Exploring wearable sensors as an alternative to marker-based motion capture in the pitching delivery

Abstract

BackgroundImprovements 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.HypothesisWe 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.MethodsA 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.ResultsIt was found that the arm slot (R = 0.975, P < 0.001), shoulder rotation (R = 0.749, P < 0.001), and stress (R = 0.667, P = 0.001 when compared to elbow torque; R = 0.653, P = 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 (R = 0.668, P = 0.001) and shoulder velocity magnitude (R = 0.659, P = 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.ConclusionThis 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.

Standing Handball Throwing Velocity Estimation with a Single Wrist-Mounted Inertial Sensor

“…The application of the inertial sensors to the professional sports has reached a large number of throws in this study and showed that increase of elbow varus torque associated with arm mechanics. Furthermore, Boddy and et al recently examined the validation of the inertial sensor to a biomechanical analysis of baseball pitching (14). They have demonstrated the differences between elbow mounted "Motus BASEBALL sensor" for the joint angle measurements.…”

“…Recently, accelerometer and IMUbased wearable technology emerged to use for overarm throwing performance assessment. Previous studies have revealed that wearable sensor's motion capture abilities, for instance, quantified the throwing activity (12), kinematic analysis of overarm throwing and comparison with lab-based systems (13,14). However, the accelerometer-based throwing velocity evaluation has only been established in one study with a low sampling frequency (250 Hz) (15).…”

IMU gyroscopes are a valid alternative to 3D optical motion capture system for angular kinematics analysis in tennis

“…In the present research, IMU signals were compared to those of an OMC system (Optitrack, NaturalPoint Inc., Oregon, USA) existing of eight cameras (model Prime 41) recording at a speed of 128 Hz. This OMC system has been used in similar studies 10,23 and considered as accurate as other well-established motion capture systems. 24 A cluster of five reflective (passive) markers of 12.7 mm diameter were fixed to the plastic rigid body, constituting the OMC gyroscope (Figure 1).…”

Validity and reliability of inertial sensors for elbow and wrist range of motion assessment

“…Additionally, their relative affordability respect to optical-tracking systems, the faster and simpler setups, and the advantage of not being confined to a laboratory setting, make them ideal tools for assessing ROM in the clinical practice (Ahmad et al, 2013). There are several reviews which explore the usage of these instruments for a variety of health purposes, yielding interesting research in biomechanics, such as the classification of age-related kinematic impairment (Greene et al, 2015;Roldán-Jiménez & Cuesta-Vargas, 2016), body mobility assessment (Iwasaki & Hirotomi, 2015), gait analysis and development of athletes' performance (Camomilla et al, 2018) and sports (Boddy et al, 2019;Wells et al, 2019).…”