Trunk muscles in young archers: Shitara 2017 study

Magnetic resonance imaging was used to measure the cross-sectional areas of four trunk muscles (rectus abdominis, the lateral abdominal muscle group, psoas major and erector spinae) in 352 high-level athletes supported by the Japanese national institute: 188 juniors aged 12 to 18 and 164 seniors aged 19 and over. Among them are 88 archers, 45 male and 43 female, chosen by the authors as the sport in which the trunk works only statically, that is, to hold the position rather than to produce the movement.

In the archers age changes very little. On the values divided by height squared, the only difference across the three bands (12-15, 16-18 and senior) is in the male rectus abdominis, larger in the seniors than in either junior band. In every other muscle in the males, and in every muscle in the females, the three bands are indistinguishable: female archers of thirteen, of seventeen and in adulthood have trunks of similar size. In the other sports, by contrast, differences between the bands are plentiful.

In the comparisons between sports the archers are almost always at the bottom. Among males aged 16 to 18 the wrestlers have larger rectus abdominis, lateral abdominals, psoas and erector spinae than the archers; among senior males the wrestlers exceed all four other sports on three of those muscles. Among girls aged 12 to 15 the gymnasts have larger lateral abdominals and erector spinae than the archers and the table tennis players; among the 16 to 18 year olds the gymnasts exceed the archers on rectus abdominis, and both table tennis players and gymnasts exceed them on the lateral abdominals; among the seniors the same picture repeats.

The reversal comes when you look at the proportions, that is, how much each muscle contributes to the trunk total. Among senior males the share of rectus abdominis is highest in the archers, ahead of Nordic combined, wrestling, basketball and table tennis, with significant differences against the table tennis players. Among girls aged 16 to 18 the archers have the lowest share of lateral abdominals of any sport and the highest share of psoas major, and among the 12 to 15 year olds the highest share of rectus abdominis. In absolute terms the male archers go from 149.4 square centimetres of total trunk muscle at 12-15 to 165.5 at 16-18 and 163.5 in the seniors; the female archers from 114.1 to 114.4 to 119.7.

The participants are athletes who had been through the fitness check at the Japanese national sport institute. The juniors belonged to their federations' national squads and were mostly placed at national championships or national games; some had competed at the Universiade or the Youth Olympics, and a proportion of the table tennis players and wrestlers were part of the Japanese Olympic Committee's academy programme. The seniors also belonged to national squads, with appearances at the Olympics, world championships, World Cup and Universiade. The sports are five for the males (archery, table tennis, Nordic combined, basketball, wrestling) and three for the females (archery, table tennis, artistic gymnastics), chosen to differ in their competitive movements: archery as the sport with no dynamic trunk movement, table tennis for trunk rotation, and the others for reasons given one by one.

Imaging used a 1.5 tesla scanner, a T1-weighted image, a body coil, a gradient echo sequence, a repetition time of 98 milliseconds, an echo time of 4.28, a field of view of 380 by 380 millimetres, a 256 by 256 matrix and a slice thickness of 10 millimetres. The athlete lay supine and still, and held their breath for 24 seconds so that movement would not blur the image. There is a single slice, taken just above the upper edge of the pelvis along the line joining the iliac crests, and the areas of the four muscles were traced on that image with dedicated software.

The analysis uses two quantities: the cross-sectional area divided by height squared, to account for differences in build between the age bands, and each muscle's share of the total area of the four. Comparisons are made with a two-way analysis of variance (age band and sport), with simple effect tests where the interaction was significant, Tukey multiple comparisons where the main effect was, and the non-parametric Kruskal-Wallis test in the four cases where variances were not homogeneous. The threshold is 5%. The text is in Japanese, with the abstract, tables and captions in English.

The archery figures come from very small cells: 8 males aged 12 to 15, 26 aged 16 to 18 and 11 seniors; 5 females aged 12 to 15, 23 aged 16 to 18 and 15 seniors. With eight or five people in a band, an age difference that fails to reach significance may simply be undetectable, and the authors acknowledge the imbalance in participant numbers across bands and sports.

The study is cross-sectional, so the three bands are three different groups of people rather than the same athlete followed over time. Where the text speaks of the trainability of the trunk at a stage of growth, it is reconstructing a developmental course from snapshots of different groups; the authors present this as an inference, leaning on longitudinal studies of other body segments.

The measure is a single slice taken at the level of the iliac crests: it tells you how large the muscle is at that point, not its volume, and says nothing about strength, endurance or the way those muscles work while shooting. The muscle is measured at rest, lying supine, far from any movement.

The idea that the archers' trunk works only statically is a criterion the authors used to pick the sports for comparison, not a finding of this study: no electromyography and no movement analysis was carried out here. And since the groups are not made up of the same people, differences such as the higher share of rectus abdominis in the senior archers may also reflect who reaches that level in that sport, not training alone. No performance measure was collected in any of the sports: there is no way to know whether the archers with the larger trunk, or with different proportions, scored more.

The useful reading for an archer lies in the contrast between two ways of looking at the same data. In absolute cross-section, and relative to height, the archers' trunk is the smallest or among the smallest of the groups compared, and in the female archers it does not change measurably from thirteen to adulthood. In proportion, by contrast, the largest slice of the trunk in senior male archers is the rectus abdominis, and in girls aged 16 to 18 the lateral abdominals are the smallest slice of the three sports.

What this does not say is that an archer should train the trunk one way or another. The study measures size, not capacity, and links none of its measures to a score: there is nothing here that would allow anyone to say that a larger rectus abdominis or smaller lateral abdominals help or hinder someone who shoots. A national-level archer's trunk resembles that of a contemporary from another sport more than it resembles a wrestler's, and in the female archers the figures stay flat right through adolescence: this is a description of the bodies that reach the top of this sport, not a programme.

One point should be kept in mind when reading these comparisons: archery is the sport the authors chose as their static reference, and every comparison in this paper follows from that choice.