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Asymmetry after Hamstring Injury in English Premier League: Issue Resolved, Or Perhaps Not?
15 June 2015 (online)
We read with interest the recent article by Barreira et al. . We agree with the authors that the “issue” is not resolved. Only one study prior has been published on unilateral running kinetics following hamstring injury by Brughelli et al. , and that paper had several limitations as correctly pointed out by Barreira et al. . Thus we were hopeful that Barreira et al.  would make a contribution to scientific knowledge in this area. We acknowledge the good intent of the authors to measure specific mechanic variables during acceleration and maximum velocity running after hamstring injury in a professional club setting. However, we were disappointed to discover that the data presented were invalid, leading to untrustworthy results. We present here our critique of Barreira et al. .
Since the vertical and horizontal ground reaction forces (GRF) were collected from the ground of the treadmill (i. e., transducers located in treadmill frame), the GRF signals need to return to zero as the athlete is in the air. It is impossible to produce any GRF when the subject is not in contact with the treadmill. There are several examples in literature for how the raw vertical GRF signal should appear during acceleration or maximum velocity running      . As seen in Figure 2 of Barreira et al. , vertical GRF remains above 300 Newtons (N) throughout the steady state phase at maximum velocity. One could confidently conclude from this signal that the transducers were not correctly calibrated or zeroed.
The horizontal GRF signal does not resemble anything presented in the literature    . Since horizontal GRF was also measured from transducers below the running belt, the signal should show distinct braking and propulsive phases during each step. There should be a negative peak below 0 N during the braking phase, and a positive peak above 0 N during the propulsive phase. See papers by Rabita et al.  and Morin et al.  for how raw data signals should appear during acceleration and maximum velocity running on a track and motorized instrumented treadmill. The invalid horizontal GRF signal presented by Barreira et al.  could have been influenced by the cross-talk phenomena between vertical and horizontal GRF signals, in addition to improper calibration and zeroing.
In Tables 2 and 3, peak horizontal GRFs at maximum velocity were reportedly 51.2–72.4 N , while the rest of the literature has reported values of ~250–500 N when athletes ran at maximum velocity on treadmills and over-ground       . This would account for ~400–600% lower values from the literature.
The horizontal GRF signal does not show the same number of peaks during the steady state phase in comparison with vertical GRF. While it can be seen that 12 peaks/steps occur from vertical GRF, horizontal GRF shows 3–4 peaks which are very hard to identify. It is not possible for a runner to produce 12 steps of vertical GRF and only 3–4 steps of horizontal GRF during the same sprint. For each step during maximum velocity running, there should be: 1 peak in vertical GRF, 1 negative peak in horizontal GRF during the braking phase, and 1 positive peak in horizontal GRF during the propulsive phase    . This is true if the force sensors were in the ground    or under a treadmill belt .
The sports science community has identified hamstring injury research as too important to have inaccurate measurements  . In the future, we encourage sports scientists and especially postgraduate students to develop software programming skills in order analyse raw data files by themselves, and better distinguish data that make no sense. Such skills will help to identify when invalid signals appear, and how to correct the problem. We also encourage journals editors and reviwers to pay particular attention to the raw signals and processed data reported, and not only to the results and conclusions drawn from them. We share the hopes expressed by Barreari et al. that a critical attitude should be adopted for the development and validation of assessment protocols in sport club settings. However, simply relying on software for data analysis from equipment companies is not accurate science, and could lead flawed results and a damaged reputation of sports science/scientists in the sports industry.
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