Achilles tendons with tendinopathy shrank right after isometric exercise

The Achilles tendon can change its dimensions under load in ways thought to relate to fluid movement within the tissue. What nobody knew was whether loading actually pushes fluid out of the tendon or simply redistributes it internally, and measuring whole-tendon volume is the way to tell the two apart. The answer matters for understanding how tendons respond to exercise, and whether painful tendons behave differently from healthy ones. This review set out to synthesise everything measured so far, in both healthy Achilles tendons and midportion Achilles tendinopathy.

The authors systematically searched five databases, MEDLINE, EMBASE, CINAHL, AMED and Scopus, from inception until May 2020 for studies measuring Achilles tendon volume before and after loading. Twelve studies were included in the pooled analysis, covering both immediate responses to a single bout of loading and longer-term responses to sustained programmes. Changes from baseline were expressed as standardised mean differences, and study quality was assessed with a modified version of the Newcastle Ottawa Scale. A secondary aim was to examine parallel changes in the tendon's cross-sectional area and length.

Healthy Achilles tendons showed negligible to small volume changes immediately after cross-country running and isometric exercise, suggesting they remain essentially isovolumetric under acute load. A 12-week isometric protocol produced a large short-term increase in healthy tendon volume, though this did not reach statistical significance. In midportion Achilles tendinopathy, the picture differed: isometric exercise produced a large and statistically significant immediate reduction in tendon volume, eccentric exercise produced a small non-significant increase, and long-term interventions produced a small significant volume reduction over the short term.

The finding that healthy tendons hold their volume under acute load suggests fluid redistributes within the tissue rather than being squeezed out, while tendinopathic tendons appear to lose volume in response to loading both immediately and over weeks. That contrast is the review's main contribution, but the evidence base is small and several pooled estimates were imprecise. It remains an open question whether these volume reductions track with symptoms, tissue healing or recovery, and what the ideal loading pattern for a painful tendon might be.

If you have a painful mid-Achilles, this is a reason to think the tendon responds to loading in a measurable, physical way, with fluid-related volume changes appearing immediately after isometric exercise. Healthy tendons, by contrast, seem to shrug off acute loading without changing volume. None of this yet tells anyone whether a smaller tendon volume is a good sign, so it is a window into tendon behaviour rather than a rehab instruction.