wulf, Dufek, et al., 2010) to 151

compare the effects of instructions provided. Five of the studies did describe the 152

most important confounders (Bredin et al., 2013; Makaruk et al., 2012; McNair 153

et al., 2000; Wu et al., 2012; Wulf & Dufek, 2009). Studies that used a within-154

subjects design did get a point when using a counterbalanced order of 155

conditions. The study of Wulf et al. (2007) used two experiments of which one 156

was counterbalanced between all three conditions (EF, IF and control group). 157

Also, Bredin et al. (2013) did not counterbalance between all three conditions. A 158

between-subjects or a within-subjects design was used in the studies. A variety 159

of EF instructions were provided that included verbal, analogy and auditory 160

modes. Table 3 shows the study characteristics of these studies and all 161

variables for which data were sought. Table 4 (Performance), 5 (Technique) 162

and 6 (EMG) show the results per study. Unfortunately, no meta-analysis 163

could be conducted due to the heterogeneity of the included type of 164

tasks. 165

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Motor learning with verbal instructions 168

Several studies showed the effectiveness of verbal instructions with an EF 169

(Bredin et al., 2013; Makaruk et al., 2012; Porter, Ostrowski, et al., 2010; Wu et 170

al., 2012; Wulf & Dufek, 2009; Wulf, Dufek, et al., 2010; Wulf et al., 2007). 171

For example, jump height was enhanced when subjects were instructed to 172

concentrate on the rungs of the Vertec (EF) instead of to concentrate on the tips 173

of their fingers (IF) when reaching as high as possible during the height jumps 174

(Bredin et al., 2013; Wulf & Dufek, 2009; Wulf, Dufek, et al., 2010; Wulf et al., 175

2007). 176

The effect of attentional focus on jump distance was determined in several 177

studies (Porter et al., 2012; Porter, Ostrowski, et al., 2010; Wu et al., 2012). 178

Instructing subjects to jump towards a target (EF) resulted in superior jump 179

distance compared to instructing subjects to extend their knees as rapidly as 180

possible (IF) (Porter 2010, Wu 2012). In addition, Porter et al. (2012) used two 181

types of an EF in a standing long jump. First, the external near condition focused 182

on jumping as far behind the start line as possible and second, the external far 183

condition focused on jumping as close as possible to a cone (ie. a target farther 184

from the body). Based on the results, the external far condition resulted in a 185

greater jumping distance compared to the external near condition (Porter et al., 186

2012). 187

One study compared an IF and EF with a control group in a training 188

protocol consisting of a nine-week training intervention (Makaruk et al., 2012). 189

Standing long jump, counter movement jump and drop jumps were tested pre- 190

and post intervention. An EF resulted in better performance and safer landing 191

technique (i.e. more knee flexion) on the standing long jump and the counter 192

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movement jump but not on the drop jump. The focus instructions of the three 193

different jumps are presented in Table 3. 194

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Analogy motor learning 196

The study of McNair et al. (2000) used a metaphorical analogy perspective 197

(Table 3). Prior to jumping from 30 cm height on a force plate, the subjects were 198

asked to picture one of these statements and feel like that object when they 199

performed their jumps. This metaphorical use of analogy did not lead to 200

significant lower GRF compared to baseline jumps. 201

202

Auditory motor learning 203

McNair et al. (2000) instructed subjects to listen to the sound of their landing 204

(EF) and use that information to assist them to land more softly from a jump 205

from a box of 30 cm height on a force plate. These instructions resulted in 206

significant lower GRF compared to the control group. 207

208

DISCUSSION 209

The purpose of this systematic review was to provide a comprehensive overview 210

of the effect of IF and EF on jump landing performance, to increase the body of 211

knowledge for ACL injury prevention programs. The results from the current 212

review suggest the potential to enhance ACL injury prevention programs by 213

adopting EF verbal instructions, analogy techniques and the use of auditory cues. 214

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The mean methodological quality score was 11.8 (range 9.5-14) (Table 2). 215

