# A POSTURAL STRATEGY AT STRING RELEASE IN ELITE ARCHERS

Andrian KUCH (1,2), Romain TISSERAND (2), François DURAND (3), Tony MONNET (2)

1. CRITT Sport Loisirs, France ; 2. Institut Pprime, CNRS/Université de Poitiers/ISAE-ENSMA, France ; 3. CREPS de Poitiers, France

## Introduction

Limiting motion around the string release instant is decisive for performance in archery [1]. Indeed, the string release changes the {archer + bow} system dynamics maintained during the aiming phase, to propel the arrow towards the target. Not taking these dynamics into account may compromise the archer's postural stability and consecutively reduce his/her performance. For efficient upright balance control associated with voluntarily initiated movements, humans integrate the consequences of the forces they will create into the motor command to ensure postural stability [2]. When the movement initiation instant is predictable, humans can time coordinated motor sequences, called Anticipatory Postural Adjustments (APA), to limit the mechanical consequences of the forthcoming movement on balance [3]. Particularly, the mechanical consequences of these muscles contractions can be observed through the displacement of the center of pressure (COP) that occurs *before* movement onset.

Elite archers are likely to have mastered the mechanical perturbation consecutive to string release. Hence, we hypothesized that their APA would be finely coordinated with string release, to limit detrimental effects on postural stability and maximize performance.

## Material and Methods

Standing on two force plates (Kistler) sampled at 1000 Hz, 6 French international level archers (3M, 3F) shot 18 arrows, at the 70m Olympic distance in an indoor range. The score of each arrow was collected.

A high-speed camera (Photron), synchronized with the force plates, captured each shot at 1000 fps, and was used to identify two instants: the clicker falling time (CFT) and the clicker reaction time (CRT). The CRT is the onset of the string release. The archers' reaction time was the duration between CFT and CRT.

Postural stability was characterized using the COP trajectory in the mediolateral (ML), i.e. the shooting, direction. On each trial, we observed a backward COP movement, starting after CFT (Fig. 1). The onset of the COP backward movement ($t_0$), its maximal backward amplitude ($COP_A$) and peak velocity ($COP_v$) were computed. If $t_0$ preceded CRT, an *early* postural strategy was identified, whereas if it occurred after CRT, a *late* postural strategy was identified. For comparison, COP trajectories were normalized by the archers' reaction time. Scores, $COP_A$ and $COP_v$ were compared between *early* and *late* strategies using a Welch's t-test, with $p < 0.05$ for significance.

## Results and Discussion

The mean reaction time was 146 ± 11 ms after CFT, which is consistent with previous studies [4].

All archers used both strategies, but they mainly used the *early* one (73 ± 8% of their shots). The average $t_0$ for the *early* strategy was 16 ± 23 ms after CFT, while it was 11 ± 14 ms after CRT for the *late* strategy. Such short duration between $t_0$ and CFT or CRT, alongside their low variability, suggest that elite level archers anticipate the forthcoming perturbation and are able to finely coordinate the display of their APA with these two specific instants during a shot.

<figure><img src="image_1.png" /><figcaption>Figure 1: COP in the ML axis.</figcaption></figure>

No statistical differences were found between the two strategies for $COP_A$ and $COP_v$ (Tab. 1). These results indicate that both strategies were identical from a biomechanical standpoint, but only displayed at two different moments in time. Additionally, despite slightly higher scores for the *early* shots, no performance difference was found between the two strategies. Thus, having a later APA does not seem to negatively affect scores in elite level archers.

<table><caption>Table 1: Compared indicators between strategies</caption><thead><tr><td></td><td>early</td><td>late</td><td>p</td></tr></thead><tbody><tr><td>scores</td><td>9.2 ± 0.8</td><td>9.0 ± 0.9</td><td>0.17</td></tr><tr><td>COP<sub>A</sub> (mm)</td><td>21 ± 7</td><td>20 ± 9</td><td>0.54</td></tr><tr><td>COP<sub>v</sub> (mm/s)</td><td>205 ± 85</td><td>209 ± 92</td><td>0.60</td></tr></tbody></table>

## Conclusion

Our findings suggest that elite level archers have automatized the perturbation caused by the string release and minimize its consequences on postural stability using a postural strategy finely coordinated with either the clicker falling time or the string release, with no differences in performance.

## References

1. Keast et al, Journal of Sports Sciences, 8:3, 203-213, 1990

2. Massion, Progress in Neurobiology, 38, 35-56, 1992

3. Bouisset et al, J. Biomech, 20:8, 735-742, 1987

4. Tinazci et al, Procedia Engineering, 13:290-293, 2011.

## Acknowledgements

This work is supported by the French National Institute of Sport and Physical Education, the French Archery Federation, the French National Research Technology Association and the French Ministry of Sports

<figure><img src="image_2.png" /></figure>