Fingertip sensitivity in archers: Kotani 2007

The authors set out to see whether years of shooting make the fingertips more sensitive, and found the opposite. In 8 university archers and 6 students who did not shoot, they measured the differential threshold, the smallest difference in pressure a person can feel between two stimuli, using very small forces of 2.0 to 7.0 grams-force on the tip of the index finger. The mean threshold was 0.63 grams-force in the archers and 0.50 in the controls; the Weber ratio, the threshold divided by the stimulus, was also higher in the archers (0.15 against 0.12). With the stronger stimuli the ratio levelled off at around 0.13 in the archers above 5 grams-force, and at around 0.10 in the controls above 4. At these intensities, in other words, the archers were slightly worse at telling small differences in pressure apart.

The skin of the fingertip was harder in the archers (49.3 against 39.8 on the durometer's relative scale) and, across all participants, harder skin went with a higher threshold. In the archers the threshold rose steadily with the stimulus (coefficient 0.93), while in the controls it fluctuated more (0.40): from this the authors infer that the archers' perception was more stable from one day to the next.

The apparatus was a jet of compressed air, controlled by an electro-pneumatic regulator and a solenoid valve and aimed at the centre of the index fingertip through a 3 mm nozzle held 5 mm away; air was used to avoid the edge effects of a solid object. Hand and wrist were strapped down, the finger rested on a clay base, and participants wore an eye mask, earplugs and headphones so that they could not judge the force from the sound of the jet. In the archers the drawing hand was tested, in the controls the right hand. The stimulus was pressure perpendicular to the fingertip, chosen because that is what the finger receives during the draw.

For each of six reference stimuli, from 2.0 to 7.0 grams-force in steps of 1.0, pairs of one-second stimuli were presented three seconds apart, with nine comparison levels, and the person said whether the second was stronger, weaker or the same: 90 trials for each reference stimulus, in random order, with a break every 20 and one day for each. The threshold was calculated with the method of complete series. Skin hardness was then measured with a durometer. The archers, all male, were 22 to 24 years old with 1 to 3 years in the university team and daily training; the controls were six male students aged 19 to 21.

The sample is tiny and the two groups differ in more than shooting: the archers were a few years older, and their drawing hand was tested while the controls' right hand was, without saying whether the two coincided. The forces used go up to 7 grams, whereas by the authors' own account drawing a bow takes on average 15 to 20 kilograms-force, which passes through the fingers: the sensitivity that matters in shooting, at those loads or as the string slides off the fingers at release, may behave differently, as the authors acknowledge. The statistical tests have 82 degrees of freedom with 14 people, so they treat repeated measurements as independent, which makes the differences look firmer than they are.

The stability credited to the archers is inferred from how closely the threshold follows the stimulus, not from a direct comparison between trials or days, and the same value of 0.93 is called a correlation coefficient in the results and a coefficient of determination in the discussion. That harder skin explains the higher threshold is a hypothesis, and the relationship is shown only in a graph. The type of bow and the use of a tab or glove, which greatly change what the finger feels, are not reported. The PDF is a scan: this record is based on text recognised from the page images.

The study does not support the idea that shooting sharpens the sense of touch in the fingers: with pressures of a few grams, the archers in this sample were slightly worse than non-archers at telling differences apart, and had harder skin on the fingertip. One possible reading is that repeated contact with the string thickens the skin and dampens the lightest pressures, but with 14 people compared at a single point rather than followed over time, it remains a hypothesis.

For anyone who shoots, little changes. At release what counts are forces of many kilograms and the string sliding off the fingers, and the study did not touch these, so it says nothing about feeling string tension during the draw or about how clean the release is. The open question is whether archers' perception really is more stable from one day to the next, which would need repeated measurements designed for that purpose.