JOURNAL OF SPORTS SCIENCES, 2015 http://dx.doi.org/10.1080/02640414.2015.1076163

Hip and upper extremity kinematics in youth baseball pitchers Taylor Holt1 and Gretchen D. Oliver2 1 School of Kinesiology, Auburn University, Auburn, AL, USA; 2Sports Medicine and Movement Laboratory, School of Kinesiology, Auburn University, Auburn, AL, USA

ARTICLE HISTORY

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ABSTRACT

The purpose of this study was to examine the relationship between dynamic hip rotational range of motion and upper extremity kinematics during baseball pitching. Thirty-one youth baseball pitchers (10.87 ± 0.92 years; 150.03 ± 5.48 cm; 44.83 ± 8.04 kg) participated. A strong correlation was found between stance hip rotation and scapular upward rotation at maximum shoulder external rotation (r = 0.531, P = 0.002) and at ball release (r = 0.536, P = 0.002). No statistically significant correlations were found between dynamic hip rotational range of motion and passive hip range of motion. Hip range of motion deficits can constrain pelvis rotation and limit energy generation in the lower extremities. Shoulder pathomechanics can then develop as greater responsibility is placed on the shoulder to generate the energy lost from the proximal segments, increasing risk of upper extremity injury. Additionally, it appears that passive seated measurements of hip range of motion may not accurately reflect the dynamic range of motion of the hips through the progression of the pitch cycle.

Introduction Baseball pitching requires coordinated muscle activation and sequential segmental movement in a proximal to distal pattern. Proximal to distal sequencing from the lower extremity to the upper extremity allows for efficient energy transfer and thus optimal performance. To transfer energy from the lower extremity to the upper extremity, the pelvis and scapula must have optimal stabilisation as well as movement efficiency (Kibler, 1998; Kibler, Press, & Sciascia, 2006). Weakness or lack of mobility in the hips as well as inefficient scapular kinematics can result in mechanical compensations during pitching, either proximally or distally in the kinetic chain (Kibler, 1998; Sciascia, Thigpen, Namdari, & Baldwin, 2012). As the pitching motion is a total body activity that requires movement efficiency, stability and energy transfer, the influence of the pelvis and scapula is paramount. It has been reported that hip range of motion in the transverse plane should be bilaterally symmetrical in baseball pitchers (Ellenbecker et al., 2007; Sauers, Bliven, Johnson, Falsone, & Walters, 2014). Deficits in stride (leg contralateral to throwing arm) hip external rotation can affect stride foot placement at foot contact. The stride foot should be directed towards home plate, and alterations in landing position greater than 25° can negatively affect proximal to distal energy transference (Kibler, Wilkes, & Sciascia, 2013; Laudner, Moore, Sipes, & Meister, 2010; Wilk, Meister, Fleisig, & Andrews, 2000). Additionally, greater stride hip passive internal rotation has been correlated to decreased scapular posterior tilt at shoulder maximum external rotation in youth pitchers (Oliver & Weimar, 2015). Proper positioning of the scapula during throwing is vital as it serves as the link between the torso and upper extremity. It is the scapula that acts to summate and transfer large forces generated

CONTACT: Gretchen D. Oliver © 2015 Taylor & Francis

[email protected]

Accepted 20 July 2015 KEYWORDS

Injury prevention; kinetic chain; lumbopelvic-hip complex; passive range of motion; dynamic range of motion

in the lower extremity and lumbopelvic-hip complex to the glenohumeral joint, elbow, wrist and on to the baseball (Kibler, 1998; Kibler et al., 2013; Lintner, Noonan, & Kibler, 2008). Hip range of motion research has exclusively utilised passive and active measures (Laudner et al., 2010; Oliver & Weimar, 2015; Sauers et al., 2014), and the importance of hip range of motion (Laudner et al., 2010; Robb et al., 2010; Sauers et al., 2014) has been investigated independently from that of shoulder range of motion (Hurd & Kaufman, 2012; Sauers et al., 2014; Wilk, Macrina, & Arrigo, 2012). However, literature is lacking on hip range of motion during the progression of the pitch cycle, specifically as it relates to the motion of the upper extremity. Therefore, the purpose of this study was twofold: (1) to examine the relationship between dynamic hip internal and external range of motion and upper extremity kinematics during baseball pitching and (2) to compare that dynamic hip rotational range of motion during pitching to passive hip rotational range of motion. It was hypothesised that during pitching significant relationships would exist between dynamic hip range of motion and upper extremity kinematics and that dynamic hip range of motion would correlate to passive hip range of motion.

