Immediate and long-term effects of mechanical loading on Achilles tendon volume: A systematic review and meta-analysis.

This paper pulled together research looking at what happens to the Achilles tendon's actual volume when it's put under load, comparing people with healthy tendons to those with midportion Achilles tendinopathy, a painful overuse condition affecting the middle part of the tendon. The idea behind it is that tendons contain fluid, and loading might either shift that fluid around within the tendon or push it out altogether, and measuring volume changes is one way to figure out which is happening. The researchers combined data from twelve studies using a statistical method called meta-analysis, which pools results across studies to get a clearer overall picture.

For healthy tendons, volume barely changed after cross-country running or isometric exercise (exercise where the muscle contracts without moving), and while a 12-week isometric program showed a fairly large increase in volume, the confidence interval crossed zero, meaning the finding wasn't statistically solid. For tendons affected by tendinopathy, the picture looked different: isometric exercise was linked to a fairly large immediate drop in volume, eccentric exercise (where the muscle lengthens under tension) showed a small increase that wasn't statistically confident, and longer-term interventions were linked to a modest reduction in volume in the short term. Put simply, healthy tendons seemed to stay roughly the same size regardless of load, while tendons with tendinopathy showed more noticeable volume shifts depending on what kind of exercise was done.

The authors frame this as evidence that healthy and problematic tendons may respond to loading differently, though many of the individual results carried wide uncertainty.

If you're dealing with Achilles tendon pain, this study looked at whether exercise actually changes the physical size (volume) of the tendon, using imaging techniques across a set of research studies. In people with healthy tendons, exercise like running or isometric holds didn't seem to change tendon volume much at all, suggesting the tendon stays fairly stable in size regardless of the load placed on it.

In people with tendinopathy, the picture was more varied. Isometric exercise was linked to a fairly large immediate reduction in tendon volume, while eccentric exercise showed a small increase, though this particular result wasn't statistically strong enough to be confident about.

Longer-term exercise programs were linked to a modest reduction in tendon volume in the short term for people with tendinopathy. These findings suggest that tendons affected by tendinopathy might respond differently to loading than healthy tendons, though the exact meaning of these volume changes for pain, healing, or function isn't addressed in this research.

It's also worth knowing that some of these results came from a fairly small number of studies, so there's still real uncertainty around how consistent these patterns would be across different people and different types of exercise.

This is a summary of research, not medical advice. Talk to your own healthcare provider about anything affecting your care.

This systematic review and meta-analysis synthesised data from twelve studies examining Achilles tendon volume changes with mechanical loading, comparing healthy tendons to those with midportion Achilles tendinopathy. The rationale is that volume change can help clarify whether load-induced fluid shifts represent redistribution within the tendon or actual fluid loss, an unresolved question in tendon mechanobiology.

In healthy tendons, standardized mean differences were negligible to small following cross-country running and isometric exercise, and although a 12-week isometric protocol produced a large effect estimate, its wide confidence interval crossing zero limits confidence in that finding. In tendinopathic tendons, isometric exercise was associated with a large and statistically significant immediate reduction in volume, eccentric exercise showed a small increase that did not reach statistical significance, and longer-term interventions were associated with a small but statistically significant reduction in volume at short-term follow-up.

Methodological quality was assessed using a modified Newcastle Ottawa Scale, and the authors note the overall body of evidence is limited by the small number of studies per comparison and considerable variability in effect estimates, several of which had confidence intervals spanning zero. The apparent divergence between isovolumetric healthy tendons and volume-reducing tendinopathic tendons under acute loading is a notable pattern, though it stops short of establishing a mechanism and the clinical implications for tendon loading protocols remain an open question for further research.