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

Background 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. Hypothesis 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. Methods 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. Results It 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. Conclusion 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

Arm Stress Comparisons Between Common Baseball Pitch Types

The purpose of this study was to examine differences in varus arm stress between baseball pitch types — fastballs versus a breaking ball of choice—with MotusBASEBALL’s motion capture arm sleeve. Twenty-eight males between the ages of 18 and 36 (21.4 ± 4.3) were asked to throw ten pitches of each pitch type — fastballs (n = 28), curveballs (n = 14), sliders (n = 14), and changeups (n = 18). Every subject threw fastballs and a breaking ball of their choice, and some subjects threw additional changeups. Sliders had the highest arm stress (54.6 ± 12.9 N·m) while curveballs had the lowest (46.8 ± 16.3 N·m). Fastball arm stress was 50.1 ± 16.8 N·m and changeup arm stress was 51.3 ± 15.5 N·m. There was no statistically significant difference between pitch types and arm stress (p-value range 0.08-0.92), although the proportion of outlier readings for arm stress was significant for sliders (proportion of outliers: 34%, p-value: 0.009 versus change-ups; p-value: 0.014 versus curveballs). In addition, pitch type was significant only in determining the velocity reading from the Motus App (p-value &lt;.0001), and was not significant in determining arm speed, arm slot, or shoulder rotation

Surface electromyographic analysis of differential effects in kettlebell carries for the serratus anterior muscles

The purpose of this study was to examine differences in the Electromyography (EMG) amplitude of the serratus anterior between 45° kettlebell carries and 90° kettlebell carries. Thirty-three men aged roughly between 19 and 23 and who were either college or professional baseball pitchers were chosen and randomly assigned to either perform the 45° kettlebell carry followed by the 90° kettlebell carry (n = 17) or the 90° kettlebell carry followed by the 45° kettlebell carry (n = 16). Each pitcher was instructed in the proper usage of the exercise and assigned a short break between the two carries. Changes in EMG amplitude were examined after proper band-pass filtering, normalization, and moving average-smoothing of the raw EMG signal. Differences of the EMG amplitude mean frequencies were examined between each subject’s individual carries and the clumped groups of all 45° and 90° carries. Among each individual comparison, eight pitchers had “large” Effect Size differences between the EMG amplitudes of their two carries, with seven of them signaling the 45° carry as the larger value. In addition, when examining the grouped mean differences of the EMG amplitudes, we found the 45° carries to be significantly higher (p-value of 0.018)