Identification of the contribution of contact and aerial biomechanical parameters in acrobatic performance

Introduction
Teaching acrobatic skills with a minimal amount of repetition is a major challenge for coaches. Biomechanical, statistical or computer simulation tools can help them identify the most determinant factors of performance. Release parameters, change in moment of inertia and segmental momentum transfers were identified in the prediction of acrobatics success. The purpose of the present study was to evaluate the relative contribution of these parameters in performance throughout expertise or optimisation based improvements. The counter movement forward in flight (CMFIF) was chosen for its intrinsic dichotomy between the accessibility of its attempt and complexity of its mastery.
Methods
Three repetitions of the CMFIF performed by eight novice and eight advanced female gymnasts were recorded using a motion capture system. Optimal aerial techniques that maximise rotation potential at regrasp were also computed. A 14-segment-multibody-model defined through the Rigid Body Dynamics Library was used to compute recorded and optimal kinematics, and biomechanical parameters. A stepwise multiple linear regression was used to determine the relative contribution of these parameters in novice recorded, novice optimised, advanced recorded and advanced optimised trials. Finally, fixed effects of expertise and optimisation were tested through a mixed-effects analysis.
Results and discussion
Variation in release state only contributed to performances in novice recorded trials. Moment of inertia contribution to performance increased from novice recorded, to novice optimised, advanced recorded, and advanced optimised trials. Contribution to performance of momentum transfer to the trunk during the flight prevailed in all recorded trials. Although optimisation decreased transfer contribution, momentum transfer to the arms appeared.
Conclusion
Findings suggest that novices should be coached on both contact and aerial technique. Inversely, mainly improved aerial technique helped advanced gymnasts increase their performance. For both, reduction of the moment of inertia should be focused on. The method proposed in this article could be generalized to any aerial skill learning investigation.

引言
以极低重复训练量教授体操技巧，是教练面临的核心挑战。生物力学、统计学或计算机模拟工具可协助教练识别影响表现的最关键决定因素。研究已证实，释放参数、转动惯量（moment of inertia）变化以及节段动量传递（segmental momentum transfers），可用于预测体操动作的成功完成情况。本研究旨在评估，在基于专业水平提升或优化改进的全流程中，上述参数对体操表现的相对贡献度。本研究选取空中前向反向动作（Counter Movement Forward in Flight，CMFIF）作为研究对象，因其兼具尝试门槛较低与掌握难度复杂的内在二元特征。

研究方法
本研究借助运动捕捉系统（motion capture system），记录了8名新手女子体操运动员与8名高水平女子体操运动员各完成3次CMFIF动作的过程。同时计算了可最大化再抓握时旋转势能的最优空中动作技术。本研究采用通过刚体动力学库（Rigid Body Dynamics Library）构建的14节段多体模型，计算记录动作与最优动作的运动学参数及生物力学参数。本研究采用逐步多元线性回归（stepwise multiple linear regression）法，明确上述参数在新手记录组、新手优化组、高水平记录组以及高水平优化组试验中的相对贡献度。最后，通过混合效应分析（mixed-effects analysis），检验了专业水平与优化改进的固定效应。

结果与讨论
仅在新手记录组试验中，释放状态的变化对表现存在显著贡献。转动惯量对表现的贡献度，从新手记录组、新手优化组、高水平记录组到高水平优化组依次递增。在所有记录组试验中，飞行过程中向躯干传递动量的贡献度占据主导地位。尽管优化改进降低了动量传递的贡献度，但向手臂传递动量的效应开始显现。

结论
研究结果表明，教练应针对新手运动员同时开展接触动作与空中动作技术的训练。与之相反，仅优化空中动作技术即可帮助高水平体操运动员提升表现。针对两类运动员，均应重点关注转动惯量的降低。本文提出的研究方法，可推广应用于各类空中技巧学习的相关研究。