Anne Benjaminse,1,2 Alli Gokeler3, 4, 5
1 University of Groningen, University Medical Center Groningen, Center for Human Movement Sciences, Groningen, Netherlands

2 School of Sport Studies, Hanze University Groningen, Groningen, the Netherlands 

3 Exercise Science and Neuroscience, Department Exercise & Health, Faculty of Science, Paderborn University, Paderborn, Germany

4 Amsterdam Collaboration for Health and Safety in Sports, Department of Public and Occupational Health, Amsterdam Movement Sciences, VU University Medical Center, Amsterdam, The Netherlands.

5 OCON Center of Orthopaedic Surgery and Sports Medicine, Hengelo, The Netherlands

Dear Editor,
We read the recent manuscript by Kal et al.1 ‘Explicit motor learning interventions are still relevant for ACL injury rehabilitation: do not put all your eggs in the implicit basket‘ with great interest. The authors did a commendable job summarizing the current literature and we highly respect them for being critical, to foster academic discussions to move science forward. We do however have some concerns regarding the methodology and interpretations made by the authors.
Confusing definition: description vs. execution
 First, the authors write: "Elite athletes have shown to successfully use explicit interventions to de-automate, and subsequently improve, problematic movements.“.2 The paper by Toner et al. is largely based on assumptions, case studies and philosophers' views.2 The authors presume that with adopting explicit learning, patients are not allowed to know the goal of the exercises.3 For clarity, the correct definition of attentional focus needs to be used: Attentional focus relates closely to what you tell your patient prior to the execution of the exercise. An internal focus would be induced to focus the attention on body parts and movements. Conversely, for an external focus, the attention is directed to the outcome or the goal of the movement. Hence, adopting an external focus is related to the planning of the movement, but has nothing to do with the processing of intrinsic feedback or bodily awareness.4 It does not mean that patients are not aware of their body movements, i.e. the ‘explicit interventions’ the authors refer to. It is important to note that bodily awareness is not necessarily inherent to an internal focus of attention.5 Leaning is not possible without bodily awareness.6 An external focus of attention simply means the patient is focusing on the intended movement effect – while preparing for the execution.6 This does not mean that one must never use body related instructions that elicit an internal focus of attention. Of course you can use it. It will just not result in most optimal motor learning.7,8
 In summary, it is important to differentiate between telling the goal of the exercise (i.e. description - general instruction) and the specific instruction provided just before the execution of the task. For example, in case of rehabilitation after ACLR: “the goal of this exercise is to improve knee bending while landing, as this promotes a softer landing, which is better for your knee.” Then, as the patient is preparing to execute the exercise, it is critical to shift their attentional focus that corresponds closest to the task goal: e.g. “focus on landing with as little noise as possible”.
Familiarity
 Second, the million-dollar question, do effects of implicit learning depend on the person’s preference? Here, again, the entire picture needs to be presented. Kal et al. conclude “Implicit or explicit learning appears most effective when it is aligned with an individual’s preference“, even though the original authors state: “Due to the well-documented external focus advantage we are hesitant to argue that direction of attentional focus does not have any influence on skilled motor performance.”.9 The preferred focus, often as a result of past experience with coaching instructions,10 is not always the most beneficial solution.11-15
 In addition, retention, a key aspect of motor learning, has not been reported.9 It is thus not possible, and it may give a skewed representation, to conclude learning is most effective when aligned with an individual’s preference, as the definition of learning is ‘a relative permanent change in a person’s capability to perform a skill’.16 To interpret results, the authors need to adhere to the correct definition of motor learning. Directly observable performance improvement is different from sustained improvements over time (retention).16 Often times explicit learning is indicated as being effective, potentially because it shows quick benefits.17
 The authors also write: “People with musculoskeletal conditions strongly prefer to consciously control movements.” But again, we need to ask the question if this preference is of benefit to them? Kal et al. should also have stated the conclusion of the paper they refer to: knee pain was more commonly reported by people with a propensity for conscious involvement in their movements.18 Thus, even it is the case that patients prefer to consciously control movements, it is not per se most effective.19 Loss of function, pain, fear of reinjury and other psychological factors, cause a shift in attentional focus towards the injured area such as the knee.19-22 The process behind this needs to be explored in further detail before we can say whether it is beneficial to consciously control movements. After musculoskeletal injuries neuroplastic changes of the central nervous system occur,23 which induces a focus on the injured joint. An ACL injury triggers neurocognitive disruption,23 demonstrating potentially maladaptive neuroplasticity within the brain whereby (pre)motor cortex areas are more active during simple movement tasks compared to uninjured individuals.24 Concluding, it would be unwise to leave patients to their own focus preference or assume they will find the focus that is optimal for them.

