Boddy Kyle J, Marsh Joseph A, Caravan Alex, Lindley Kyle E, Scheffey John O, O'Connell Michael E
Research and Development, Driveline Baseball, Inc, Kent, WA, USA.
PeerJ. 2019 Jan 24;7:e6365. doi: 10.7717/peerj.6365. eCollection 2019.
Improvements in data processing, increased understanding of the biomechanical background behind kinetics and kinematics, and technological advancements in inertial measurement unit (IMU) sensors have enabled high precision in the measurement of joint angles and acceleration on human subjects. This has resulted in new devices that reportedly measure joint angles, arm speed, and stresses to the pitching arms of baseball players. This study seeks to validate one such sensor, the MotusBASEBALL unit, with a marker-based motion capture laboratory.
We hypothesize that the joint angle measurements ("arm slot" and "shoulder rotation") of the MotusBASEBALL device will hold a statistically significant level of reliability and accuracy, but that the "arm speed" and "stress" metrics will not be accurate due to limitations in IMU technology.
A total of 10 healthy subjects threw five to seven fastballs followed by five to seven breaking pitches (slider or curveball) in the motion capture lab. Subjects wore retroreflective markers and the MotusBASEBALL sensor simultaneously.
It was found that the arm slot ( = 0.975, < 0.001), shoulder rotation ( = 0.749, < 0.001), and stress ( = 0.667, = 0.001 when compared to elbow torque; = 0.653, = 0.002 when compared to shoulder torque) measurements were all significantly correlated with the results from the motion capture lab. Arm speed showed significant correlations to shoulder internal rotation speed ( = 0.668, = 0.001) and shoulder velocity magnitude ( = 0.659, = 0.002). For the entire sample, arm slot and shoulder rotation measurements were on a similar scale, or within 5-15% in absolute value, of magnitude to measurements from the motion capture test, averaging eight degrees less (12.9% relative differences) and nine degrees (5.4%) less, respectively. Arm speed had a much larger difference, averaging 3,745 deg/s (80.2%) lower than shoulder internal rotation velocity, and 3,891 deg/s (80.8%) less than the shoulder velocity magnitude. The stress metric was found to be 41 Newton meter (Nm; 38.7%) less when compared to elbow torque, and 42 Nm (39.3%) less when compared to shoulder torque. Despite the differences in magnitude, the correlations were extremely strong, indicating that the MotusBASEBALL sensor had high reliability for casual use.
This study attempts to validate the use of the MotusBASEBALL for future studies that look at the arm slot, shoulder rotation, arm speed, and stress measurements from the MotusBASEBALL sensor. Excepting elbow extension velocity, all metrics from the MotusBASEBALL unit showed significant correlations to their corresponding metrics from motion capture and while some magnitudes differ substantially and therefore fall short in validity, the link between the metrics is strong enough to indicate reliable casual use. Further research should be done to further investigate the validity and reliability of the arm speed metric.
数据处理的改进、对动力学和运动学背后生物力学背景的更深入理解以及惯性测量单元（IMU）传感器的技术进步，使得在测量人体关节角度和加速度方面能够实现高精度。这催生了一些新设备，据报道这些设备可以测量关节角度、手臂速度以及棒球运动员投球手臂所承受的应力。本研究旨在通过基于标记的运动捕捉实验室来验证其中一种传感器，即MotusBASEBALL单元。
我们假设MotusBASEBALL设备的关节角度测量值（“手臂槽位”和“肩部旋转”）将具有统计学上显著的可靠性和准确性，但由于IMU技术的局限性，“手臂速度”和“应力”指标将不准确。
共有10名健康受试者在运动捕捉实验室中先投出五到七个快球，随后投出五到七个变速球（滑球或曲球）。受试者同时佩戴反光标记和MotusBASEBALL传感器。
发现手臂槽位（= 0.975，< 0.001）、肩部旋转（= 0.749，< 0.001）以及应力（与肘部扭矩相比时= 0.667，= 0.001；与肩部扭矩相比时= 0.653，= 0.002）测量值均与运动捕捉实验室的结果显著相关。手臂速度与肩部内旋速度（= 0.668，= 0.001）和肩部速度大小（= 0.659，= 0.002）显示出显著相关性。对于整个样本，手臂槽位和肩部旋转测量值与运动捕捉测试的测量值在规模上相似，或绝对值相差5 - 15%，平均分别少8度（相对差异12.9%）和9度（5.4%）。手臂速度的差异要大得多，平均比肩部内旋速度低3745度/秒（80.2%），比肩部速度大小低3891度/秒（80.8%）。与肘部扭矩相比时，应力指标低41牛顿米（Nm；38.7%），与肩部扭矩相比时低42 Nm（39.3%）。尽管在大小上存在差异，但相关性非常强，表明MotusBASEBALL传感器在日常使用中具有高可靠性。
本研究试图验证MotusBASEBALL在未来研究中的应用，这些研究关注MotusBASEBALL传感器的手臂槽位、肩部旋转、手臂速度和应力测量。除了肘部伸展速度外，MotusBASEBALL单元的所有指标与其在运动捕捉中的相应指标均显示出显著相关性，虽然一些大小差异很大，因此在有效性方面存在不足，但指标之间的联系足够强，表明在日常使用中具有可靠的性能。应进一步开展研究以进一步探究手臂速度指标的有效性和可靠性。
