Can Exercise Affect Tendons — Even in Identical Twins?

The body's invisible force transfer

Think of the muscle as an engine and the tendon as part of the transmission between the engine and the movement. The Achilles tendon connects the calf muscles to the heel bone and is an important part of how force is transferred around the ankle joint.

The analogy is useful, but the body is not a machine with simple spare parts. Tendon properties are influenced by biology, loading history, age, and several other factors that cannot be summarised in a single measure.

What is tendon stiffness?

Tendon stiffness is a mechanical property that describes how much a tendon resists deformation when loaded. The word "stiff" therefore does not mean the same as rigid in the everyday sense.

Tendon stiffness is also not the same as flexibility, pain, muscle strength, or the tendon's ability to withstand a future injury. A higher measured value cannot on its own say that the tendon is better, healthier, or stronger.

In the study, stiffness was estimated with a handheld device that applied a small mechanical impulse to the skin and recorded the tissue's oscillation response. The authors describe the result as a surrogate measure, not a complete mapping of the tendon's material properties.

Why identical twins are interesting

Identical twins share essentially the same DNA sequence. When researchers compare twins within the same pair, they can reduce some of the genetic variation that otherwise makes human studies difficult to interpret.

That does not mean the effect of exercise becomes isolated. The twins may differ in work, previous injuries, illness, how they use their bodies, and activity during childhood. A co-twin comparison is valuable, but it does not turn a cross-sectional study into a controlled exercise experiment.

How the study was conducted

Researchers included 40 identical twin pairs: 19 female and 21 male pairs. Mean age was around 40 years, but the age range was broad.

Participants answered questions about regular physical activity, type of activity, weekly training volume, and — when they could remember — how many years they had engaged in the activity.

In the study, a participant was counted as active if they had engaged in at least 60 minutes of regular physical activity per week for at least one year, regardless of intensity. This is a research definition, not a training recommendation.

What the researchers actually found

Among the 40 pairs, both twins were inactive in eight pairs, both were active in 25 pairs, and activity levels differed in seven pairs.

When researchers compared the pairs where both were active with the pairs where both were inactive, Achilles tendon stiffness was only slightly higher in the active group. The difference was not statistically significant.

The more notable finding only emerged in the smaller comparison within the seven pairs where one twin was active and the other inactive.

The finding from the seven discordant pairs

In the seven activity-discordant pairs, the active twins had on average approximately 28% higher measured Achilles tendon stiffness than their inactive co-twins. The difference was statistically significant in the study's analysis.

The result is interesting because the comparison is made within genetically very similar pairs. But seven pairs is a very small basis. The percentage describes this particular sample and must not be used as a promise of what activity does to every twin's tendon.

Because the measurement was made at a single point in time, the study can show an observed difference — not the time order or that physical activity alone caused it.

What the study did not measure

The device did not measure the tendon's cross-sectional area or other morphological changes. The study also did not measure ligaments, bone, the nervous system, muscle strength, or performance capacity.

It did not examine whether participants had less pain, fewer injuries, or better rehabilitation outcomes. The result therefore cannot be used as evidence that a stiffer tendon is more resistant to injury.

The researchers did not collect activity history from childhood and adolescence. Early differences in movement and loading therefore cannot be ruled out as part of the explanation.

Activity with and without a flight phase

The researchers also divided activities according to whether they regularly involved a flight phase — such as running or jumping where both feet leave the ground — or not.

Larger differences in measured Achilles tendon stiffness were seen in pairs where at least one twin engaged in a flight-phase activity. This may suggest that activity type matters, but the analysis was coarse and was not based on a clean comparison where one twin had a flight-phase activity and the other did not.

Only two pairs met the stricter comparison. The classification also did not account for exact impact forces, frequencies, or tendon loading. The result should therefore be treated as a hypothesis-generating signal.

Why stiffer does not automatically mean stronger or healthier

It is tempting to translate a higher stiffness value into "stronger tendon". This article does not do that. Strength concerns how much load tissue can tolerate before injury, while stiffness describes the relationship between load and deformation.

The two properties may be related in some contexts, but they are not interchangeable. The same caution applies to words like healthier, more flexible, more explosive, and more injury-resistant.

The study therefore gives no basis for ranking twins' tendon health or recommending how anyone should train, prevent injuries, or rehabilitate an Achilles tendon.

Two twins — different use of the body

What is fascinating is not that one twin can have a "better" tendon than the other. It is that genetically very similar people can still show different mechanical measurements after different lives and activity habits.

The finding fits a broader TwinPare question: how does the body we are born with interact with the life we actually live? Here the study offers a small, cautious puzzle piece — not a finished answer.

The fun truth about twins

Twins often compare height, fitness, strength, and who gets sore muscles first. Achilles tendon oscillation responses end up lower on the family top list.

Yet the study shows why the invisible details can be the most interesting. Two nearly identical genetic starting points can still carry traces of how the bodies have been used.

That does not make one twin the winner. It makes the comparison an unusually good research question.

The TwinPare perspective

At TwinPare we want to make twin research understandable without making it bigger than it is. This article therefore tells both the story of the interesting 28% finding and the fact that it rested on seven pairs.

That distinction matters. An engaging number can spark curiosity, but only when sample, measurement method, and limitations follow along does it become useful knowledge.

The twin study suggests that habitual activity may be associated with measured Achilles tendon stiffness. It does not show that training automatically builds stronger, healthier, or more injury-resistant tendons.

Source and limitations

This article is based on a peer-reviewed study published 5 January 2022 in Frontiers in Physiology, volume 12 (2021), article 777403. It covered 40 identical twin pairs and was exploratory and cross-sectional.

Activity was classified with questionnaires and was partly based on participants' memory. Only seven pairs had one active and one inactive twin, and childhood and adolescent activity history was not collected.

The handheld oscillation device gave a surrogate measure of stiffness at one measurement point. The study did not measure the tendon's cross-sectional area, complete material properties, pain, injury history, or future injury risk.

The result supports cautious phrasing about an association between habitual physical activity and measured Achilles tendon stiffness within this small sample. It does not prove that activity caused the difference and gives no individual training or rehabilitation advice.