Softball players with shoulder injuries exhibit upper-body compensatory strategies compared to healthy controls: a wearable inertial sensors study

INTRODUCTION: Softball is a sport gaining popularity in Europe and United States, with slow-pitch being more popular than fast-pitch softball (Lear Grantham et al., 2015; Thompson et al., 2018). In particular, shoulder and elbow joints constitute the main sites of injuries in professional softball players (Diffendaffer et al., 2023; Friesen et al., 2022) due to recurring stresses resulting from repetitive throw movements (Krajnik et al., 2010). Although several studies have been performed in baseball pitch movement, there is scarcity of literature regarding the softball throw and other common movements (Diffendaffer et al., 2023). It's important to note that pitchers are not the sole players susceptible to injuries. Indeed, injuries sustained during softball practice are more commonly linked to throwing activities not directly related to pitching, such as those carried out by position players (Krajnik et al., 2010; Minetos et al., 2023). The most common shoulder injuries among softball players are muscle-tendon strain, tendinitis, and subluxation (Marshall et al., 2007; Minetos et al., 2023). In severe cases, upper body injuries may necessitate surgery, extending the rehabilitation period. Moreover, after severe injuries or surgery, it is expected that players may alter their throw kinematics due to disuse or changes in muscular and tendon strength balance. To design and prescribe a sport-specific strength and conditioning program targeted at reducing the risk of injury, boosting performance, and improving health and lifespan, it is essential to comprehend the mechanics of softball task that mainly lead to an injury (Diffendaffer et al., 2023; Escamilla et al., 2017; Hamer et al., 2021; Krajnik et al., 2010). The aim of the study was to assess the kinematic differences in the upper limb and trunk during the throwing movement between healthy softball field players and nd those who had experienced a previous shoulder injury. It was hypothesized that residual kinematics differences and compensatory strategies would emerge in players that had suffered a shoulder injury compared to the healthy ones. METHODS: In this cross-sectional study, 11 first-division softball players were enrolled. Among them, 5 reported experiencing a shoulder injury that required surgery. The players completed 10 trials, each involving throwing the ball to a target player positioned 10 meters away. The three with the highest spin velocity were retained for further analysis. Kinematic data on the shoulder, elbow, and trunk were collected using a set of 11 wearable inertial sensors (Xsens MTw Awinda). For the analysis, the motor task was divided into 2 phases: Pickup, which starts from ball grabbing to reaching the throw position; Pass, which covers the motion from the hand far back to the completion of the throw. The XSENS Analyze software was used to identify valid trials (complete and correct execution of the two phases) and specific events defining the different stances of interest. Primarily, the data from the different exercises were separated, and the limb used to pick up and throw the ball was identified (Diffendaffer et al., 2023; Escamilla et al., 2017). The phases were extracted from the data and normalized using 3 points of interest. The initial point was defined as the beginning of the stance, set at the 0% frame, while the endpoint of the stance was established as the 100% frame. An additional point was used to define the 50% frame of the stance. For each phase, the peak angle kinematics defined as the maximum rotation value, the minimum rotation value, and the range of motion (ROM) for the shoulder, elbow, wrist, head, pelvis, and trunk joint were computed. However, only the data for the shoulder, elbow, and trunk were kept for the study. The final kinematics parameters analyzed and presented were peaks joint angle and range of motion. The data processing was carried out using MATLAB R2020a, and the data extraction and analysis were performed through a custom MATLAB script. The normal distribution of the data was verified through the Shapiro-Wilk test. The normally distributed continuous data were presented as mean ± standard deviation with 95% confidence interval (CI), while the categorical data were presented as a percentage over the total. The Student’s t-test was employed to compare the demographics between the two groups. The two-tailed Student’s t-test within the Statistical Parametric Mapping was used to inspect the differences in upper limbs and trunk kinematic waveforms between the healthy and injured athletes (p<0.05). Frequencies of each response were determined by simple addition. Responses by each reviewer carried equal weighting. This section usually concludes with information on the reliability of the measurements, data analysis, and the statistical procedures used. RESULTS: The comparison of the samples’ characteristics did not show significant differences between the healthy and shoulder-injured groups (Tab. 1). In the Pickup phase (Tab 2.), the shoulder showed a higher internal-/external rotation ROM in healthy players than in injured ones (p<0.001). The same results were observed for the elbow flex-/extension ROM (p<0.01). Moreover, healthy players showed a higher tilt ROM of trunk than the injured ones (p<0.01). Similarly, during the Pass phase (Tab. 3), the shoulder internal/external rotation and flex-/extension ROMs of healthy players were wider than the injured ones (p<0.001). DISCUSSION: The present study aimed to investigate the biomechanical differences between healthy and injured first-division softball players in a common throwing movement performed in an ecological environment by means of wearable inertial sensors. The most significant finding of this study was that shoulder-injured players exhibited less ROM than their healthy counterparts during a standard throw, a movement commonly performed during practice and matches. This confirms the hypothesis that, despite having returned to play at the pre-injury level, injured players may voluntarily or unconsciously execute the motor task in a more conservative manner than healthy players. Similar to our findings, Hamer et al. (Hamer et al., 2021) found that high school and collegiate baseball pitchers who had undergone ulnar collateral ligament reconstruction showed significantly lower degrees of elbow extension when compared with healthy controls. However, they did not found differences in throwing biomechanics or passive ROM in baseball pitchers before and after ulnar collateral ligament reconstruction (Hamer et al., 2021). A reduced ROM in the lumbopelvic-hip complex and shoulder complex, particularly in internal/external rotation and abduction, could be potential risk factors associated with injuries during the season (Paul et al., 2021). To the best of our knowledge, this was the first study to investigate the differences between healthy and injured non-pitcher softball players. Most research has traditionally focused on pitchers (Hamer et al., 2021), given their higher incidence of shoulder injuries. Nevertheless, in absolute terms, non-pitcher softball players reported more injuries than pitchers. This discrepancy may be attributed to the larger number of non-pitchers than pitchers. However, exploring the common injuries among softball players who are not pitchers is a significant area of concern. Krajnik et al. (Krajnik et al., 2010) reported that 55% of injuries occurred during softball practice (not competition). Specifically, softball players, excluding pitchers, showed a higher percentage (65%) of injuries during practice than during competition (Krajnik et al., 2010). The present study has several limitations: first, the limited sample size given by the pilot nature of this study. Despite several biomechanical studies having been performed with 10 subjects or less, a more extensive cohort could have led to a better interpretation of the differences between healthy and injured players in the investigated movements. Second, the rehabilitation protocol and the time from injury were not controlled and differed among the injured players. However, all players belonged to the same team and the same staff organized their return to sport in the same way after clearance by the medical doctors. Third, the frequency of the wearable sensor might have been at the lower bound to produce a proper analysis of such high-speed movements. A further investigation of the potential biases due to the technology limitations is mandatory. CONCLUSION: In players who have experienced injuries and undergone surgery, the ROM during a common sport-specific motor task may be limited voluntarily or unconsciously. Coaches and trainers should focus on minimizing compensatory movements in softball players after receiving clearance from medical professionals. It is crucial to provide guidance for the return-to-sport process following shoulder injuries.