Skeletal muscle is known to be set at an over-stretched length in clinical tendon transfer. Such chronic stretching of skeletal muscle increases the serial sarcomere number required for muscle adaptation. Passive tension of the muscle must be affected during the adaptation. Thus, the objective of this study was to clarify the origin of increased passive tension after stretched tendon transfer in an animal model. The distal tendon of the extensor digitorum of the second toe was transposed to the extensor retinaculum at 3.7 mm of muscle sarcomere length. The contralateral muscle served as control. Muscle passive length-tension curves were measured at 1 week and 4 weeks after the transfer to the bilateral muscles. After functional measurements were taken, the muscles were dissected into fiber bundles and single muscle fibers to measure their passive mechanical properties with a micro force transducer. Passive tension was increased in the transferred muscle with a steeper inclination and leftward shift of muscle length-tension curve. Elastic modulus of the transferred fiber bundle increased at both time points, while those of the transferred single fibers increased only at 1 week. Results of the study suggest that the transferred muscle increased passive tension mainly due to proliferation of extracellular connective tissue within the muscle. Increased passive tension was a characteristic feature for the transferred muscle, which may ultimately represent a target for therapeutic intervention to optimize muscle function.