It is recognized that this modified scoring list is arbitrary. However, the authors 216

felt that weighing the included studies’ scoring was necessary to compare across 217

studies. To add insight relative to the strength of the differences noted between 218

the variables of interest, the ES was calculated. The mean ES was 0.77 (SD 219

0.49) and can be considered moderate. The results included many different 220

outcome variables like reduced movement noise, less electromyographic (EMG) 221

activity, less co-contraction, lower GRF, more knee flexion and more CoM 222

displacement after adopting an EF. These results are in favor of decreasing the 223

risk of an ACL injury (Onate et al., 2005; Sarafrazi et al., 2012). 224

225

The current review showed when athletes adopt an EF in jumping a 226

decrease in GRF during landing which could result in a lower ACL injury risk 227

(McNair et al., 2000; Onate et al., 2001). Most ACL injuries occur with the knee 228

near full extension during a sharp deceleration or while landing a jump (Smith, 229

Ford, Myer et al., 2007), which generates significantly higher GRFs than a soft 230

landing. Landing with a larger knee flexion angle decreases forces on the ACL 231

and therefore potentially reduce the risk of an ACL injury (Devita & Skelly, 1992; 232

McNitt-Gray, 1991; McNitt-Gray, Hester, Mathiyakom et al., 2001). Recent 233

research has shown that instructions that provide an EF result in an increase in 234

knee flexion angles when landing (Makaruk et al., 2012). Increased knee flexion 235

angles will also generate more CoM displacement and can result in lower 236

posterior and vertical GRFs (Blackburn & Padua, 2009), probably caused 237

by the muscles being in a more advantageous position to absorb kinetic 238

energy (Podraza & White, 2010). In addition, large knee flexion angles result 239

in a potential of an increased maximum force production or a more optimized 240

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coordination (Wulf & Dufek, 2009; Wulf et al., 2007). An increase in maximum 241

force production will lead to better performance (jumping distance or jumping 242

height). 243

All included studies, except the one from McNair et al. (2000), have 244

demonstrated that using verbal instructions targeted on a cue (eg. Vertec ring or 245

cone) to direct a performers’ attention externally, significantly enhances jump 246

landing performance. Furthermore, research showed that adding a distance 247

effect, which means the attention is directed further away from the body, 248

enhances motor learning and performance (McNevin, Shea, & Wulf, 2003; Porter 249

et al., 2012). There is evidence pertaining beneficial effects of offering an EF in a 250

plyometric training protocol (Makaruk et al., 2012). As jump landing technique 251

improved, these results are promising for implementing a verbal EF in ACL injury 252

prevention programs. 253

Even though only one study was included on auditory cues, the use of an 254

auditory cue may be successful to decrease potential risk factors of an ACL 255

injury, as other studies show. Instructing subjects to listen to the sound of their 256

landing and use that information to land more softly in subsequent landings 257

resulted in a significant decrease in GRF (McNair et al., 2000). This is in 258

agreement with a study that showed a 20% to 26% decrease in peak GRFs when 259

auditory cues are provided (Prapavessis, McNair, Anderson et al., 2003). The 260

latter study was done with children (age 9±0.89 years) and therefore, not 261

generalizable to adults. Furthermore, Mizner et al. (2008) showed that an IF (i.e. 262

bend your knees when landing) in combination with an auditory cue (minimize 263

the sound of the landing) resulted in a significant longer landing time, lower GRF 264

after instructions, greater peak knee flexion angles, less knee valgus angle and 265

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lower external knee abduction moments comparing pre- and post-instruction 266

(Mizner, Kawaguchi, & Chmielewski, 2008). 267

The metaphorical cue used by McNair et al. (2000) to investigate the 268

effects of analogy techniques, did not result in decreased GRF. This can possibly 269

be explained by the subjects having difficulty with identifying with these 270

statements. Instructions should be simple as complex feedback can 271

hamper motor learning (Marchant, Clough, & Crawshaw, 2007). However, 272

analogy instruction when presented in the right context, could lead to 273

improvements in motor skills. Olsson et al. (2008) trained athletes experienced 274

in high jumping over a 6 week period (Olsson, Jonsson, & Nyberg, 2008). In 275

several mental imagery sessions, jumping instructions were presented in which 276

subjects had to imagine specific parts of their jumps like the take-off. This type 277

of imagery lead to significant improvement on bar clearance suggested that 278

imagery may help to improve a critical component of a complex motor skill. 279

Imagery could also reduce ACL injury risk factors in more controlled conditions. 280

Imagery (ask subjects to close their eyes, focus on the ground and imagine the 281

correct landing task (ie. positive feedback)) could also increase knee flexion 282

angles and decrease knee valgus angles (Sarafrazi, Abdulah, & Amiri-Khorasani, 283

