TRUNK LEAN DURING A SINGLE-LEG SQUAT IS ASSOCIATED WITH TRUNK LEAN DURING PITCHING

Abstract

Background

Impaired trunk motion during pitching may be a risk factor for upper extremity injuries. Specifically, increased forces about the shoulder and elbow have been observed in pitchers with excessive contralateral trunk lean during pitching. Because of the difficulty in identifying abnormal trunk motions during a high-speed task such as pitching, a clinical screening test is needed to identify pitchers who have impaired trunk motion during pitching.

Hypothesis/Purpose

The purpose of this study was to determine the relationship between the degree of lateral trunk lean during the single-leg squat and amount of trunk lean during pitching and if trunk lean during pitching can be predicted from lean during the single-leg squat.

Study Design

Controlled Laboratory Study; Cross-sectional.

Methods

Seventy-three young baseball pitchers (11.4 ± 1.7 years; 156.3 ± 11.9 cm; 50.5 ± 8.8 kg) participated. An electromagnetic tracking system was used to obtain trunk kinematic data during a single-leg squat task (lead leg) and at maximum shoulder external rotation of a fastball pitch. Pearson correlation coefficients for trunk lean during the single-leg squat and pitching were calculated. A linear regression analysis was performed to determine if trunk lean during pitching can be predicted from lean during the single-leg squat.

Results

There was a positive correlation between trunk lean during the single-leg squat and trunk lean during pitching (r = 0.53; p<0.001). Lateral trunk lean during the single-leg squat predicted the amount of lateral trunk lean during pitching (R² = 0.28; p < 0.001).

Conclusions

A moderate positive correlation was observed between trunk lean during an SLS and pitching. Trunk lean during the single-leg squat explained 28% of the variance in trunk lean during pitching.

Level of Evidence

Diagnosis, level 3

INTRODUCTION

Shoulder and elbow injuries are prevalent in youth and adolescent baseball pitchers.^1-4 The injury incidence over the course of a season in youth baseball pitchers has been reported to be as high as 28.7%, with a large majority of injuries occurring to the upper extremity.¹ Furthermore, ulnar collateral ligament reconstruction has become one of the most commonly performed procedures in pitchers⁵ and have doubled since 2000.⁶

Pitching is a complex activity that requires coordinated and controlled transfer of energy from the legs, through the trunk, to the shoulder, and then distal to the hand for ball release.^7-9 It has been estimated that the proximal segments of the hip and trunk contribute 50% of kinetic energy and force during dynamic overhead activities.^7,8 As such, trunk stability during pitching is important not only for postural control but to also generate and transfer force from the lower extremity to the upper extremity.¹⁰ Previous modelling simulations have identified 10° of contralateral trunk lean (lean away from the throwing arm) as the ideal trunk position to minimize varus moments about the elbow at maximum shoulder external rotation.¹¹ In support of this premise, Oyama and colleagues¹² found that high school pitchers with excessive lateral trunk lean (>10 °) away from the throwing arm, at maximum shoulder external rotation, exhibited increased varus moment at the elbow and glenohumeral internal rotation moment about the shoulder. Solomito et al.¹³ examined the relationship between the amount of lateral trunk lean and upper extremity forces in college pitchers, and found that increased varus moment at the elbow and glenohumeral internal rotation moment increased as trunk lean increased at maximum shoulder external rotation of the pitching motion. These increased moments are potentially dangerous and could lead to injury.

Given the importance of trunk stability to the biomechanics of pitching, identifying individuals who exhibit altered trunk motion during pitching is important. However, the highly dynamic nature of pitching makes it difficult for clinicians to visually identify altered trunk mechanics during pitching without sophisticated motion analysis equipment or high-speed cameras. As such, there is a need for a clinical test to screen for the potential for altered trunk motion in pitchers. Screening tests have previously been developed to identify movement deficits in lateral trunk lean in pitchers.¹² Specifically, the single-leg squat (SLS) has been used as a screening test to assess trunk control, however it is unknown if trunk lean in the frontal plane found during the SLS will inform trunk stability during pitching.¹⁴ Clinical screening tests that may be suggestive of altered pitching mechanics may help to decrease upper extremity injury rates.

