Bow stability and posture in archers: Sarro 2020 study

Eight Brazilian archers shot six arrows in a laboratory, standing on two force plates and with a marker on the bow, and the authors compared, for each archer, the arrow with the highest score and the one with the lowest. The question was twofold: during aiming, do the bow and the body move more in the worse arrows, and do the two movements go together?

In the better arrow the body swayed less. The displacement of the centre of pressure in the direction along the line of shooting went from 0.67 centimetres in the worse arrow to 0.36 in the better one, and the total length of the path of the centre of pressure from 61.66 to 26.64 centimetres. The bow moved less too: the total path of the marker on the bow fell from 28.79 to 17.75 centimetres. Aiming, finally, lasted less: 2.07 seconds in the better arrow against 3.55 in the worse one. All these differences came out as statistical on the Wilcoxon test. The amplitudes of bow displacement in the three directions, taken one by one, did not separate the two arrows, and nor did the displacement of the centre of pressure in the other direction.

The second part of the result is the new one. In the highest scoring arrow the displacement of the body towards and away from the target went together with the displacement of the bow in the same direction (correlation 0.83) and with the vertical displacement of the bow (0.80); in the lowest scoring arrow the same correlations fell to 0.46 and were not statistical. The authors read this as a synchronisation between body sway and bow sway in the more accurate shots, and from it they take the idea that training balance and bow stability together may help performance.

The participants were eight recurve archers, five men and three women, with a mean age of 29 (standard deviation 10.75) and 6.56 years of practice on average, competing at national level and training at least four times a week. They used their own bow, their own clothing and the footwear they normally train in. In the laboratory, on an official indoor 400 millimetre ten-ring face set at 13 metres, they warmed up with their own routine, then took twelve shots with about thirty seconds of rest between them, then, when they chose to, the six arrows that were recorded. Foot position was kept the same for all shots.

Bow stability was measured with a reflective marker on the lower end of the riser, filmed by two Basler cameras at 100 hertz and reconstructed in three dimensions. The aiming phase was bounded by eye, by the same operator for every shot: it starts at the first frame in which the hand is anchored on the jaw and the elbow has stopped moving, and ends at the first frame in which the string leaves the hand. The coordinates were filtered with a Butterworth low-pass at 15 hertz, and from them the authors derived the excursion of the bow in the three directions (towards the target, sideways, vertical) and the total length of the marker's path.

Postural control was measured with two Kistler force plates of 600 by 400 millimetres set into the floor, one under each foot, synchronised with the cameras and sampled at 100 hertz. The global centre of pressure was computed by combining the two plates, filtered at 10 hertz, and summarised as the amplitude of displacement in the two horizontal directions and the length of the path. The comparison between the better and the worse arrow used the Wilcoxon signed-rank test, and the relations between bow variables and body variables the Pearson correlation.

Eight archers and six arrows each: the correlation of 0.83 between body sway and bow sway rests on eight points, one per archer, and with numbers like these a high value moves easily if one participant is dropped. The comparison, moreover, is between the best and the worst of six arrows, that is, between two extremes of a small sample: the two arrows differ in score by construction, and part of the difference in sway may come from the extremes having been picked.

The authors flag two limits: the laboratory, with the target at 13 metres because that was the longest distance available, five metres short of the 18 used in competition, and the small number of shots, chosen to avoid fatigue. They add a third in the discussion, namely that the coupling between body and bow may be a feature of non-elite archers, and remains to be checked on top-level athletes. The aiming phase was bounded by eye, and bow stability comes from a single marker on the lower end of the riser, so rotation and translation of the bow are not separated.

There is also an inconsistency in the labels: in the methods text the displacement of the centre of pressure called COPY is the one across the line of shooting and COPX the one towards and away from the target, while the caption of Table 1 describes them the other way round, the first anteroposterior and the second mediolateral. This record follows the methods. No result bears on cause: in the better arrows body and bow moved together, and whether the synchronisation produces the good shot or merely accompanies it is unknown. The proposal to train balance and bow stability together is the authors' conclusion, not something the study tested. The file available is the accepted author version, posted in April 2020; the print issue is from 2021.

The idea that an archer should stay still is obvious, and this study makes it more precise: in the better arrows it was not the amplitude of bow movement in one direction that made the difference, but the total amount of ground covered, by the bow and by the body, over the time spent aiming. The path of the centre of pressure in the worse shots was more than twice that in the better ones, and the path of the bow about one and a half times.

The part that adds something concerns the relation between the two. In the better arrows the bow moved backwards and forwards together with the body; in the worse ones the bow went its own way. Put differently: a bow that stays aligned with the body swaying underneath it behaves differently from an arm correcting on its own account. The study does not explain how that is achieved, nor whether those who shoot better do so because they achieve it.

Two numbers should be handled with care. The first is the duration of aiming, shorter in the better shots (2.07 against 3.55 seconds): that is a difference between the best and the worst of six arrows in eight archers, and other studies on higher-level archers report otherwise, so there is no aiming time to follow. The second is the distance: 13 metres in a laboratory, not 18 indoors and not 70 outdoors. In the registry it counts as evidence on national-level archers for claims about stability during aiming, with a sample of eight people.