2012). It should be noted that this imagery technique was both environmental- 284

and body oriented. 285

Benefits of adopting an EF are not only seen relative to IF conditions, but 286

also in comparison to control conditions (Marchant, Greig, & Scott, 2009; Wulf, 287

Weigelt, Poulter et al., 2003). Despite the obvious benefits, athletes are inclined 288

to adopt an IF even when they are not explicitly instructed to do so (Porter, Wu, 289

et al., 2010). This is an interesting and underexposed given clinicians and 290

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coaches should be aware of. Also, coaches mostly use an IF (84,6%) to instruct 291

their athletes which shows that coaches might not be familiar with the beneficial 292

effects of using an EF (Porter, Wu, et al., 2010). It has been suggested that 293

coaches view motor learning as too theoretically driven (Ericsson & Williams, 294

2007). There is a need for better implementation and education materials for 295

clinicians and coaches that highlights relevant motor learning issues (Steffen, 296

Meeuwisse, Romiti et al., 2013; White, Otago, Saunders et al., 2013). 297

An IF results in an increase of co-contraction of agonists and antagonists, 298

which in turn may cause “freezing” by limiting the degrees of freedom of 299

movements, and in the recruitment of unnecessary motor units within muscles, 300

which adds “noise” to the motor system (Lohse & Sherwood, 2012). An EF 301

enhances movement economy while maintaining or even improving performance 302

(Vance et al., 2004; Zachry et al., 2005). Movements will be a more automatic, 303

reflex-type mode of control that leads to faster and more finely tuned movement 304

responses, potentially reducing the risk of an ACL injury. 305

In addition to the findings of the included studies, motor learning with an 306

EF is suggested to be more resilient under psychological (Beilock & Carr, 2001; 307

Gray, 2004) and physiological fatigue (Masters, Poolton, et al., 2008a; Poolton, 308

Masters, & Maxwell, 2007b) conditions. Because fatigue has been proposed to be 309

a contributor to ACL injuries (Benjaminse, Habu, Sell et al., 2008; McLean, Fellin, 310

Suedekum et al., 2007; Rodacki, Fowler, & Bennett, 2001), it is suggested that 311

the decreasing capacity of controlling body movements when fatigued or 312

stressed will be less prominent when appropriate landing techniques have been 313

taught with an EF. 314

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Instructions recommendations in ACL injury prevention programs 316

Clinicians and coaches should keep the consequences of their instructions in 317

mind when instructing an athlete. Verbal instructions have a direct effect on 318

motor performance and therefore effect the risk for sustaining an ACL injury 319

(Benjaminse et al., 2014). When instructions given are focussed on a target 320

outside someone’s body (EF) (eg. a cone for distance jumping, a Vertec ring for 321

vertical jumping or watching someone performing the task) all included studies 322

found superior results compared to instructions given are internally directed (IF) 323

(eg. related to the position of specific body parts). Particularly, increasing the 324

distance of an external focus of attention relative to the body, performance 325

increases (Wu, Porter 2012), without applying injurious loads at the knee. 326

In addition to the findings of this review, numerous research show that 327

visual feedback is an efficient tool in ACL injury prevention by improving knee 328

kinetics and kinematics (Myer, Stroube, DiCesare et al., 2013; Onate et al., 329

2005; Onate et al., 2001; Parsons & Alexander, 2012). These studies did not use 330

an IF or control group to compare these effects or presented a combined IF and 331

EF at a time. However, all augmented feedback groups produced a positive 332

change in landing biomechanics. Another way of a visual offer of an EF is to 333

instruct subjects through demonstration and practice (dyad training) 334

(Benjaminse et al., 2014). It has also been shown that subjects significantly 335

decrease GRF compared with baseline jumps after EF instructions and through 336

demonstration by a coach and practice of the perfect landing technique (Cronin, 337

Bressel, & Fkinn, 2008). 338

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In addition, it is worthwhile to examine positive and self-controlled versus 339

pre-determined feedback schedules in future research (Andrieux, Danna, & Thon, 340

2012). Positive feedback enhances interest and enjoyment and encourages 341

subjects to raise their goals and expectancies for future performance (Singer & 342

McCaughan, 1978; Wulf, Shea, & Lewthwaite, 2010). Negative feedback may 343

hamper motor learning and be less effective (Chiviacowsky & Wulf, 2007; Wulf, 344

