Displacement of the thorax and head are directly related to displacement of the shoulder girdle. In the event of a frontal crash, the child will interact with the CRS harness, forcing the clavicles posterior, along with the entire shoulder girdle complex. Currently, the 3-year-old ATD shoulder complex may not fully capture this movement. By understanding how the shoulder and clavicle move, we can better predict the overall effect of impacts on the thorax, cervical spine, and head. To our knowledge there have been no studies that have quantified anterior-posterior clavicular displacement of 2-4 year old children. This study seeks to understand the role of chest clip position and harness tightness on clavicular displacement in 2-4 year old children.

Data was collected from 23 children, 2-4 years old, after obtaining IRB approval and parental consent. Anthropometry measurements were taken and clavicular motion was captured using an electromagnetic tracking system (trakSTAR, Ascension, Shelburne, VT). Surface electrodes were placed on the:​ sternum, spinous process of T1, distal end of the right clavicle, right acromion process (of scapula), and right deltoid tubercle (of humerus). Anatomical segments were digitized using MotionMonitor (Innovate Sports Training, Inc., Chicago, IL). Following instrumentation, anterior and posterior passive excursion of the shoulder was measured while the child was holding onto a hand-strap connected to a hand-held digital dynamometer (Baseline Instruments, White Plains, NY). Children were passively pulled into shoulder flexion and extension with 5-10 pounds of force. Each measurement was repeated twice. Children were placed in a 5-point CRS (Evenflo Triumph, Evenflo Company Inc.). Clavicular displacement was measured using 4 conditions:​ proper harness tightness and chest clip position, proper harness tightness with low chest clip position, loose harness with proper chest clip position, and loose harness with low chest clip position. Loose harness conditions were standardized on each child using a 6.3cm block. Each CRS testing condition was repeated 3 times. Anthropometry measurements and CRS scenarios were repeated using a 3 year-old ATD for comparison.

Basic anthropometry was collected. Anterior-posterior displacement (x-axis) of the clavicle was calculated relative to the sternum in all CRS conditions. Displacement x conditions (CRS-Normal, CRSLow, CRS Lose-Normal, CRS Loose-Low) were analyzed using MatLab (The MathWorks, Natick, MA). Volunteer data was compared to the 3 year-old ATD. A total of 23 volunteers (2-4 years-old) and one ATD were tested. Volunteers were significantly smaller across most anthropometric measurements. Maximum anterior and posterior displacement of the clavicle and anterior maximum displacement of T1 were significantly greater in the volunteer population. The effect of chest clip and harness tightness on clavicular movement was also analyzed, showing that the location of the chest clip was a greater factor on anterior displacement of the thorax compared to harness tightness.

These data may benefit future 3 year-old ATD shoulder designs or computer models. Understanding clavicular movement relative to chest clip position will help to better predict the effect of impacts on thorax, cervical spine, and head excursion in motor vehicle crashes.