Tendon elongation in the free tendon is evident in patients with and without persistent muscle weakness following an Achilles tendon rupture

Incomplete recovery with persistent muscle weakness is frequently observed following Achilles tendon rupture. The mechanisms for this weakness remain unclear, but tendon elongation has been suggested as a contributing factor. The aim of this study was to compare tendon and muscle morphology in high- and low-functioning patients more than 2 years after non-surgical treatment of a total Achilles tendon rupture. Forty-six patients underwent screening, including a standardized heel-rise work test on both legs. Based on this test, a heel-rise index (HRI) for total muscle work, categorized 29 patients into a low- (HRI&#x2009;<&#x2009;33%) or high-functioning (HRI&#x2009;>&#x2009;67%) group. Both groups underwent bilateral magnetic resonance imaging to assess tendon and muscle morphology, and the main variable was tendon elongation. High-functioning patients were on average 13 years younger than the low-functioning patients (p&#x2009;<&#x2009;0.001). Free tendon length was longer on the injured side in both groups, 3.59 and 5.19&#x2009;cm in the high- and low-functioning group (p&#x2009;=&#x2009;0.053). Tendon cross-sectional area was significantly larger in the high-functioning group compared to the low-functioning group (383% vs. 256% after normalization to the uninjured side, p&#x2009;=&#x2009;0.005). The soleus muscle had notable differences between the groups, as low-functioning patients had a smaller mediolateral diameter (p&#x2009;=&#x2009;0.002), a more pronounced muscle length difference (p&#x2009;=&#x2009;0.009) and a higher atrophy grading. Additionally, there were significant correlations between age, HRI and tendon size. Free tendon length after rupture may play a role in muscle weakness. However, tendon elongation does not necessarily lead to low function, as it was also evident in the high-functioning group. These findings may be important, as they suggest that tendon elongation is not the sole determinant of functional outcome and that other factors may contribute to muscle performance after rupture. Further research is needed on the role of age in muscle function following Achilles tendon ruptures. Level III.