2012; Wulf & Lewthwaite, 2010). It provides negative competence information 345

and therefore, decreases intrinsic motivation and beliefs about personal 346

capability (Badami, VaezMousavi, Wulf et al., 2011; Chiviacowsky & Wulf, 2002, 347

2005). During ACL injury prevention programs, it would therefore be effective to 348

give feedback after good instead of after poor trials and address the correct 349

instead of faulty movement patterns. Furthermore, self-controlled feedback 350

schedules benefit learning because they are more tailored to the performers’ 351

needs (Wu & Magill, 2011; Wulf, Raupach, & Pfeiffer, 2005) and therefore, these 352

schedules will increase motivation and active involvement in the learning process 353

(Andrieux et al., 2012; Chiviacowsky, Wulf, & Lewthwaite, 2012). 354

Study limitations 355

This systematic review provides an overview of the literature regarding different 356

ways of offering an EF in combination with a jump-landing task. However, some 357

limitations need to be addressed. All but one (Makaruk et al., 2012) studies 358

included both male and female athletes, which makes it difficult to compare 359

results between gender and make gender specific recommendations, which is 360

desirable (McLean, 2008). Future research aimed at instructions and feedback 361

optimization for ACL injury prevention programs should examine males and 362

females separately. 363

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Some included studies used relatively small groups in their experiments 364

(Wulf & Dufek, 2009; Wulf, Dufek, et al., 2010; Wulf et al., 2007). All subjects 365

were older than 18 years, which means we cannot generalize the results to 366

children. However research showed that adopting an EF is also beneficial for 367

children (Chiviacowsky, Wulf, & Avila, 2013; Chiviacowsky, Wulf, de Medeiros, 368

Kaefer, & Tani, 2008; Chiviacowsky, Wulf, de Medeiros, Kaefer, & Wally, 2008; 369

Chow, Koh, Davids et al., 2013; Prapavessis et al., 2003). This is promising, as 370

children are at an optimal age for motor skill learning. 371

None of the included studies was performed blinded which could make the 372

study expectation a confounding factor. Lastly, the use of within and between 373

subject designs could have caused or prevented different confounding factors in 374

the included studies (eg. no inter-individual differences in a within subject 375

design, however earlier instructions can affect study results if not 376

counterbalanced). 377

378

Practical implementation in the field 379

In light of the current evidence, clinicians, athletic trainers and coaches are 380

encouraged to use an EF to 1) increase the athlete’s performance and 2) reduce 381

the risk of ACL injury. This outcome confirms and can be used along with 382

the advice to emphasize performance-enhancement benefits when 383

implementing ACL injury prevention programs (Joy et al., 2013). All 384

included studies found superior results for using an EF in relation to IF or no 385

instruction. Instructions related to technique for increased safety and 386

performance during jumping and landing should be established within training 387

regimens. Based on this review, it is advised to use EF instructions, which may 388

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17

have the potential to further reduce risk factors for ACL injury. Verbal, auditory 389

and analogy instructions do not require specialized equipment, making them 390

simple and cost effective to implement. Visual feedback on the other hand could 391

be effectively delivered with the use of a simple handy cam or with dyad training. 392

Physical therapists, athletic trainers and coaches should be aware of the benefits 393

of positive and self-controlled feedback. As several studies demonstrate a 394

missing link between the literature and the practical field (Durham, Van 395

Vliet, Badger et al., 2009; Porter, Wu, & Partridge, 2010), we hope this 396

review contributes in helping clinicians, athletic trainers and coaches to 397

implement these recommendations. Future research should focus on 398

optimizing the implementation of EF instruction in ongoing ACL injury prevention 399

programs as part of regular training sessions to decrease the risk of an ACL 400

injury (Benjaminse et al., 2014). 401

402

403

• Ethical Approval: Not applicable 404

• Funding: Not applicable 405

• Conflict of Interest: There are no conflicts of interest406

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Table 1 Search terms per database

Pubmed (1966 to May

2013)

Cinahl (1981 to May 2013)

PsycINFO (1989 to May

2013)

(jump*[tiab]) AND ("Attention"[Mesh] OR external focus[tiab] OR Landing forces [tiab] OR concentration[tiab] OR attention[tiab] OR video [tiab] OR imagery [tiab])

(TI jump* OR AB jump*) AND (Attention OR TI external OR TI Landing forces OR AB Landing forces OR AB external OR TI focus OR AB focus OR TI concentration OR AB concentration OR TI attention OR AB attention OR TI video OR AB video OR TI imagery OR AB imagery)

(TI jump* OR AB jump*) AND (Attention OR TI external OR TI Landing forces OR AB Landing forces OR AB external OR TI focus OR AB focus OR TI concentration OR AB concentration OR TI attention OR AB attention OR TI video OR AB video OR TI imagery OR AB imagery)

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Table 2 Methodological quality of the included studies.

