Medial gastrocnemius muscle fascicle function during heel-rise after non-operative repair of Achilles tendon rupture

Background To better understand muscle remodelling in dynamic conditions after an Achilles tendon rupture, this study examined the length of medial gastrocnemius muscle fascicles during a heel-rise at 6- and 12-months after non-operative ATR treatment. Methods Participants (15 M, 3F) were diagnosed with acute Achilles tendon rupture. Medial gastrocnemius subtendon length, fascicle length and pennation angle were assessed in resting conditions, and fascicle shortening during bi- and unilateral heel-rises. Findings Fascicle shortening was smaller on the injured side (mean difference [95% CI]: −9. 7 mm [−14.7 to −4.7 mm]; −11.1 mm [−16.5 to −5.8 mm]) and increased from 6- to 12 months (4.5 mm [2.8–6.3 mm]; 3.2 mm [1.4–4.9 mm]) in bi- and unilateral heel-rise, respectively. The injured tendon was longer compared to contralateral limb (2.16 cm [0.54–3.79 cm]) and the length decreased over time (−0.78 cm [−1.28 to −0.29 cm]). Tendon length correlated with fascicle shortening in bilateral (r = −0.671, p = 0.002; r = −0.666, p = 0.003) and unilateral (r = −0.773, p ≤ 0.001; r = −0.616, p = 0.006) heel-rise, at 6- and 12-months, respectively. In the injured limb, the change over time in fascicle shortening correlated with change in subtendon length in unilateral heel-rise (r = 0.544, p = 0.02). Interpretation This study showed that the lengths of the injured tendon and associated muscle can adapt throughout the first year after rupture when patients continue physiotherapy and physical exercises. For muscle, measures of resting length may not be very informative about adaptations, which manifest themselves during functional tasks such as unilateral heel-rise.