Muscle synergies in archery: Vendrame 2021 study

Two archers with more than ten years' experience, a 47-year-old man without disability and a 34-year-old W1 Paralympic athlete who uses a wheelchair because of spina bifida with spastic tetraparesis, shot three ends of six arrows at 10 metres while the activity of five muscles was recorded. Muscle synergies were extracted from the signals, meaning groups of muscles that switch on together with the same time course: for both archers three were enough to describe the shot.

The first, built on the upper trapezius of both sides, mainly serves to raise the bow and is similar in the two athletes (correlation between the time courses 0.87). The second has an even closer time course (0.91) but is shifted by 12% of the shot cycle: in the non-disabled archer it goes with raising the bow, in the Paralympic archer with the start of the draw. The third covers drawing and aiming (0.90, with a 14% shift): on the string side the non-disabled archer relies mainly on the posterior deltoid, the Paralympic archer on the biceps. On the bow side, from the start of the draw the former holds the bow with the anterior deltoid alone, while the latter keeps using the upper trapezius as well until release. The authors link these differences to the Paralympic archer's way of drawing, with the elbow bent and the arm only slightly raised, and to the seated position.

Both shot with the bow in the left hand, the Paralympic archer seated in his wheelchair and the other standing. A Vicon system followed two markers on each wrist to split every shot into phases: start of the bow raise, highest bow position, full draw, release and bow lowering. Surface EMG (Cometa MiniWave, 1000 samples per second) recorded the anterior deltoid and upper trapezius on the bow side, and the biceps, posterior deltoid and upper trapezius on the string side. The signals, filtered and turned into envelopes, were normalised to each muscle's mean peak and resampled to 200 points per shot cycle. Synergies were extracted by non-negative matrix factorisation, repeated 50 times to avoid local solutions, keeping the smallest number of synergies that explained at least 97% of the variance; the two archers' time courses were compared by cross-correlation.

There are two athletes, and the authors themselves present the work as a test of the method, not a description of how Paralympic archers shoot. Only five muscles were recorded, so the synergies depend heavily on which ones were chosen: the middle and lower trapezius and the forearm muscles, for instance, which matter in the draw and release, are missing. The differences between the two archers mix three things the study cannot separate: the disability, the seated position and personal style. The paper does not say what type of bow they used, nor does it report scores or accuracy, so it is not known whether the two strategies produce different results on the target. In one passage the anterior deltoid is attributed to the string side, where it was not recorded.

For archers without a disability the study finds, in one archer, a familiar pattern: during drawing and aiming the work on the string side falls mainly to the posterior deltoid, and on the bow side to the anterior deltoid. It is of more interest to anyone coaching archers with a disability, because it shows that an athlete who draws with the arm only slightly raised, probably to make up for impaired motor control, finds a different combination of muscles (here the biceps instead of the posterior deltoid, and the upper trapezius kept active until release) and that EMG makes it visible. Whether that compensation works well or loads the shoulder more over time, the study does not say.