Study Level of evidencea

Randomizationb Control groupc

Similarity baselined

Selection criteriae

Settingf Demographic informationg

Blindedh Statistical analysisi

Percentage missingj

Outcome variablesk

Confoundersl Total score (max = 16)

McNair et al. 2000

4 1 1 1 1 1 1 0 1 1 1 1 14

Wulf et al. 2007

3 1/21 1 0 0 1 1 0 1 1 1 0 91/2

Wulf et al. 2009

4 1 0 0 0 1 0 0 1 1 1 1 10

Porter et al. 2010

4 1 0 0 1 1 1 0 1 1 1 0 11

Wulf et al. 2010

4 1 0 0 0 1 1 0 1 1 1 0 10

Makaruk et al. 2012

4 1 1 1 1 1 1 0 1 1 1 1 14

Porter et al. 2012

4 1 1 0 1 1 1 0 1 1 1 0 12

Wu et al. 2012

4 1 0 1 0 1 1 0 1 1 1 1 12

Bredin et al. 2013

3 0 1 1 1 1 1 0 1 1 1 1 12

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a. Oxford Center for Evidence Based Medicine levels of evidence (level 1 = 5 points; level 2 = 4 points; level 3 = 3 points; level 4 = 2 points; level 5 = 1 point).

b. Acceptable method of randomization or counterbalanced within-subjects design (1 point).

1. Note: study of Wulf et al. (2007) used two experiments, only one was counterbalanced between all conditions.

c. The use of a control group (1 point).

d. Groups at baseline were similar (1 point).

e. Inclusion and exclusion criteria clearly described (1 point).

f. Described treatment protocol (replicable) (1 point).

g. Age (mean or median and standard deviation or range) and gender reported (1 point).

h. Assessor, subjects and intervention is blinded (1 point).

i. Statistical analysis: for variable of interest details given on mean or median, standard deviation or confidence intervals and predictive value (1 point).

j. All included subjects measured and, if appropriate, missing data or withdrawals from study reported or explained (1 point).

k. Outcome variable clearly defined and method of examination of outcome variable adequate (1 point).

l. Most important confounders and prognostic factors identified and adequately taken into account in design study (1 point).

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Table 3 Study characteristics including an internal- and external focus of attention in jumping performance.

Study Participants Task Internal focus External focus Design Feedback Outcome measures

Key

findings

McNair et al. 2000.

80 (27 male, 53 female); mean age 24.0 ± 7.0 years.

Jump 8 times from a box of 300 mm in height on a force plate.

“When you do your next jump, position yourself on the balls of your feet with bent knees just prior to landing. On landing, lower the heels slowly to the ground and bend the knees until well after the landing”.

Auditory cue: “Listen to the sound of your landing and use that information to assist them to land more softly in subsequent landings”.

Imagery rehearsal:: “bubbles floating down toward the ground”; “feathers floating down towards the ground”; “leaves floating down towards the ground”; snowflakes floating down towards the ground”.

Randomized between subjects design.

Not described. GRF Auditory cue and lower limb instructions are effective ways to sig. reduce GRF. No sig. difference in imagery- and control group.

Wulf et al. 2007.

Exp 1.

Ten (1 male and 9 female); mean age 23.0 ± 5.81 years.

Jump five times as high as possible by hitting the Vertec..

“Concentrate on the tips of your fingers, reaching as high as possible during the jumps”.

“Concentrate on the rungs of the Vertec, reaching as high as possible during the jumps”.

Within-participant design.

Not described. Jump-and-reach height.

An external focus results in a sig. higher jump-and-reach height than an internal focus or no focus instruction.

Wulf et al. 2007.

Exp 2.

Twelve (5 male and 7 female); mean age 23.6 ± 1.16 years.

Jump five times as high as possible by hitting the Vertec..