Verbal and visual feedback
 The authors pose that explicit verbal feedback on movement has been identified as a key active ingredient in primary ACL injury risk reduction.25 However, again correct interpretation of scientific studies is crucial here. All these training programs are conducted in a relatively controlled setting (e.g. with supervision from researcher) with no retention and transfer effects measured.25 Also, no studies thus far have comparison included with implicit verbal feedback conditions.
 The authors also state that it is unclear whether improved biomechanics can be attributed to implicit learning per se as this can not proven through self-report.26 Indeed, self-report, although done often,27-29 is not the best way to provide objective evidence.30 But, it is widely known that model learning, as we did in this study,26 or observing and imitating movements facilitates an athletes’ exploration and search for their own most optimal functional task solutions,34-36is part of an implicit learning strategy.34 Studies using Electroencephalography and Transcranial Magnetic Stimulation conclude that movement outcome and efficiency are enhanced when an external focus of attention is promoted, 31-33, likely due to improved neural strategies, such as cortical inhibition.. When providing sound designed research with only the attentional focus instruction being different across groups, including a control group, one can conclude that difference across groups is based on that factor. Vice versa, how can we know athletes in previous research have indeed adopted an internal focus of attention?
 In sum, we feel the authors should have presented the entire picture to the readers based on sound synthesis of the literature and proper use of definitions.

