Neuromechanical adaptations in the gastrocnemius muscle after Achilles tendon rupture during walking

Although some Achilles tendon rupture (ATR) patients regain function in low-force levels activities, it is not yet well known how neuromuscular and structural alterations after ATR manifest during everyday-locomotion. This study assessed medial gastrocnemius (MG) fascicle shortening during walking 1-year after ATR. Additionally, we explored neuromuscular alterations in lateral gastrocnemius (LG), soleus and flexor hallucis longus (FHL) muscles. We observed 3.1 pp (95 %CI 0.8-5.4 pp) higher average and 14.5 pp (95 %CI 0.5-28.5 pp) higher peak LG surface electromyography amplitude in the injured compared to the un-injured during walking, but no differences were observed in soleus or FHL. The injured limb fascicles were 12.9 mm shorter while standing compared to the un-injured limb. In absolute terms, MG shortening in the injured limb was 2.8 mm (95 %CI 0.96-4.6 mm) smaller compared to the un-injured limb. However, when normalized to standing fascicle length, the amount of shortening was not different between the limbs. Our results showed that 1-year after ATR, MG muscle had remodelled, which manifested as shorter fascicle length during standing. During walking, injured and un-injured MG fascicles showed similar shortening relative to the standing fascicle length, suggesting that MG could function effectively at the new mechanical settings during everyday locomotion.