Skeletal muscles are overstretched following limb lengthening procedures. Muscles can adapt to this lengthening by adding new sarcomeres in series. Recent developments in limb lengthening provide adult patients more opportunities to undergo limb lengthening procedures. The purpose of this study was to clarify the difference in muscle adaptation between adult and young groups using a rabbit model of limb lengthening. Five mature (10-43 month old) and 6 immature (3-4 month old) white rabbits underwent tibial osteotomy. After a 1-week lag phase, tibial lengthening was applied at a rate of 1.4 mm/day for 2 weeks. Animals were euthanized after the completion of lengthening. Both hindlimbs were immersed in buffered formalin with the ankle and knee at a right angle. Muscle belly length, muscle fiber bundle length and sarcomere length were measured, and sarcomere number and internal tendon length were calculated in five representative muscles around the lengthened segment. Muscle belly length increased in all the lengthened muscles compared with the corresponding contralateral muscles regardless the group. Aponeurosis length increased significantly in one muscle for the adult group and three muscles for the young group. Sarcomere length tended to be longer or was significantly longer in the lengthened muscles. Serial sarcomere number significantly increased in 3 lengthened muscles in the adult group with a more conspicuous increase in the amount, while this occurred in 2 muscles in the young group with only a tiny increase. The predominant processes of skeletal muscle adaptation to the limb lengthening are sarcomere number addition in muscle fibers for the adult group and elongation of internal tendon for the young group.