The purpose of the study was to determine if there was a relationship between the degree of lateral trunk lean during the SLS and the degree of the lean at maximum shoulder external rotation of the pitching motion. It was hypothesized that the degree of lateral trunk lean during the SLS would be positively correlated to the degree of lateral trunk lean at maximum shoulder external rotation of the pitching motion. The second aim of this study was to determine if lateral trunk lean during the SLS could predict lateral trunk lean during pitching. Understanding the relationship between the amount of trunk lean during the SLS and the amount of trunk lean during pitching may provide important information related to the ability of the SLS to detect altered trunk mechanics during pitching.

METHODS

Participants

This study was a cross-sectional design performed in a controlled laboratory environment. Participants were recruited from local recreational leagues and through the use of flyers. Seventy-three youth baseball pitchers (11.4 ± 1.7 years; 156.3 ± 11.9 cm; 50.5 ± 8.8 kg) volunteered for this study. The only inclusion criterion was that participants were free from any injury within the past six months. An injury was defined as missing at least one practice or game. The Auburn University Institutional Review Board approved this study. Prior to data collection, all testing procedures were explained to each participant as well as their parent[s]/legal guardian[s] and informed consent and participant assent was obtained. Participants were instructed to not throw 48-hours prior to arrival for testing.

An a priori power analysis was performed for a between group comparisons for independent groups. A sample size of 29 participants was needed to achieve 80% power to determine if a correlation coefficient differs from zero, with an effect size 0.77¹⁵, and an alpha level of 0.05.

Procedures

The MotionMonitor™ (Innovative Sports Training, Chicago, IL, USA) synched with an electromagnetic tracking system (Track Star, Ascension Technologies Inc., Burlington, VT, USA) was used to collect kinematic data at 100 Hz. Kinematic sensors were attached to the following locations: [1] the posterior/medial aspect of the torso at T1, [2] posterior/medial aspect of the pelvis at S1, [3] distal/posterior aspect of the upper arm, [4] the flat, broad portion of the acromion of the scapula, [5] distal/posterior aspect of the forearm, [6-7] bilateral distal/posterior aspect of the thigh, [8-9] bilateral distal/posterior aspect of the lower leg, and [10] the mid-foot at the 3^rd metatarsal of the non-throwing foot (Figure 1).¹⁶ Sensors were attached to the skin using PowerFlex cohesive tape (Andover Healthcare, Inc., Salisbury, MA, USA).

Figure 1.

Kinematic sensor placement and set-up.

Following the application of the electromagnetic sensors, an additional sensor was attached to a stylus that was used to digitize boney landmarks to define segments of the trunk, humerus, forearm, femur, and shank.^17,18 Specifically, the medial and lateral aspect of each joint was identified and digitized and the midpoint of the two points was calculated to determine the joint center.¹⁹ A link segment model was then developed through digitization of bony landmarks used to estimate the joint centers for the knee, hip, shoulder, thoracic vertebrae 12 (T12) to lumbar vertebrae 1 (L1), and C7 to thoracic vertebrae 1 (T1). The trunk segment was defined as the digitized space between the T12 and L1 spinous processes, whereas the knee was defined as the midpoints of the digitized medial and lateral femoral condyles. The shoulder and hip joint centers were estimated using the least squares rotation method.^19,20 First, the humerus and the femur were moved in small arcs while respectively maintaining no movement of the scapula and pelvis. Then the shoulder and hip joint centers were calculated as the point on the humerus and femur that moved the least according to a least-squares algorithm. The variation in the measurement of the joint center had to have a root mean square error of less than 0.001 m to be accepted.

The global coordinate system was defined with the y-axis vertical (upward direction), the x-axis was perpendicular to the y and z-axes and directed anterior/posterior (forward direction), and the z-axis was directed in the medial/lateral direction and orthogonal to the x and y-axes (towards the right). Raw data regarding sensor orientation and position were transformed to locally-based coordinate systems for each respective body segment. Euler angle decomposition sequences were used to describe both the position and orientation of the trunk following the International Society of Biomechanics standards.^21,22 All segment axes systems were the same as the global coordinate system. The trunk segment was defined by the T1 and S1 sensors. The y-axis of the trunk was the line connecting the midpoint between the xiphod process and C7 vertebrae (positive direction upward).²³ The z-axis of trunk was defined as the plane formed by the suprasternal notch, C7, and the midpoint between the xiphoid process and T8 vertebrae (positive direction towards the right).²³ The x-axis was the common line perpendicular to the z and y-axes (positive direction anterior/forward).²³ The Euler angle decomposition sequence for the trunk segment was ZX’Y’’ with the second rotation trunk lean. Trunk lean was defined as movement in the frontal plane angular excursion of the thorax segment relative to the global reference frame (Figure 2).^12,13

Figure 2.