Methods Participants A total of 31 youth baseball pitchers (10.87 ± 0.92 years; 150.03 ± 5.48 cm; 44.83 ± 8.04 kg) were recruited for participation. Selection criteria included coach recommendation, years of pitching experience and freedom from injury within the past 6 months. Coach recommendation and years of pitching experience were required to ensure that the participants

301 Wire Rd, Auburn University, Auburn, AL, USA.

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T. HOLT AND G. D. OLIVER

were competitive pitchers, and the pitchers in the current study averaged 3.25 ± 1.03 years of competitive pitching experience. Despite freedom from injury within the past 6 months being a selection criterion, no participant reported having suffered an upper extremity injury nor did they experience upper or lower body pain/stiffness following extensive throwing bouts. University Institutional Review Board approval of all testing protocols was obtained, and all testing procedures were explained to each participant prior to data collection in addition to their assent and their parent(s)/legal guardian(s) informed consent being obtained.

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Procedures All participants were tested during their fall throwing season and had not thrown prior to their passive range of motion measurements. A single examiner conducted the measurements of bilateral hip rotational passive range of motion using a Baseline® Digital Inclinometer (Medline Industries Inc., Mundelein, IL, USA). Hip passive range of motion (internal and external rotation) was measured with the participant in a seated position, with knees flexed to 90°, allowing the legs to comfortably hang off the edge of the table (standard athletic training treatment table), and with hands resting comfortably on the table to assist with trunk stabilisation (Figure 1). A towel was placed under the femur being tested to allow for 90° of hip flexion, and the digital inclinometer was aligned along the shaft of the participant’s tibia. The examiner supported the femur, to eliminate accessory motion, and passively rotated the lower shank (internally and externally) until a resistive end feel was achieved in the hip. At the point of resistive end feel without the production of accessory hip movement, the passive range of motion measurement was recorded. Kinematic testing included pitching maximal effort fastballs the age regulation distance of 14.02 m from mound to home

Figure 1. Measurement of passive hip internal and external rotational range of motion.

plate. Upper extremity and hip kinematic data were collected throughout testing, but only recorded for the first three-pitched strikes. On average, it took participants five pitches to record three strikes. The same investigator, who possessed 13 years of competitive softball experience, called all balls and strikes during testing. Kinematic data for each participant were averaged across the first three-pitched strikes, and the mean value for each kinematic variable was used for analysis. Ball speeds were measured using a JUGS radar gun (OpticsPlanet, Inc., Northbrook, IL, USA) that was positioned in the direction of the pitch. Prior to testing, each participant was allotted unlimited time to stretch and perform their individual warm-up routine. During warm-up, participants were required to perform test trials pitching from the mound in order to gain familiarity with the testing environment and to ensure proper stride foot-landing placement on the force plate anchored into the throwing surface. Following stretching, the average length of throwing warm-up was 10 min.