Not described. Jump-and-reach height, CoM displacement.

An external focus results in sig. higher jump-and-reach height and sig. more CoM displacement compared to internal focus or no focus instruction.

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Wulf et al. 2009.

Ten students (4 male and 6 female); age range 20–30 years.

Jump ten times as high as possible by hitting the Vertec.




Frequency: several times between trails. Positive/negative: negative, only asymmetric landings were noticed. Self-controlled (y/n): n

Jump-and-reach height, force production, CoM displacement.

An external focus results in a sig. higher jump-and-reach height, more force production and more CoM displacement compared to internal focus

Porter et al. 2010.

One-hundred- and-twenty (two groups): internal focus (n=60); age 22.12 ± 4.05 years and external focus (n=60); age 21.72 ± 2.72 years.

Five standing long jumps.

‘‘When you are attempting to jump as far as possible, I want you to focus your attention on extending your knees as rapidly as possible.’’

‘‘When you are attempting to jump as far as possible, I want you to focus your attention on jumping as far past the start line as possible.’’

Between-participant design.

Frequency: before each jump. Positive/negative: not described. Self-controlled (y/n): n

Jump distance An external focus results in a sig. further jumping distance in the standing long jump compared to internal focus.

Wulf et al. 2010.

Eight (3 male and 5 female); age 22.6 ± 2.50 years.

Jump ten times as high as possible by hitting the Vertec.




Frequency: before each jumping trail. Positive/negative: not described. Self-controlled (y/n): n

Jump height, EMG activity.

An external focus results in sig. higher jump height and lower EMG activity compared to internal focus.

Makaruk et al. 2012.

Exp. 1

Thirty-six (male) ; age 22.3 ± 1.1 years.

Nine-week plyometric training protocol with a standing long jump.

“Reach your heels as far as you can”, “extend the knees and ankles as rapidly as possible” and “reach your knees as high as you can”.

“Jump behind the line”, “push against the ground as forcefully as possible”, “push against the box as forcefully as possible” and “touch the hanging ball”.

Between subjects design.

Frequency: before each set of jumps. Positive/negative: not described. Self-controlled (y/n): n

Jump distance, jump height

An external results in sig. further jumping distance and height during the standing long jump during a nine-week plyometric training protocol.

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Exp. 2

Thirty-six (male) ; age 22.3 ± 1.1 years..


“Reach your fingers as high as you can”.

‘Touch the hanging ball”.



Jump height, vertical GRF, knee flexion

An external results in greater jump height, more vertical GRF and more knee flexion during the counter movement jump during a nine-week plyometric training protocol.


Exp. 3

Thirty-six (male) ; age 22.3 ± 1.1 years.


“Reach your fingers as high as you can”.

“Touch the hanging ball”.



Jump height Receiving no focus instruction results in a sig. greater jump height during the drop jump during a nine-week plyometric training protocol.

Porter et al. 2012.

Thirty-eight (male); age 20.7 ± 2.2 years.

Eight standing long jump trials.

“When you jump, focus on extending your knees as rapidly as possible.”

External near : “When you jump, focus on jumping as far past the start line as possible”.

External far: “When you jump, focus on jumping as close to the cone as possible”.

Within-participant design


Jump distance An external focus results in a further jumping distance. Increasing the distance of an external focus of attention results in a further increased distance.

Wu et al. 2012.

Twenty-one (10 male and 11 female); age 21.3 ± 1.74 years.

Four standing long jumps.

“Jump as far as you can” and “while you are jumping, I want you to think about extending your knees as rapidly as possible”.

“Jump as far as you can” and “I want you to think about jumping as close to the green target as possible”.



Jump distance An external focus results in a sig. further jumping distance compared to internal focus.

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Bredin et al. 2013

Sixteen (male; n=8); age 26.3 ± 3.9 years and female (n=8); age 25.0 ± 4.0 years.

Seven physical fitness test (including vertical jump) were completed on three different days (separated by one week).




Not described. Jump height An external focus results in a sig. higher jumping performance compared to internal focus.