Trunk lean.

SLS testing protocol

Participants performed the SLS task on the lead leg, which is the leg contralateral to the throwing arm during pitching. The lead leg was selected as the support leg in the SLS because weight is transferred to this leg as the pitching motion progresses. Participants were instructed to place their hands on their hips and squat as low as possible before returning to a full upright position. Participants performed one practice trial and then a subsequent SLS trial was recorded for analysis. Cadence during the SLS was self-selected and not controlled for in this study. The trunk position in the frontal plane at 45 ° of knee flexion was selected for analysis.²⁴ Reliability for assessing lateral trunk lean during a SLS has been reported as excellent (ICC=0.86).²⁵ The standard error of measurement (SEM) for trunk lean was 1.5 °, and the minimal detectable change (MDC) at 95% confidence interval was 4.2 ° for the SLS.

Pitching protocol

Participants were allotted an unlimited time to perform their individual warm-up routine to gain familiarity with pitching with the sensors. Once the participant's completed the warm-up, testing began. Kinematic data were collected during three accurate two-seam fastball pitches. All participants reported pitching two-seam fastballs frequently during games and were confident in performing this pitch in a laboratory environment. An accurate pitch was defined as a pitch passing through the strike zone and was determined by a trained investigator. Participants threw to a catcher from their age-regulated pitching distance (46ft/14.02m). The fastest accurate pitch was selected for analysis.¹¹ The position of maximum shoulder external rotation was the discrete point of the pitch that was selected to measure trunk lean towards and away from the throwing arm (Figure 3a & 3b). This event of pitching motion represents the end of the cocking phase (lead foot contact to maximum external rotation). Maximum shoulder external rotation was selected because maximum trunk lean has been reported to occur at this event of the pitching motion.¹³ Excellent reliability has been established for two-dimensional video analysis of angular measures of lateral trunk lean in high school pitchers at maximum external rotation (ICC_2,k = 0.90; SEM = 3.2 °).¹⁵

Figure 3.

Sagittal (A) and frontal (B) plane depiction of maximum shoulder external rotation of the pitching motion.

Statistical analyses were performed using SPSS software (version 22; SPSS Inc., Chicago, IL, USA), with an alpha level set a priori at p ≤ 0.05. A Pearson correlation coefficient was calculated to determine the relationship between trunk lean in the SLS and lateral trunk lean in pitching. A linear regression was performed to assess the predictive ability of lateral trunk lean during the SLS on lateral trunk lean during pitching. The dependent variable entered into the model was lateral trunk lean during the SLS, and the independent variable was trunk lean during pitching.

RESULTS

Negative lateral trunk lean values indicate lean away from the throwing arm. A significant positive correlation between trunk lean during the SLS and trunk lean during pitching was observed (r = 0.53; p<0.001) (Figure 4). Mean lateral trunk lean during pitching was -17.1 ° ± 13.0 ° (away from the throwing arm) and -5.8 ° ± 10.4 ° during the SLS. Lateral trunk lean during pitching could be predicted from the amount of lateral trunk lean in the SLS (R² = 0.28; p < 0.001). No missing data were present.

Figure 4.

Relationship between SLS trunk lean and trunk lean during pitching. Negative values indicate lateral trunk lean away from the throwing arm.

DISCUSSION

Screening tests have been developed to identify lower extremity/upper extremity movement impairments and potential injury risk.^26,27 The current study sought to characterize the relationship between lateral trunk lean during the SLS and the amount of lateral trunk lean observed during pitching. The first hypothesis was confirmed as a significant positive correlation between lateral trunk lean during the SLS and lateral trunk lean during pitching was observed. The results of this study suggest that the SLS may be useful as a screening tool to identify greater degrees of lateral trunk lean during pitching.

