The flexing the arrow takes on does not grow in proportion to how fast the fingers open
2 studies
The summary today
The only data come from a shooting robot with a competition recurve bow, built in Japan in 2015 with motorised fingers that open at different speeds, and with the arrow's flexing filmed at high speed. The authors expected the arrow to flex more as the fingers opened faster, and that is not what happened: with the motor at 30, 90 and 150 rpm the peak flexing amplitude is 7.07, 7.44 and 7.23 millimetres, so the maximum sits at the middle speed. Opening the fingers statically, up to the smallest angle at which the string slips away, gives the smallest flexing of all, 6.94 millimetres, eight hundredths of a millimetre from the archer used as a reference, at 6.86. The flexing frequency, by contrast, does not move, between 83.7 and 88.5 Hz in every condition. The differences are a few tenths of a millimetre, only just above the reported standard deviations (between 0.18 and 0.23 millimetres), with only three speeds, no stated number of shots and no statistical test, on a machine: the data say the relationship is not proportional on that robot, not that it really has a peak. A 1997 simulation does not vary the opening speed: it says only that in the model the sideways push the arrow receives matters more than the exact way the string leaves the fingers.
Also phrased as: "snapping the fingers open at release"; "active or passive release and arrow flex"; "release speed"; "the fingers letting go of the string"; "shooting robot and release"
In practice
Archers are taught that a clean release is one where the fingers relax rather than snap open, because an active, fast opening would disturb the string and make the arrow flex more.
Evidence
-
1997
Context Model Published later
A simulation of the arrow leaving a modern recurve, compared with high-speed film of an archer: the string slipping off the fingers is described by a simple function, with 3.5 mm of contact and a peak sideways displacement of the nock of 2.29 mm, the latter tuned to the data together with an efficiency coefficient. According to the authors the arrow's motion after release is not very sensitive to the exact shape of that function and depends mainly on the sideways momentum received during release; no sensitivity values are reported.
On the mechanics of the arrow: Archer's Paradox · B.W. Kooi, J.A. Sparenberg
-
2015
For Equipment tests Published later
Motorised fingers with the motor at 30, 90 and 150 rpm (1.125, 3.375 and 5.625 degrees per second): peak flexing amplitude 7.07, 7.44 and 7.23 millimetres. The maximum sits at the middle speed, against the authors' expectation that it would rise with speed.
2A2-T08 Development of Archery Robot : Development of finger mechanism for controllable the bending vibration · Jun Wakatabe, Chisato Kanamori, Ryu Ando, Takeshi Miyazaki
-
2015
Context Equipment tests Published later
Opening the fingers statically up to the smallest angle at which the string slips away gives the smallest flexing of the four conditions: 6.94 millimetres (standard deviation 0.23) against 6.86 for the reference archer, a difference of 0.08 millimetres. In the same tests the arrow's flexing frequency does not depend on how the fingers open: 88.30 Hz at 30 rpm, 83.68 at 90, 85.96 at 150, 88.47 with static opening, against 86.82 for the reference archer.
2A2-T08 Development of Archery Robot : Development of finger mechanism for controllable the bending vibration · Jun Wakatabe, Chisato Kanamori, Ryu Ando, Takeshi Miyazaki
Status history
- 1997 Not studied The first source close to the question is a simulation of the archer's paradox on a modern recurve. In the model the string slips off the fingers over 3.5 millimetres, pushing the nock sideways by 2.29, a value tuned so that the calculation matches film of an archer, and the authors note that the arrow's motion after release depends little on the exact shape of that slip and a great deal on the sideways push the arrow receives. It is a theoretical pointer, and nobody has yet varied the speed at which the fingers open.
- 2015 Reported A shooting robot with motorised fingers tests three opening speeds: with the motor at 30, 90 and 150 rpm the peak arrow flexing is 7.07, 7.44 and 7.23 millimetres, with the peak at the middle speed rather than at the highest, where the authors had expected it. Static opening gives the smallest flexing, 6.94 millimetres, almost the same as the reference archer, and the flexing frequency does not change. It is a test on equipment, so direct for this claim, but a single one, on three speeds, with means and standard deviations but no statistical test, in conference proceedings: the evidence in favour is there and it is thin.
Related
Related articles
- Fishtailing and porpoising: what arrow flight saysDoes your arrow wag sideways or wave up and down? What causes fishtailing and porpoising, which to fix first, and how to confirm it with a bare shaft test.
- The archer's paradox: the physics of the flexing arrowAt full draw the arrow points to one side and still reaches the centre: how it flexes and clears the riser in the first twenty milliseconds, and how spine matches it to the bow.
- The release in archery: anatomy of a few millisecondsHow the archery release works according to the measurements: how long it lasts, how the fingers open, what the arrow does and what you can train.
Research lines
Cite
Campagna, M. (2026). The flexing the arrow takes on does not grow in proportion to how fast the fingers open (Version 3). In Evidence Registry. Retrieved 16 October 2026, from https://matteocampagna.com/en/registry/equipment/finger-opening-speed-arrow-flex
@misc{campagna2026_velocitaaperturaditaflessione,
author = {Campagna, Matteo},
title = {The flexing the arrow takes on does not grow in proportion to how fast the fingers open},
year = {2026},
howpublished = {Evidence Registry, matteocampagna.com},
url = {https://matteocampagna.com/en/registry/equipment/finger-opening-speed-arrow-flex},
note = {Version 3; accessed 2026-10-16}
}
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