Centre of Mass Position in Gymnastics Balances – Are the Textbook Illustrations Correct?

Abstract

This study examined if the centre of mass (CM) position of static balances in gymnastics textbooks were correctly illustrated. Out of 108 gymnastics textbooks screened for figures that illustrated CM positions in static balance postures, 33 images from four textbooks were retrieved for analysis. In each image, the actual CM was calculated using the segmentation method based on measured segmental lengths and body segmental inertial parameters. The offset distance between the illustrated and calculated CM was quantified to indicate the accuracy of the book illustration. To facilitate comparisons among images of different sizes, the offset distance was normalised to the leg length of the body as a percentage error. An error of less than 10% was considered small, 10 to 20% moderate, and greater than 20% large. Results showed that the average error was 12.8% (ranging from 0% to 39.0%). The error was small in 17 (52%) images, moderate in 11 (33%) images, and large in 5 (15%) images. Calculated CM were often (26 out of 33, or 78.8%) superior to the illustrated positions. One common misconception was that the illustrated CM tended to lie within the body in postures where the calculated CM was outside the body (e.g. half headstand, seated pike stretch). These incorrect CM illustrations and concepts in gymnastics textbooks may misguide the pedagogical practices for physical education teachers and coaches. To better reflect the true CM of gymnastics postures, future reference materials should incorporate anatomical and biomechanical expertise in the preparation of figures.