During pitching, lateral trunk lean away from the throwing arm functions to position the shoulder in the proper arm slot (position of the shoulder relative to the trunk) in preparation for ball release. Lateral trunk lean increases as the pitching motion progresses from foot contact to maximum shoulder external rotation.¹³ In the current study, pitchers had an average -17.9 ° of lateral trunk lean away from the throwing arm during pitching. These results agree with Solomito et al.¹³ who reported -18 ° of lateral trunk lean away from the throwing arm at maximum shoulder external rotation in collegiate pitchers. The importance of identifying the relationship between lateral trunk lean and moments about the shoulder and elbow has been established in high school¹² and collegiate¹³ pitchers. Increased lateral trunk lean away from the throwing arm can impact shoulder and elbow moments. Solomito et al.¹³ reported that for every 10 ° increase in trunk lean there was an increase in elbow varus moment of 3.7 Nm and a 2.5 Nm increase in glenohumeral internal rotation moment. While the current study did not examine the moments about the shoulder or elbow, the SLS may have potential value in identifying baseball pitchers who may exhibit increased lateral trunk lean and shoulder/elbow moments during pitching. The results of this study valuable for clinicians and coaches who work with youth pitchers to identify potential trunk kinematic deficits during pitching that can be targeted with individualized interventions through the use of a SLS screening test. For example, if a pitcher has excessive trunk lean during a SLS then they may have similar trunk lean during pitching which warrants an intervention program to correct.

Lateral trunk lean during the SLS explained 28% of the trunk lean observed in pitching. Therefore, other factors likely contributed to the degree of lateral trunk lean observed during pitching that were outside the scope of this current study. Maintaining trunk stability during dynamic movements is dependent on neuromuscular control and strength of the trunk and lower extremity musculature.²⁸ Muscle performance deficits in lumbo-pelvic musculature may contribute to the lateral trunk lean that was observed during the SLS and pitching. Specifically, decreased strength of the gluteus medius and maximus may also affect the pitchers’ ability to maintain and control a neutral trunk position however further research is needed to substantiate this theory. Popovich and Kulig²⁹ have shown that females classified by weak hip muscle strength had significantly greater trunk lean during a single-leg landing task than participants classified as strong. Activation of the erector spinae, gluteus medius and maximus, external oblique, and rectus abdominis muscles was also significantly greater in individuals with weak hip musculature.²⁹ The findings of Popovich and Kulig²⁹ suggest that individuals with weak hip musculature may exhibit altered neuromotor control of the trunk to maintain stability during single-leg tasks. The implication of these results during a landing task should be applied with caution as similar results in baseball players performing a SLS have not been examined.

Decreased hip strength and control have been shown to result in increased hip adduction and knee valgus during single-leg tasks.^30-33 In single-leg tasks individuals with weak hip abductors lean towards the stance limb to reduce the demand on the abductors.^34,35 The lateral lean in the SLS and pitching may occur as a compensation strategy in pitchers with weak hip abductors. Compensations in trunk motion to maintain energy transfer during pitching may contribute to upper extremity injury rates. Chaudhari and colleagues reported that professional pitchers with poor lumbopelvic control, as measured during a single-leg balance task, were 2.2-3 times more likely to miss ≥ 30 days during the season due to injury.³⁶ As such, lumbopelvic and lower extremity control may be important in reducing injury in baseball pitchers. The SLS test may prove to be an easy and beneficial way to screen for lumbo-pelvic instability and compensatory trunk motions in pitchers, however further work in this area is needed.

There are several limitations of this study that need to be considered when interpreting the results. This study only examined the role of trunk stability, in a single plane, and fastball kinematics in youth pitchers. The results of this study may only be generalizable to youth pitchers from a selected area of the country. Other factors such as knee valgus, hip abduction, pelvic drop, or pelvic and trunk rotation also may differentiate pitchers with altered trunk stability. This study did not measure strength of the lumbar or gluteal muscles and thus, the generalizations being made as to why the trunk lean occurred are purely speculation at this time. The relationship between trunk deficits and shoulder injuries were not examined because no pitchers in this sample reported a history of upper extremity injury. The next step of this work is to track injuries during the season and examine the relationship between trunk stability and upper extremity injury. Future research should aim to characterize the risk of poor trunk stability on upper extremity injury. Other potential contributing factors should also be considered, such as muscular performance impairments and range of motion deficits at the shoulder that can enable a comprehensive characterization of risk of injury.

CONCLUSION

The results of the current study indicate that pitchers with trunk lean during the SLS screening test have greater trunk lean during pitching. Increased lateral trunk lean has previously been reported to result in increased moments at the shoulder and elbow that may contribute to injury. Lateral trunk lean during the SLS is similar in magnitude to the amount of lean in pitching. Implementing the SLS may allow clinicians to identify pitchers with increased lateral trunk lean during pitching without performing a three-dimensional motion analysis of their pitching mechanics. Identifying pitchers with trunk deficits can enable the targeting of prevention programs of those modifiable impairments associated with these trunk deficits and to reduce the torque about the elbow and decrease the risk of injury.