31.10.2026 · 5 min read
In acrobatic sports — gymnastics, circus, pole sport — spinal mobility is valued almost as highly as strength. A common notion is that the more flexible someone is, the more "pliable", and the stronger, the more "stiff". From this a conclusion is sometimes drawn that flexibility and strength get in each other's way. The new study tested a more modest version of the question: is global spinal mobility related to trunk strength — and, for now, not in athletes but in ordinary schoolchildren.
The participants were 226 healthy schoolchildren from Valencia (Spain): 96 pupils from primary school (ages 9–11) and 130 from secondary school (ages 12–16). In a single session during a physical education lesson they underwent an assessment of spinal mobility and movement speed with an inertial sensor, the Biering–Sørensen test of back-extensor endurance and an isometric pull with dynamometry. Pain was also asked about on the Wong-Baker scale.
Older pupils turned out to be stronger than younger ones, and mobility was higher in girls. But the link between mobility and strength was almost absent: in three of the four subgroups, correlations of mobility with a composite strength index were non-significant. In middle-school boys a small negative association was observed (r = −0.25), but after adjustment for anthropometry it ceased to be significant for the overall index and remained only for time in the Biering–Sørensen test.
Passive flexibility shows the range available with external assistance. Active flexibility is the ability to get into a position unaided. Strength through range means being able to produce force at a specific amplitude, and control means managing the trajectory and position under load.
A person can have a large passive range and insufficient control at its end. Another can be strong in mid-range but lack the necessary mobility. A single global test does not describe the whole system. The authors note that the role of neural and coordination factors was not measured.
The study did not include adults, athletes or extreme positions on the pole. It therefore does not refute the value of flexibility and does not define exercises for pole sport. It only supports a practical principle: range should be developed together with strength and control in the positions that matter.
Strength training itself can improve range of motion if the exercise is performed through a suitable range. Stretching can also affect strength depending on dose and context. These are not competing camps. In practice this means: if a person bends well but cannot hold the position, they need work on active control; if they are strong but stiff, work on mobility without losing strength.
The correlation coefficient r shows how closely two quantities are related: 0 means no relationship, 1 or −1 a perfect one. A value of r = −0.25 in middle-school boys means a weak negative association: with greater mobility, the time in the Biering–Sørensen test is slightly shorter. Such a value explains only a small share of the differences between children and partly disappeared after adjustment for height and weight. Between mobility and movement speed in primary school a moderate association was found (r about 0.44–0.45) — but these are different characteristics of movement, not strength.
It did not measure the flexibility of individual segments of the spine, active control in extreme positions or the technique of performing elements, and did not include training athletes. Biological maturation and habitual physical activity were not assessed, and these factors influence both flexibility and strength. So the work supports neither the conclusion "flexibility gets in the way of strength" nor "they develop in step".
It makes sense to assess several qualities separately: passive range, active range, strength in the position and stability. If one quality lags, it should be developed deliberately rather than hoping it will catch up by itself through another. This is especially important in sports where injury risk rises at the end of range.
Standardised tests and subgroup analysis are useful, but the work has many limitations. The design is cross-sectional: it shows neither causality nor changes after training. The subgroups are small (about 47–79 people), power was estimated at about 0.60, and no correction for multiple comparisons was applied. The two strength tests hardly correlated with each other, so the "composite index" is a descriptive summary, not a single quality. Biological maturity and habitual activity were not assessed. Active control at extreme range was not measured.
Finally, the age of the participants matters: in children, mobility and strength change quickly with growth, while in adults, especially those who train, the relationships may be different. So the subgroups in which correlations were non-significant say, rather, that at this age there is no simple universal dependence between global mobility and trunk strength.
Flexibility is not equal to strength, but it is not their opposite either. For pole sport it is more sensible to train available range, strength and control as linked tasks, without transferring the findings of a school-age study directly to adult athletes.
Tags: children, spinal mobility, trunk strength, flexibility, pole sport
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