11 e-Letters

published between 2019 and 2022

• Response to “Important Nuances for Non-Linear Modeling”

We thank Shrier et al. for a thoughtful expansion on the topic of non-linearity.1 The comments from the authors provide valuable insights and detail to both the handling and the interpretation of fractional polynomials and splines, and may interest readers who seek more information than the short introduction in Bache-Mathiesen, et al. 2.

We are especially grateful for elaborating on the interpretation of restricted cubic splines, and the solution of adding a small constant (i.e. 0.1) to all values to handle the value “0” when using fractional polynomials. These topics could not be sufficiently addressed within the limited wordcount of the original article, and we encourage readers to consider these comments.

Conflict of Interest:
None declared.

References
1. Shrier I, Wang C, Stokes T, et al. Important Nuances for Non-Linear Modeling. BMJ Open Sport & Exercise Medicine 2021
2. Bache-Mathiesen LK, Andersen TE, Dalen-Lorentsen T, et al. Not straightforward: modelling non-linearity in training load and injury research. BMJ Open Sport & Exercise Medicine 2021;7(3):e001119. doi: 10.1136/bmjsem-2021-001119

• Important Nuances for Non-Linear Modeling

We would like to thank Bache-Matiesen et al.(1) for their thoughtful article on non-linear modelling in sport medicine. Our own study on the non-linear relationship between acute: chronic workload ratio (ACWR) and injury risk in children was published as a preprint (2) and recently accepted by the American Journal of Epidemiology.(3) Below, we highlight some additional underlying principles in non-linear modelling that readers should understand.

GENERAL CONCEPTS
Models are based on information, which includes both data and assumptions. Simple linear models are more prone to bias because they assume a data generating process that is likely incorrect. The flexibility of non-linear models leads to less risk of bias, but also less precision. The optimal choice between bias and uncertainty depends on the context.(4)

Bache-Matiesen describe three non-linear modelling options: quadratic modelling, fractional polynomials (FP), and restricted cubic splines (RCS, where knots are determined by either data driven or a priori methods). These all fall under generalized additive models (GAMs), or generalized additive mixed models (GAMMs; if one uses “random effects” to adjust for repeated measures on participants).

FP methods use a single polynomial function over the entire range of exposures to predict the outcome. Quadratic models are special cases of FP (with exponents of 0, 1 and 2) and are too restrictive to be generally recommended. RCS separate data i...

• Interesting study

Thanks to the authors for providing some preliminary data on the potential effectiveness of bike-fitting to reduce pain and discomfort in cycling.
Very interesting study, but probably some questions are worth comments from authors:
1- Was there a proper ethics approval for this study? It seems that data was obtained retrospectively from clinical records.
2- How transferable to training is 100W of cycling?
3- Would authors be able to disclose the source of the 'measurement reference values'? Elaborating the criteria used for changes in bike-fit is critical to understand how and why cyclists improved their posture on the bike.

It would have been nice to see a control group to determine how much of the perceived changes are from placebo-effect.

• Response to Best test for diagnosing PEBSI

Dear Dr Obedkova

Thank you for your e-letter of 5th April 2021 regarding our publication ‘Diagnostic accuracy of MRI for identifying posterior element bone stress injury in athletes with low back pain: a systematic review and narrative synthesis. BMJ Open Sport & Exercise Medicine 2020;0:e000764. doi:10.1136/ bmjsem-2020-000764’. We welcome your comments and interest in this research as it further highlights the importance of evidence based safe and ethical practice. Our own interest and rationale for this review stems from extensive working with young adults and adolescents involved in elite sport with low back pain.

We take on board your comments, although where good evidence to support one modality over another is lacking, the merits of different approaches concerning risk v benefit must be central to clinical decision making. In this instance, evidence based practice in the UK overwhelmingly supports the use of MRI as the first line investigation; recognising SPECT/CT involves ionising radiation, and that a safe alternative exists with MRI. When undertaken and interpreted correctly, MRI permits the sensitive detection of posterior element bone stress injury in the vast majority of cases and should therefore be used as the first line investigation. SPECT/CT should therefore be reserved for those small minority of cases where (following MRI) diagnostic doubt remains, where there are underlying complexities (such as previous same-level stress injuries)...

• Best test for diagnosing PEBSI.

Dear colleagues, I have read with a great interest this systematic review “Esh R, Grødahl LHJ, Kerslake R, et al. Diagnostic accuracy of MRI for identifying posterior element bone stress injury in athletes with low back pain: a systematic review and narrative synthesis”[1] and would like to make some comments. Further research, as authors concluded, are always desired and I wish there are appropriate studies in the next future to decide if MRI could be a modality of choice for diagnosing PEBSI and not SPECT/CT as it is by now. But the referred articles from this review do not support this assertion for the following reasons.
First, there are just 2 articles from 4 included in this review compared MRI and SPECT/CT and just in 1 of them (Juvenil spondilolysis: a comparative analysis of CT, SPECT and MRI) the results of both techniques were concordant, although the main causes of discrepancy were between MRI and SPECT/CT for diagnoses of stress reaction in absence of overt fracture and distinguishing incomplete fracture from intact pars of complete defects (22 patients and 40 pars defect) [2].
Second, in the 2nd article the authors mention that sensitivity and specificity of MRI was 80 and 100% respectively skipping the SPECT/CT results and author’s conclusion that in the original article was following: “These results suggest that there is a high rate of active spondylolysis in active athletes with low back pain. MRI is inferior to bone scintigraphy (with SPECT)/c...