REFERENCES
1. Kal E, Ellmers T, Diekfuss J, et al. Explicit motor learning interventions are still relevant for ACL injury rehabilitation: do not put all your eggs in the implicit basket! Br J Sports Med 2021.
2. Toner J, Moran A. Exploring the orthogonal relationship between controlled and automated processes in skilled action. Rev Philos Psychol 2020;1–17.
3 Toner J, Moran A. Enhancing performance proficiency at the expert level: considering the role of “somaesthetic awareness. Psychol Sport Exerc 2015, 16, 110e117.
4. Wulf G, Why did Tiger Woods shoot 82? A commentary on Toner and Moran. Psychol Sport Exerc 2016;337-8.
5. Milley KR, Ouellette GP. Putting attention on the spot in coaching: shifting to an external focus of attention with imagery techniques to improve basketball free-throw shooting performance. Front Psychol 2021;16:12.
6. Wulf G. An external focus of attention is a conditio sine qua non for athletes: a response to Carson, Collins, and Toner (2015). J Sports Sci 2016;34:1293-5.
7. Ille A, Selin I, Do MC, Thon B. Attentional focus effects on sprint start performance as a function of skill level. J Sports Sci 2013;31:1705-12.
8. Winkelman NC, Clark KP, Ryan LJ. Experience level influences the effect of attentional focus on sprint performance. Hum Mov Sci 2017;52:84-95.
9. Maurer H, Munzert J. Influence of attentional focus on skilled motor performance: performance decrement under unfamiliar focus conditions. Hum Mov Sci 2013;32:730–40.
10. Porter JM, Wu WFW, Partridge JA. Focus of attention and verbal instructions: Strategies of elite track and field coaches and athletes. Sport Sci Rev 2010;19:199–211.
11. Guss-West C, Wulf G. Attentional focus in classical ballet: a survey of professional dancers. J Dance Med Sci 2016;20:23-9.
12. Stoate I, Wulf G. Does the attentional focus adopted by swimmers affect their performance? Int J Sports Sci Coach 2011;6:99–108.
13. Abdollahipour R, Wulf G, Psotta R, et al. Performance of gymnastics skill benefits from an external focus of attention. J Sports Sci 2015;33:1807–13.
14. LoSarah L, Jagodinsky A, Torry M, et al. Effects of attentional focus cues on lower extremity kinematics during inside of the foot soccer trap among expert soccer players. Int J Sports Sci Coach. 2021;
15. Maloney MA, Gorman AD. Skilled swimmers maintain performance stability under changing attentional focus constraints. Hum Mov Sci 2021;77:102789.
16. Schmidt RAWC. Motor learning and performance. Champaign: Human Kinetics; 2005.
17. Masters R. Knowledge, knerves and know-how: The role of explicit versus implicit knowledge in the breakdown of a complex motor skill under pressure. Br J Psychol 2011;83:343-58.
18. Selfe J, Dey P, Richards J, et al. Do people who consciously attend to their movements have more self-reported knee pain? An exploratory cross-sectional study. Clin Rehabil 2015;29:95-100.
19. Gray R. Differences in attentional focus associated with recovery from sports injury: Does injury induce an internal focus? J Sport Exerc Psychol 2015;37:607-16.
20. Hsu CJ, Meierbachtol A, George SZ, et al. Fear of reinjury in athletes: implications for rehabilitation. Sports Health 2017;9(2):162–167.
21. Ardern CL, Taylor NF, Feller JA, et al. Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery. Am J Sports Med 2013;41:1549-1558.
22. Podlog L, Dimmock J, Miller J. A review of return to sport concerns following injury rehabilitation: practitioner strategies for enhancing recovery outcomes. Phys Ther Sport 2011;12:36-42.
23. Grooms D, Appelbaum G, Onate J. Neuroplasticity following anterior cruciate ligament injury: a framework for visual-motor training approaches in rehabilitation. J Orthop Sports Phys Ther 2015;45(5):381-93.
24. Needle AR, Lepley AS, Grooms DR. Central nervous system adaptation after ligamentous injury: a summary of theories, evidence, and clinical interpretation. Sports Med 2017;47:1271–88.
25. Sugimoto D, Myer GD, Barber Foss KD, et al. Critical components of neuromuscular training to reduce ACL injury risk in female athletes: meta-regression analysis. Br J Sports Med 2016;50:1259–66.
26. Welling W, Benjaminse A, Gokeler A, et al. Enhanced retention of drop vertical jump landing technique: a randomized controlled trial. Hum Mov Sci 2016;45:84–95.
27. Ruocco AC, Direkoglu E. Delineating the contributions of sustained attention and working memory to individual differences in mindfulness. Person Ind Diff 2013;54,226–30.
28. Scheibner HJ, Bogler C, Gleich T, et al. Internal and external attention and the default mode network. Neuroimage 2017;48:381-89.
29. Porter JM, Nolan RP, Ostrowski EJ, et al. Directing attention externally enhances agility performance: a qualitative and quantitative analysis of the efficacy of using verbal instructions to focus attention. Front Psychol 2010;1:216.
30. Zhou ZW, Lan XQ, Fang YT, et al. The inter-regional connectivity within the default mode network during the attentional processes of internal focus and external focus: An fMRI study of continuous finger force feedback. Front Psychol 2019;10:2198.
31. Mak TCT, Young WR, Wong TWL. Conscious control of gait increases with task difficulty and can be mitigated by external focus instruction. Exp Aging Res 2021;47:288-301.
32. Suzuki LY, Meehan SK. Attention focus modulates afferent input to motor cortex during skilled action. Hum Mov Sci 2020;74:102716.
33. Kuhn YA, Keller M, Ruffieux J, et al. Adopting an external focus of attention alters intracortical inhibition within the primary motor cortex. Acta Physiol (Oxf) 2017;220:289-299.
34. Benjaminse A, Otten E. ACL injury prevention, more effective with a different way of motor learning? Knee Surg Sports Traumatol Arthrosc 2011;19:622-7.
35. Otte FW, Davids K, Millar SK, et al. When and how to provide feedback and instructions to athletes? How sport psychology and pedagogy insights can improve coaching interventions to enhance self-regulation in training. Front Psychol 2020;11:1444.
36. Al-Abood SA, Bennett SJ, Hernandez FM, et al. Effect of verbal instructions and image size on visual search strategies in basketball free throw shooting. J Sports Sci 2002;20:271-8.

Dear Editor:

Alvares et al. [1] conducted a study to compare performance-related measures such as cardiopulmonary exercise capacity and muscle strength in non-athlete transgender women (TW) undergoing long-term gender-affirming hormone therapy to non-athlete cisgender men (CM) and non-athlete cisgender women (CW). The authors report higher absolute VO2peak (L/min) and muscle strength (kg) in TW compared to CW and lower than CM. The authors conclude that their “…findings could inform policy and help in decisions about the participation of transgender women in sporting activities”.