GRF = Ground Reaction Force, CoM = Center of Mass

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Table 4. Results performance

Outcome variable

Task Study IF Mean ± SD (n)

EF Mean ± SD (n)

CTRL Mean ± SD (n)

Absolute difference IF - EF

P-value Effect size IF - EF (95% CI)

Jump distance (cm) Standing long jump Porter 2010 177.33 ± 40.97 (60) 187.37 ± 42.66 (60) 10.04 0.003 0.24 (-0.60 - 0.12)

Makaruk 2012 264.00 ± 14.00 (12) 265.00 ± 12.00 (12) 261.00 ± 10.00 (12) 1.00 0.009 0.08 (-0.87 - 0.73)

Porter 2012 204.40 ± 26.00 (38) EXN 216.20 ± 24.80 (38) EXF 224.20 ± 22.50 (38)

207.20 ± 25.90 (38) EXN 11.80 EXF 19.80

<0.01 <0.01

0.46 (-0.92 - 0.00) 0.81 (-1.27 - 0.34)

Wu 2012 139.50 ± 46.70 (21) 153.60 ± 38.60 (21) 14.10 <0.01 0.33 (-0.93 - 0.29)

Jump and reach height (cm)

Jump and reach task with Vertec

Wulf 2007 5.23 ± ? (10) 6.08 ± ? (10) 5.21 ± ? (10) 0.85 <0.01 NA

23.20 ± ? (10) 24.50 ± ? (10) 23.70 ± ? (10) 1.30 <0.01 NA

Wulf 2009 30.40 ± 3.04 (10) 31.90 ± 3.23 (10) 1.50 <0.05 0.48 (-1.35 - 0.43)

Wulf 2010 31.00 ± 3.18 (8) 32.40 ± 3.05 (8) 1.50 <0.05 0.45 (-1.42 - 0.57)

Bredin 2013* M 38.46 ± 7.69 (16) F 20.77 ± 6.92 (16)

M 50.00 ± 9.23 (16) F 30.77 ± 7.69 (16)

M 42.31 ± 6.92 (16) F 22.31 ± 6.15 (16)

M 11.54 F 10.00 <0.05 1.36 (-2.09 - -0.56) 1.37 (-2.10 - -0.57)

Jump height (cm) Standing long jump Makaruk 2012 39.00 ± 4.00 (12) 44.00 ± 4.00 (12) 38.00 ± 4.00 (12) 5.00 <0.000 1.25 (-2.08 - -0.34)

Counter movement jump

Makaruk 2012 41.00 ± 5.00 (12) 45.00 ± 5.00 (12) 43.00 ± 5.00 (12) 4.00 NS 0.80 (-1.60 - 0.06)

COM displacement (cm)


Wulf 2007 36.00 ± ? (10) 38.70 ± ? (10) 36.40 ± ? (10) 2.70 <0.05 NA


Wulf 2009 26.20 ± 2.10 (10) 29.50 ± 1.50 (10) 3.30 <0.05 1.81 (-2.76 - -0.70)

Contact time (sec) Counter movement jump

Makaruk 2012 0.38 ± 0.02 (12) 0.42 ± 0.03 (12) 0.37 ± 0.02 (12) 0.05 NS 1.57 (-2.42 - -0.61)

IF = internal focus; EF = external focus; CTRL = control; SD = standard deviation; CI = confidence interval; NA = not applicable, ie. not being able to calculate as essential data were not provided; * = values determined out of figure; M = male; F = female; EXN = external focus near; EXF = external focus far; NS = not significant; COM = Center of Mass

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Table 5. Results kinematics and kinetics

Outcome variable


EF Mean ± SD (n)

CTRL Mean ± SD (n)


P-value Effect size IF - EF (95% CI)

Knee flexion (°) Counter movement jump

Makaruk 2012 88.30 ± 3.20 (12) 93.40 ± 2.20 (12) 88.90 ± 4.00 (12) 5.10 0.005 1.86 (-2.74 - -0.85)

Drop jump Makaruk 2012 85.40 ± 2.60 (12) 90.00 ± 3.10 (12) 85.20 ± 2.40 (12) 4.60 0.046 1.61 (-2.47 - -0.64)

Force (N) Standing long jump Wu 2012 1453.70 ± 299.70 (21) 1429.80 ± 289.10 (21) 23.90 NS 0.08 (-0.53 - 0.68)

Counter movement jump

Makaruk 2012 1718.00 ± 119.00 (12) 1733.00 ± 106.00 (12) 1633.00 ± 79.00 (12) 15.00 0.004 0.13 (-0.93 - 0.67)

