Archery shot biomechanics: Ahmad 2014 study

This is a preliminary instrumentation study: the authors put four devices on an archer at once and show what can be recorded during a shot. There is a single subject, an experienced coach of 23, right-handed, 180 centimetres and 84 kilos, shooting recurve indoors while standing on a force plate. The instruments are two high-speed cameras synchronised at 150 frames per second with 12 markers on wrists, elbows and shoulders, an EMG module at 1000 Hz on the deltoid and the finger flexor of the drawing arm, a force plate under the left foot, and a blood volume pulse sensor on the finger to derive heart rate and variability bands.

On the draw force line, the subject's elbow is not aligned: from above the measured angle is 45 degrees, from the side the line is almost straight with a slight deviation. The authors note that an elbow off the line is common in recurve archers and that, with a small distance from the line, the position remains acceptable.

On the EMG, the two muscles follow the same course across the three phases: low amplitude while setting the bow, highest at full draw and while aiming, with the finger flexor above 2.5 millivolts between 0.7 and 1.4 seconds, then a rise at 1.6 seconds and an abrupt collapse at release. The deltoid has the same profile at higher amplitudes.

On the plate, the load on the left foot reaches a maximum of 44.12 kilos out of 84 of total body weight, with the three phases placed between 0 and 1.2 seconds for the draw, 1.2 and 2.8 for the aim and 2.8 and 4 for the release. The load is not constant during the draw and the aim, the archer stabilises it as the shot approaches, and immediately after release the weight on the left foot drops sharply for an instant.

On the cardiac side, the mean heart rate over the three attempts is 112.10 beats per minute and the high-frequency band accounts for 46.88% of the power, with a low-to-high frequency ratio of 0.80. Over the three scored arrows, the high band is 47.83, 40.19 and 50.51 and the scores are 8, 7 and 8.

The subject is an experienced coach, selected by the authors, who shot indoors with a recurve bow standing with the left foot on the force plate. The measurements were all taken at the same time, so part of the equipment was worn: the EMG and the finger sensor.

The two EPIX cameras were arranged to record the top and sagittal views during draw, aim and release, synchronised and set to 150 frames per second. To reconstruct the draw force line, 12 reflective markers were applied to the wrists, elbows and shoulders, and the angles and distances were obtained with XCAP software.

Surface EMG, with a Shimmer module, recorded the right deltoid and right finger flexor at 1000 Hz: the authors explain that they chose those two because they are the muscles where the activity concentrates and the ones that can lead to injury. The signal was collected across the three phases of draw, aim and release.

The AMTI force plate recorded the ground reaction force. Only the left foot was on the plate, because the platform is small relative to the width of the archer's stance. Data were collected for five seconds, of which the first four were used, that is the actual duration of the shot, and analysed with BioAnalysis software to obtain the weight distribution millisecond by millisecond. The subject's weight and height had been measured beforehand.

The biofeedback uses a blood volume pulse sensor applied to the finger, which shines infrared light through it and measures the reflected light, which changes with each heartbeat. The device was calibrated for about 60 seconds; the archer then shot, releasing after three seconds of aiming counted on a stopwatch, and the points were read from a photograph of the target. The analysis is with BioGraph Infiniti software.

There is no statistical analysis: the study reports traces, single values and three scored attempts.

There is one subject, and three scored arrows. That settles what can be taken from the study: a demonstration of instruments, not a transferable result. The authors present it as a preliminary study, but the conclusions go much further: they write that all the biomechanical parameters considered are associated with the archer's performance, and that statement, on one archer and three arrows, has no basis. No correlation was calculated, although the aim stated in the abstract is precisely to correlate the biomechanical measures with performance; no statistical test appears in the text.

The link between concentration and score is the weakest point. It rests on three arrows: high-frequency band 47.83 with 8 points, 40.19 with 7 and 50.51 with 8. Three arrows separate nothing, and two of them have the same score. From this the authors conclude that the frequency level is directly linked to performance.

In the shooting data there are two inconsistencies a reader can check by hand. The text says that during the shot the weight sits more on the right foot than the left, but the only measurement available gives a maximum of 44.12 kilos on the left foot out of 84 of total body weight, that is more than half: the measurement and the sentence say opposite things. And the description of the three arrows speaks of a first arrow in the yellow and a third close to the yellow, while the scores are 8 and 8: on a competition face the 8 is in the red, not the yellow.

Other details weaken the reading. The time axis of the two EMG graphs is labelled in seconds in one case and in minutes in the other, though both describe the same shot. The EMG amplitude is given in raw millivolts, with no normalisation to a maximum contraction, so it is not comparable with other studies nor rigorously between the two muscles, and it is precisely the two muscles that are compared. The heart rate of 112 beats is called high relative to a «normal rate of 90», which is not a normal resting value. The reference for good balance is set at half the body weight on each foot, that is 42 kilos, assumed rather than measured: the archer's stance is not symmetrical with respect to the shooting line, and a single plate under one foot does not allow the distribution to be derived.

On the draw force line, the 45 degree angle is read from a single frame with markers, with no repetitions and no estimate of error, and the reasoning closes on itself: the subject was chosen because he is an experienced coach, and from the fact that his elbow is only slightly off the line the authors conclude that he is an experienced and knowledgeable archer. The introduction, finally, presents some coaching-school statements as established principles, for instance that forces should be maximised on the bones and minimised on the muscles, and the closing prospect, that beginners can be brought to elite level in a short time, comes from no data in this work.

No conclusions come out of this work, but a picture of the timings inside a single shot does, and that is the useful part. On the plate, the draw takes up about the first second and two tenths, the aim from one and two tenths to two and eight tenths, the release and what follows the last seconds. On the EMG, the activity of the drawing arm's muscles is low while setting the bow, highest at full draw and while aiming, and collapses abruptly at release. It is the sequence anyone filming themselves recognises, set alongside two measures that video does not give.

The most interesting finding is what happens after the release: the weight on the left foot drops sharply for an instant. It is a measure of the recoil passing through the body, and it says that the shot does not end when the arrow leaves. It is also worth noting that during the draw and the aim the load was not constant and that the archer stabilised it as the shot approached: steadiness is not a position you take up, it is something built in the seconds beforehand.

On the draw force line, the observation that stands is the authors' own: in this archer the elbow was not aligned, by 45 degrees seen from above, and they regard that as a common condition in recurve and acceptable as long as the distance from the line is small. It is a coaching-school indication, not a measurement of what a misalignment costs in points.

Two things, by contrast, cannot be taken away. That concentration measured at the finger predicts the score: it rests on three arrows, with scores of 8, 7 and 8. And any conclusion about the distribution of weight between the two feet: the plate was under one foot only, and the paper's text says the opposite of what its one measurement says.