Arm muscles in compound archers: Ariffin 2017

Two right-handed compound archers, one skilled and one who rarely shot, were recorded with surface electromyography on six muscles per arm while shooting five arrows: flexor carpi radialis, extensor digitorum, brachioradialis, middle deltoid and middle trapezius on both arms, plus the biceps on the string arm and the triceps on the bow arm. Each muscle's signal was converted into a force estimate.

During the draw, on the string arm, the skilled archer produced by this estimate 280 N in the biceps, 213.9 N in the deltoid and 123.4 N in the trapezius; the occasional archer 371.1, 164.9 and 163.8 N. On the bow arm peak triceps values were similar, 337 N for the skilled archer and 323 N for the other. In the authors' reading the skilled archer spread the draw across two highly active muscles and the other relied on one, which would leave the latter more exposed to fatigue and hence to injury. How the forces evolved over time is shown only in charts.

Before shooting, the authors recorded a maximal voluntary contraction, performed by flexing the arms, to serve as a reference. Each archer then completed five shots with rests in between, and the shot was divided into six phases: set-up, pre-draw, draw, anchor, hold and aim, and release. The signal was filtered, rectified, normalised to the maximal contraction and averaged over the five trials.

To go from signal to force, the authors multiply the percentage activation by each muscle's theoretical maximum force, calculated as 25 newtons per square centimetre times the muscle's cross-section taken from the literature. The two archers averaged 82 kg in weight and 166 cm in height. Draw weight, draw length, distance, electrode placement, sampling rate and how the phases were identified are not reported.

With one archer of each kind, the comparison describes two people, not two levels: the authors themselves write that at least ten subjects per group would be needed. It is not stated whether the two used the same bow and draw weight, yet the force the draw requires depends mainly on that.

The force estimate has an internal problem. With 25 N/cm² and the cross-sections used in the paper, the theoretical maximum for the biceps is about 298 N and for the trapezius about 156 N: the occasional archer's 371.1 N in the biceps and 163.8 N in the trapezius exceed the maximum, so activation during the shot exceeded the reference. This happens when the reference contraction was not maximal for that muscle, and here it was an arm flexion movement, poorly suited to the trapezius or the triceps, for instance. Fatigue was not measured, and the step from fatigue to serious injury is the authors' hypothesis.

For compound archers the study offers no solid ground. The clearest result, that the occasional archer used more biceps during the draw, recalls the common view that less experienced archers pull more with the arm than with the back, although the same archer also had the higher trapezius estimate; but with two people, unknown draw weights and a force estimate that exceeds the limits of its own model, it remains an observation about these two archers. It says neither how much biceps is too much nor that a very active biceps leads to injury. The paper's contribution lies mainly in its attempt to record both arms across the six phases of a compound shot.