• A dog is not a standardised intervention

Nice study, and great to see you including dog ownership in your COI statements! A plea from a veterinary surgeon interested in dog walking though: please collect and report data about the dogs. A dog isn't a standardised intervention like a Fitbit. Their exercise capacity and exercise desire will vary with breed, age, comorbidities, body condition score, behaviour yet no data about the dogs is included in this publication. In addition, research suggests that owning an ill or behaviourally problematic dog can be very stressful. These canine attributes may have introduced a level of unaccounted for heterogeneity into your intervention arm which may have confounded your results. I'd be happy to point any researchers in this field towards the relevant canine literature, or assist with dog aspects of a study design.

• Conclusions questioned?

Dear authors, I have read your paper with interest, and I have a couple of remarks that I think needs to be discussed to hopefully stimulate to more studies in this important field.
It is a weakness that there are very few randomised trials on surgical treatment of tendinopathy. To then draw conclusions from few studies is difficult and not always possible.
From this systematic review it is shown that 10 out 12 trials are on tendinopathy in the upper extremities (shoulder and elbow-althogether 998 tendinopathies) and only 2 on tendinopathy in the lower extremities (patellar and Achilles-althogeteher 60 tendinopathies). The authors have unfortunately missed to include 2 randomised studies on surgical treatment of patellar tendinopathy, published by Dr Willberg et al. Althogether, most information is about the shoulder and elbow, and very little about the Achilles and patellar tendons. Anyhow, when conclusions are drawn I get the impression that the authors put all tendinopathies in one group. This might be strongly misleading since there is very little information about the lower extremity tendons (only 2 studies), and conclusions cannot be drawn for Achilles and patellar tendinopathy. Furthermore, such conclusions might lead to that we miss possible differences in load response between upper and lower extremity tendons? The upper extremity non weight-bearing tendons might respond different compared to the lower extremity weight bearing tendons, as it is fo...

• Response to the viewpoint of Stöllberger and Finsterer: Side effects of and contraindication for whole body electromyostimulation

In their recent viewpoint and article, Stöllberger and Finsterer 1 2 criticize the inadequate regulation of “whole body-electromyostimulation” (WB-EMS), potentially responsible for a variety of adverse effects recently reported. 3-6 Indeed, in contrast to locally applied EMS, the stimulation of all, or at least most, major muscle groups characterizes WB-EMS. Consequently, given that even locally applied EMS might cause severe rhabdomyolysis and hospitalization 7, it is obvious that a technology able to stimulate simultaneously up to 2600 cm2 of muscular area entails a much larger risk of triggering unintended side effects 3-6 at least when inadequately applied. 8
Particularly with regard to the WB-EMS safety guidelines published in 2016 by our national WB-EMS consortium8, Stöllberger and Finsterer2 complain that the enquiring about contraindications and the requirements for a licensed WB-EMS trainer are not adequately specified. Overlapping with the publication of the article of Stöllberger and Finsterer, however it should be noted, that a German standard (DIN 33961-5, 9) was recently released which includes both contraindications for WB-EMS application in commercial, non-medical settings 10 and the requirements for the qualification of EMS trainers. Of importance, the latter was also specified by the 2019 revised German Radiation Protection Statutes (NiSV) a mandatory guideline published by the German “Bundesministerium für Umwelt, Naturschutz und nukleare Sicherhei...

• Longer career, longer follow up, immortality bias

The article Mortality of Japanese Olympic athletes: 1952 – 2017 cohort study currently fails to account for a probable source of non-negligible bias. Whilst the main finding, stating that there is a decreased mortality rate among Japanese Olympians appears to be methodologically reliable, the authors also state that “higher mortality was observed among those who participated in the Olympics twice and three times or more compared with those who participated just once,” which we believe to be a potentially inaccurate finding.
A delayed entry exists at baseline, as those with longer careers tend to be older than those with shorter careers. For instance, someone who has participated in 3 Olympic Games started her/his Olympic career 12 years before those who have participated only once. Thus, there might be an important period effect underlying these findings. Even if the authors have adjusted the analysis by age, this is not equivalent to adjusting for period, as earlier periods in time are related with higher mortality rates, which could explain the findings.
Additionally, another problem often seen in survival studies is ignoring when a death event occurs. This is problematic because deaths will be observed more frequently in subjects with longer follow-up times. In the presence of time-dependent bias, the hazard ratio is artificially underestimated and the length bias leads to an artificial underestimation of the overall hazard [2]. Therefore, faulty interpretat...