INTRODUCTION 염색체 이상으로 인해 나타나는 다운증후군은 작은 키, 특 이한 굴 생김새, 낮은 지능, 운동 발달 지연 등의 특징을 가 지고 있다(Sherrill, 1998). 특히 다운증후군의 일반적 특성으로 려져 있는 운동 발달 지연은 모터뉴런(motoneuron) 활성 저 하와 장 반사의 문제로 인해 비정상적인 근육 활성화가 유 발되는 근긴장저하(hypotonia)와 인대 이완(ligament laxity) 기인한다(Carr, 1970; Gilman, Bloedel & Lechtenberg, 1981). 인대 의 이완은 다운증후군 동들게서 주로 나타나는 대표적 특 징으로 근긴장저하 현상과 관련이 있다고 려져 있다(Hayes & Batshaw, 1993; Morris, Vaughan & Vaccaro, 1982). 이러한 근 긴장저하와 인대 이완은 하지 불정성의 원인이 되며, 다운 증후군 환자의 비정상적인 움직임의 원인이 될  있다(Caselli, Cohen-Sobel, Thompson, Adler & Gonzalez, 1991). 다운증후군 동은 근긴장저하로 인하여 낮은 하지 근력을 KJSB Korean Journal of Sport Biomechanics 2019; 29(3): 137-143 http://dx.doi.org/10.5103/KJSB.2019.29.3.137 http://e-kjsb.org eISSN 2093-9752 ORIGINAL Vertical Stiffness and Lower Limb Kinematic Characteristics of Children with Down Syndrome during Drop Landing 드롭랜딩 동작 시 다운증후군 아동들의 수직 강성과 하지 운동학적 특성 Dohoon Koo 1 , Hyokju Maeng 2 , Jonghyun Yang 3,4 1 Department of Physical Education, Seoul National University, Seoul, South Korea 2 Department of Kinesiology and Health, Georgia State University, Atlanta, U.S 3 Department of Physical Education, Incheon National University, Incheon, South Korea 3 Sport Science Institute, Incheon National University, Incheon, South Korea Received : 20 May 2019 Revised : 21 August 2019 Accepted : 21 August 2019 Corresponding Author Jonghyun Yang Department of Physical Education, Incheon National University, 29-306, 119 Academy- ro, Yeonsu-gu, Incheon, 22012, South Korea Tel : +82-32-835-8846 Fax : +82-32-835-4510 Email : j.yang@inu.ac.kr Objective: Ligament laxity and hypotonia are characteristics of Down syndrome patients. The aim of this study was to compare the landing pattern between Down syndrome patients and typically developing subjects. To compare the landing pattern, variables related to ligament laxity and hypotonia i.e. vertical stiffness and lower extremities kinematics were investigated. Method: Five subjects with Down syndrome (age: 14.6±1.8 years, mass: 47.6±6.94 kg, height: 147.9±6.0 cm) and six able-bodied subjects (age: 13.2±0.4 years, mass: 54.7±6.7 kg, height: 160.1±9.8 cm) participated in this study. Results: The vertical displacement of the center of mass, vertical reaction force, leg stiffness and range of ankle angle range among Down syndrome patients were significantly different than typically developing group. The youth with Down's syndrome appeared to receive greater vertical impact force at landing than normal youth. Conclusion: The differences in the biomechanical characteristics suggest the delay in motor development among Down syndrome patients and an increased risk of injury to the lower extremity during movement execution such as drop landing. Keywords: Down syndrome, Drop landing, Vertical stiffness Copyright ○ C 2019 Korean Journal of Sport Biomechanics This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by- nc/3.0/) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.