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

The Achilles tendon (AT) may experience changes in dimensions related to fluid flow under load. The extent to which fluid flow involves redistribution within or flow out of the tendon is not known and could be determined by investigating volume changes. This study aimed to synthesize data on immediate and long-term effects of loading on tendon volume among people with a healthy AT and midportion Achilles tendinopathy (MAT). A secondary aim was to synthesise data from the included studies investigating parallel change in cross-sectional area and length. Systematic electronic search was performed in MEDLINE, EMBASE, CINAHL, AMED, and Scopus from inception until May 2020. Standardized mean differences (SMDs) were calculated for intervention-induced changes from baseline for all outcomes. Methodological quality was assessed using modified version of Newcastle Ottawa Scale (NOS). Twelve studies were included in meta-analysis. For healthy AT, there were negligible to small changes in volume following cross-country running (−0.33 [95% CI = −1.11 to 0.45] (P = 0.41)) and isometric exercise (0.01 [95% CI = −0.54 to 0.55] (P = 0.98)) and a large increase at the short-term with 12-week isometric protocol (0.88 [95% CI = −0.10 to1.86] (P = 0.08)). For MAT, there was an immediate large reduction in volume with isometric exercise (−1.24 [95% CI = −1.93 to −0.55] (P = 0.0004)), small increase with eccentric exercise (0.41 [95% CI = −0.18 to 1.01](P = 0.18)) and small reduction at the short-term with long-term interventions (−0.46 [95% CI = −0.87 to −0.05] (P = 0.03)). This meta-analysis suggests that healthy AT remain isovolumetric with acute interventions while MAT exhibit immediate and short-term volume reductions in response to different interventions.