Permanent loss of muscle function seen after an Achilles tendon rupture may partly be explained by tendon elongation andaccompanying shortening of the muscle. Muscle fascicle length shortens, serial sarcomere number is reduced, and the sarco-mere length is unchanged after Achilles tendon transection (ATT), and these changes are mitigated with suturing. The methodinvolved in this study was a controlled laboratory study. Two groups of rats underwent ATT on one side with a contralateral con-trol (CTRL):A) ATT with 3 mm removal of the Achilles tendon and no suturing (substantial tendon elongation), andB) ATT withsuture repair (minimal tendon elongation). The operated limb was immobilized for 2 wk to reduce load. Four weeks after surgerythe rats were euthanized, and hindlimbs were analyzed for tendon length, gastrocnemius medialis (GM) muscle mass, length, fas-cicle length, sarcomere number and length. No differences were observed between the groups, and in both groups the Achillestendon length was longer (15.2%,P<0.001), GM muscle mass was smaller (17.5%,P<0.001), and muscle length was shorter(8.2%,P<0.001) on the ATT compared with CTRL side. GM fascicle length was shorter (11.2%,P<0.001), and sarcomere num-ber was lower (13.8%,P<0.001) on the ATT side in all regions. Sarcomere length was greater in the proximal (5.8%,P<0.001)and mid (4.2%,P¼0.003), but not distal region on the ATT side. In this animal model, regardless of suturing, ATT resulted intendon elongation, loss of muscle mass and length, and reduced serial sarcomere number, which resulted in an“overshoot”lengthening of the sarcomeres.
NEW & NOTEWORTHY
Following acute Achilles tendon rupture, patients are often left with functional deficits. The specific rea-son remains largely unknown. The shortened muscle leads to reduced fascicle length, in turn leading to adaptation by reducedserial sarcomere numbers. Surprisingly, this adaptation appears to“overshoot”and lead to increased sarcomere length. Thepresent animal model advances understanding of how muscle sarcomeres, which are difficult to measure in humans, areaffected when undue elongation takes place after tendon rupture.