Aiming path and score in archery: Lin 2005 study

Twelve archers from the men's team of the National College of Physical Education and Sports in Taiwan shot 12 arrows at 70 metres with a laser pointer fixed to the riser. The light spot, projected onto a panel about 4 metres away, showed where the bow was pointing in the last second and a half before the release; a second camera recorded where each arrow landed. For each archer and each tenth of a second, the authors calculated the correlation between the movement of the laser spot and the position of the arrow on the target, taking the horizontal and vertical directions separately.

In 10 of the 12 archers the bow moved more horizontally than vertically; the exceptions were archers 3 and 8, and the ratio between the two ranged from 0.30 to 6.56. The link between aim and impact varied a good deal from one archer to another. Horizontally, archers 2, 3 and 8 had a positive correlation throughout the second and a half, meaning the arrow went the way the bow was pointing, while for archers 5, 9 and 11 it was negative all the way through, most strongly for archer 9. Vertically, at the last instant before the release, eight archers had a positive correlation and four (3, 7, 8 and 10) a slightly negative one. In some archers the correlation changed sharply in the final tenths of a second, which the authors read as unstable aiming in that phase.

The camera filming the laser spot recorded 60 fields per second, so the second and a half before the release gives 90 points, grouped into 15 units of a tenth of a second each. The release is defined as the last visible point, because the riser's vibration makes the light vanish from the image. Each archer's aiming point is the mean of the last 30 points (half a second) across the 12 arrows. The coordinates, extracted with the APAS motion analysis system, were corrected for the camera angle and then expressed as offsets from the aiming point and from the centre of the target.

Before the test each archer's arrows were weighed and selected with an 18-metre test, keeping those that grouped best, and numbered. Each archer had three arrows to bring the laser panel within their aiming range, then 12 arrows in four ends of three, with no more than 40 seconds per arrow; foot positions were marked on the ground. Correlations were calculated in Matlab, archer by archer, over the 12 arrows.

Each correlation rests on 12 arrows, too few for a stable coefficient: with twelve pairs of values a correlation of around 0.58 is needed to reach significance, and the paper reports neither the values nor any tests, only charts. The standard deviations of the aiming movement are also given without units. The laser shows where the riser is pointing, not how the arrow leaves: between aim and impact come the release, the bow's reaction and 70 metres of flight in the wind, which the paper does not mention.

The counts are not consistent across the paper. The abstract and conclusions speak of four archers with an always positive horizontal correlation and three with an always negative one, and of six and two vertically; the text names three and three horizontally and, for the vertical direction, describes only the last instant, with eight positive and four negative. Reading a negative correlation as a technical fault to correct is the authors' interpretation: with twelve arrows a negative sign can turn up by chance. The bow type is not stated; the 70-metre distance suggests recurve. Scores are not analysed.

The study is an early example of a tool many archers now know: a sensor on the bow that traces the aim before the release. Its firmest finding is descriptive. In most archers the bow moves more horizontally than vertically, and the link between where the bow points in the last second and where the arrow lands varies a great deal from one person to the next.

It does not show that an archer with a negative correlation has a problem to fix, or which part of the last second and a half matters most for the score. For anyone using this kind of tool it leaves a point of method: twelve arrows are not enough to draw conclusions about an individual archer.