However, the authors interpreted their findings on the basis of the absolute data they present and not the relative data that was controlled for body mass and fat-free mass (FFM), as would be appropriate for comparisons of such performance metrics (e.g., aerobic capacity and muscle strength). By focusing on the absolute data, the authors over-emphasise differences between comparison groups (e.g., TW and CW) that are clearly driven by differences in anthropometry. For example, when the data reported in Table 2 [1] are corrected for body mass and fat-free mass (FFM), differences in aerobic capacity and strength between TW and CW disappear. Yet, in the section “WHAT THIS STUDY ADDS” [1], which is the primary focus of many readers, the authors omit the results that control for body mass and FFM, instead leaving the reader with the misleading message that “[t]he mean strength and VO2peak in non-athlete TW … were higher than those in non-athlete CW.”

Relatedly, we question the authors’ reliance on expressing lung volume relative to body mass and FFM, when expressing relative to height would be more informative. The important relationship between height and indices of cardiovascular function/capacity is widely recognised [2]. However, Alvares et al. only report the heights of participants in their supplementary data [1] – which will mostly be overlooked by many reading the paper. Our own analysis of the supplementary data shows that height differences between TW and CW could fully account for the absolute differences in lung capacity. Again, the key finding here would be an absence of difference in relative aerobic capacity of cisgender versus transgender women.

In addition to the biased message, there are also many other basic errors in this manuscript that questions its value as published to inform policy. In the interest of brevity our main concerns are listed below as follow:

1. We question why one of the TW participants had a very high level of testosterone on the day of testing (TT=637 ng/dL). The authors justified the inclusion of this subject’s testing results on the basis of her having had a lower testosterone level six months prior. However, this is a cross-sectional study, meaning this participant should have been excluded. Without access to the raw data it is impossible to know the impact of the inclusion of this data in the comparison between groups.

2. VO2peak rather than VO2max is listed as the measure of aerobic capacity, even though VO2peak is known to underestimate the value of VO2max by 0.1-0.4 l/min [3]. It is unknown whether this represent a semantic error, or the authors have indeed measured VO2peak.

3. The authors rely on the International Physical Activity Questionnaire (IPAQ) to assess the physical activity levels of participants and match subjects in the comparison groups for meaningful data interpretation. However, the IPAQ is considered a valid instrument for measuring physical activity in large study populations, and not for such small studies needing validity at the individual level. This makes the interpretation of the comparisons between “matched” groups very difficult, if not impossible.

4. The number of participants reported in the abstract (i.e. 15 TW, 13 CM and 14 CW) does not match the number reported later in the article. The units of measurement for maximum aerobic capacity are clearly erroneous as physiologically implausible (e.g. Table 2 and VO2peak (L/min) was 2606±416.9 in TW, 2167±408.8 in CW and 3358±436.3 in CM).

5. The authors imply that gender dysphoria is a mental disorder, stating that “the criteria for gender dysphoria diagnosis were in accordance with the Diagnostic and Statistical Manual of Mental Disorder.” However, Gender Dysphoria is no longer classed as a mental health disorder [4].

In conclusion, we suggest that, as presented, the paper by Alvares et al. [1] cannot be used to “inform policy and help in decisions about the participation of transgender women in sporting activities”. Instead, the scientific community including these authors should be encouraged to conduct and publish high quality studies involving trained transgender individuals (and athletes where possible) and involving sport-specific measures to inform policy and guidelines [5].

References

1. Alvares LAM, Santos MR, Souza FR, Santos LM, Mendonça BB, Costa EMF, Alves MJNN, Domenice S. Cardiopulmonary capacity and muscle strength in transgender women on long-term gender-affirming hormone therapy: a cross-sectional study. Br J Sports Med. 2022 Nov;56(22):1292-1298.

2. HEPPER NG, FOWLER WS, HELMHOLZ HF Jr. Relationship of height to lung volume in healthy men. Dis Chest. 1960 Mar;37:314-20. PMID: 14401182.

3. Smirmaul BP, Bertucci DR, Teixeira IP. Is the VO2max that we measure really maximal? Front Physiol. 2013 Aug 5;4:203.

4. Rodríguez MF, Granda MM, González V. Gender incongruence is no longer a mental disorder. Journal of Mental Health & Clinical Psychology. 2018;2(5).

5. Martowicz M, Pape M, Budgett R, Mascagni K, Engebretsen L, Dienstbach-Wech L, Pitsiladis Y, Pigozzi F, Erdener U. Position Statement: IOC Framework on Fairness, Inclusion and Non-Discrimination on the Basis of Gender Identity and Sex Variations. British Journal of Sports Medicine. In review.