To investigate the effect on intramuscular connective tissue and passive range of joint motion by the stress produced in limb lengthening. Methods:An animal model of limb lengthening was established in the tibia of rabbits. Distraction was initiated at a rate of 1 mm/d and 2 mm/d in two steps respectively,and both proceeded until 10%and 20% of the tibia length was achieved. Muscle samples were harvested at the time when distraction ended and at the 4th week of consolidation after the distraction. Scanning electron microscope was applied to observe the morphological changes of the perimysium. The goniometer,which we made for this study,was used to measure the passive range of joint motion.Results:The collagen fibers were partitioned in bundles, crimped and interconnected closely and orderly.In the regime of 1 mm/d distraction with 10% lengthening,no apparent changes of the collagen fiber and passive range of joint motion was demonstrated. When tibia was increased to 20%,the crimped fibers showed a tendency of being straightened while the passive range of joint motion was reduced. The findings remained the same at the 4th week of consolidation. In the regime of 2 mm/d distraction with 10% lengthening,the crimped structure of the collagen fibers in the perimysium disappeared and the fibers were almost straightened. Additionally,the interconnection of the collagen fibers became loosened and interstice was presented among the fibers. At the 4th week of consolidation,the restoration to the original crimped structure was not completed. When the lengthening ratio was increased to 20%,the collagen fibers were straightened completely. This condition remained unchanged throughout all 4 weeks. The passive range of joint motion was reduced dramatically in the regime of 2 mm/d distraction.Conclusion:The ultrastructure of perimysium and the passive range of joint motion in the regime of 1 mm/d lengthening shows the condition closest to the normal ones. The regime of 2 mm/d lengthening may cause an apparent change in the ultrastructure of perimysium and passive range of joint motion.