Tenouchi in kyudo: Yamamoto 2008 study

Te-no-uchi is the way the bow is held in kyudo, and tsunomi the push from the base of the thumb that twists it about the grip at the moment of release. The authors measure no archers: they mount Japanese bows on a mechanical shooting rig and deliberately vary the twist to see where the arrow lands. They used two bows, of 13 and 15 kgf, and a single duralumin arrow, shot at 28 metres, the distance set by the Japanese kyudo federation. After every shot they measured the horizontal distance from the centre of the target to the point of impact with a tape measure.

The graph that summarises the trials plots the torque applied to the bow, from about 2 to 14 newton metres, against the lateral deviation of the impact, and gathers two sets of data, from 2004 and 2005. The relationship between the two is not a straight line: the deviation does not fall in proportion to the twist. The impact closest to the centre comes when the bow's support point sits exactly below the nocked arrow. The authors' conclusion has two parts. An ever stronger tsunomi does not keep improving the shot, because some balance is needed between the push on the bow and the twist; comparing the two extremes, however, a strong twist leaves smaller deviations than a weak one. With no twist at all, they explain in the theoretical section, the string pushes the tail of the arrow towards the centre of the bow and gives it a torque that sends it to the right of the line of aim.

The rig was built from L-section steel, sized from an archer of average height, and holds the bow without a human hand. A projecting post acts as the support: when it is aligned with the centre of the bow's body the torque is zero, and moving the support point changes the twist applied. The post is drilled and threaded with a wire that stops the bow flying off immediately after the release. In place of the kyudo glove, the yugake, a metal fitting holds the string at three points, top, middle and bottom, and rotates freely on the rig, so that the string pushes the arrow along the line aimed at the bow's support point. The release is made by opening the clamp.

The trials were run outdoors, on the kyudo range of the University of the Ryukyus, with a crosswind that could not be screened off. The bow was held at draw for 5 to 10 seconds before each release. Aim was set by hanging a weighted yellow thread from above the target and sighting from behind the nocked arrow: the authors note that the arrow drops in flight, and that over the 28 metres, about half a second, it loses roughly the height of an archer.

This is a two-page conference proceedings paper rather than a peer-reviewed article, and the data show it. The number of arrows shot at each setting is not given, no figures appear in a table, and the results can only be read off a graph that combines two sets of trials a year apart. The authors themselves write that the crosswind introduced scatter in the points of impact.

The rig is not a hand. A clamp releasing the string does not reproduce fingers leaving it, and a rigid support has none of the give of a palm: here the twist is a quantity set by the experimenters, not the outcome of a movement. There are two bows, light ones by kyudo standards, and a single arrow, so spine and tuning never vary. Only the lateral deviation of single shots is measured, not the spread of a group, and elevation is not considered. No archer takes part in the study, so the step from how the rig behaves to how a trained hand behaves remains the authors' hypothesis. The text is in Japanese, with an English abstract.

The study is about kyudo, where the bow is long, asymmetrical and has neither pressure button nor sight, and it measures one thing: how far the impact moves sideways as the twist applied to the grip changes. What matters to archers shooting other bows is the shape of that relationship. There is no proportionality between twist and point of impact, so the idea that pushing harder with the base of the thumb brings the arrow steadily closer to the centre finds no support in the data; and the best shot comes when the bow's support sits below the arrow, which is to say when the push passes where the arrow passes.

None of this says how the bow should be held, or how much te-no-uchi matters to a flesh-and-blood archer. A mechanical rig shows how the equipment responds to a twist chosen by the experimenters, not how an archer learns to metre it. For recurve archers the link stays indirect: there too the bow hand introduces a torque, but no comparison between the two disciplines is made in this work.