Drop jump Makaruk 2012 2216.00 ± 164.00 (12) 2243.00 ± 120.00 (12) 2151.00 ± 181.00 (12) 27.00 0.011 0.19 (-0.98 - 0.62)

GRF (BW) Jump from a 30 cm box

McNair 2000* 2.71 ± 0.43 (20) Sound 2.57 ± 0.36 (20) Imagery 2.86 ± 0.57 (20)

3.07 ± 0.57 (20) Sound 0.14 Imagery 0.15

S 0.71 (-1.34 - 0.06)

Impulse (Ns) Jump and reach task with Vertec

Wulf 2009 169.90 ± 14.56 (10) 191.40 ± 12.62 (10) 21.50 0.05 1.58 (-2.51 - -0.52)

Ankle moment (Nm/kg)


Wulf 2009* 2.89 ± 0.33 (10) 3.11 ± 0.28 (10) 0.22 <0.01 0.72 (-1.59 - 0.21)

Knee moment (Nm/kg)


Wulf 2009* 2.72 ± 0.28 (10) 2.89 ± 0.33 (10) 0.17 <0.01 0.56 (-1.42 - 0.36)

Hip moment (Nm/kg)


Wulf 2009* 1.00 ± 0.33 (10) 1.33 ± 0.28 (10) 0.33 <0.01 1.08 (-1.97 - -0.10)

Overall joint moment (Nm/kg)


Wulf 2009* 1.41 ± 0.097 (10) 1.57 ± 0.055 (10) 0.16 <0.01 2.03 (-3.01 - -0.88)

IF = internal focus; EF = external focus; CTRL = control; SD = standard deviation; CI = confidence interval; NA = not applicable, ie. not being able to calculate as essential data were not provided; * = values determined out of figure; M = male; F = female; EXN = external focus near; EXF = external focus far; NS = not significant; S = significant; GRF = ground reaction force; BW = body weight; N = Newton

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Table 6. Results electromyography

Outcome variable


EF Mean ± SD (n)

CTRL Mean ± SD (n)


P-value (main effect)

Effect size IF - EF (95% CI)

EMG (RMSE) TA Jump and reach task with Vertec

Wulf 2010*

155.00 ± 15.00 (8) 145.00 ± 20.00 (8) 10.00 <0.05 0.57 (-0.46 - 1.53)

EMG (RMSE) BF 85.00 ± 30.00 (8) 60.00 ± 25.00 (8) 25.00 <0.05 0.91 (-0.17 - 1.88)

EMG (RMSE) VL 190.00 ± 35.00 (8) 175.00 ± 25.00 (8) 15.00 <0.05 0.49 (-0.53 - 1.46)

EMG (RMSE) RF 130.00 ± 20.00 (8) 125.00 ± 20.00 (8) 5.00 <0.05 0.25 (-0.75 - 1.22)

EMG (RMSE) LG 110.00 ± 50.00 (8) 115.00 ± 40.00 (8) 5.00 <0.05 -0.11 (-1.09 - 0.88)

Muscle onset time (sec) TA Jump and reach task with Vertec

Wulf 2010*

0.57 ± 0.03 (8) 0.56 ± 0.03 (8) 0.01 0.822 0.33 (-0.67 - 1.30)

Muscle onset time (sec) BF 0.41 ± 0.05 (8) 0.44 ± 0.03 (8) 0.03 0.822 0.73 (-1.70 - 0.32)

Muscle onset time (sec) VL 0.41 ± 0.03 (8) 0.41 ± 0.05 (8) 0.00 0.822 0.00 (-0.98 - 0.98)

Muscle onset time (sec) RF 0.36 ± 0.03 (8) 0.35 ± 0.03 (8) 0.01 0.822 0.33 (-0.67 - 1.30)

Muscle onset time (sec) LG 0.26 ± 0.03 (8) 0.24 ± 0.03 (8) 0.02 0.822 0.67 (-0.37 - 1.63)

IF = internal focus; EF = external focus; CTRL = control; SD = standard deviation; CI = confidence interval; * = values determined out of figure; RMSE = root-mean-square error; TA = tibialis anterior; BF = biceps femoris; VL = vastus lateralis; RF = rectus femoris; LG = lateral gastrocnemius.

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Conflict of Interest Statement

• Ethical Approval: Not applicable

• Funding: Not applicable

• Conflict of Interest: There are no conflicts of interest

Novel methods of instruction in ACL injury prevention programs, a systematic review

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