Kinematics An electromagnetic tracking system (Flock of Birds® Ascension Technologies Inc., Burlington, VT, USA) synced with The MotionMonitor™ (Innovative Sports Training, Chicago, IL, USA) was used to collect all kinematic data. The Flock of Birds® Ascension system has been validated for tracking humeral movements. Trial-by-trial intra-class coefficients for axial humeral rotation in unloaded and loaded conditions have been produced in excess of 0.96 (Ludewig & Cook, 2000). Field distortion in electromagnetic tracking systems have been shown to be the cause of error in excess of 5° at a distance of 2 m from an extended range transmitter (Day, Murdoch, & Dumas, 2000), but this error has been reduced from near 10° to as low as 2° by increasing instrument sensitivity following system calibration (Meskers, Fraterman, Van der Helm, Vermeulen, & Rozing, 1999; Périé, Tate, Cheng, & Dumas, 2002). Therefore, prior to data collection, the current system was calibrated using previously defined methods (Day et al., 2000; Meskers et al., 1999; Périé et al., 2002). Once calibrated, the magnitude of error in determining the position and orientation of the electromagnetic sensors within the calibrated world axis system was less than 0.01 m and 3°, respectively. Eleven electromagnetic sensors were affixed to each participant’s skin at the following locations using PowerFlex™ cohesive tape (Andover Healthcare, Inc., Salisbury, MA, USA) for secure placement throughout testing: (1) the posterior/medial aspect of the torso at T1, (2) posterior/medial aspect of the pelvis between S1 and S2, (3) the flat, broad portion of the acromion on the throwing arm scapula, (4) proximal/lateral aspect of the throwing upper arm, (5) distal/posterior aspect of the throwing forearm, (6–7) bilateral distal/lateral aspect of the upper leg, (8–9) bilateral distal/lateral aspect of the lower leg and (10–11) bilateral dorsum of the foot at the distal second metatarsal (Myers, Laudner, Pasquale, Bradley, & Lephart, 2005; Myers, Oyama, & Hibberd, 2013; Oliver, 2013). A twelfth sensor was attached to a stylus and used to digitise bony landmarks described by the International Shoulder Group of the International Society of

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Table I. Description of bony landmarks palpated and digitised. Bony process palpated and digitisation

Bony landmark Thorax Seventh cervical vertebra [C7] Thoracic vertebra 12 [T12]

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Eighth thoracic vertebra [T8] Suprasternal notch Xiphoid process Humerus Medial epicondyle Lateral epicondyle Glenohumeral joint centre of rotation Forearm Radial styloid process Ulnar styloid process Thigh Medial femoral condyle Lateral femoral condyle Acetabulofemoral joint centre of rotation Shank Medial malleolus Lateral malleolus

Most dorsal aspect of the spinous process Most dorsal aspect of the spinous process Most dorsal aspect of the spinous process Most cranial aspect of sternum Most distal aspect of sternum Medial/distal aspect of condyle Lateral/distal aspect of condyle Rotation method* Lateral/distal aspect of radial styloid Medial/ distal aspect of ulnar styloid Medial/distal aspect of condyle Lateral/distal aspect of condyle Rotation method* Medial/distal aspect of malleolus Lateral/distal aspect of malleolus

Notes: *Centre of glenohumeral rotation was not digitised. The rotation method estimated joint centre using lest of squares algorithm for the point moving the least during a series of short rotational movements (Meskers et al., 1999).

Biomechanics (Wu et al., 2005). Two points defined the longitudinal axis of a segment, and a third point defined the plane. A second axis was defined as orthogonal to that plane, and a third axis was defined as orthogonal to the first and second axes. For between-participant reliability, the same investigator performed the digitisation throughout all testing sessions (Table I). Digitising bony landmarks allowed for raw sensor data in the global coordinate system to be transformed to the anatomically based local coordinate system. Data describing position and orientation of electromagnetic sensors were collected at 100 Hz. Raw data were independently filtered along each global axis using a fourth-order Butterworth filter with a cut-off frequency of 13.4 Hz (Oliver, 2013; Oliver & Keeley, 2010). Neutral stance was the y-axis in the vertical direction, horizontal and to the right of y was the z-axis, and anterior was the x-axis. Euler angle decompositions were used to determine femoral orientation relative to the

Table II. Angle and orientation decomposition sequences. Segment Hip Rotation 1 Rotation 2 Rotation 3 Scapular Rotation 1 Rotation 2 Rotation 3

Axis of rotation

Angle

Z X′ Y″

Flexion [+]/extension [–] Adduction [+]/abduction [–] Left: internal rotation [–]/external rotation [+] Right: internal rotation [+]/external rotation [–]

Y X′ Z″

Protraction [+]/retraction [–] Upward rotation [–]/downward rotation [+] Anterior tilt [–]/posterior tilt [+]

Notes: Prime (′) and double prime (″) notations represent previously rotated axes due to the rotation of the local coordinate system, resulting in all axes within that system being rotated. (Rotation about the X axis also results in rotation of both Y and Z axes, resulting in a new system of X′Y′Z′.) Subsequent rotation are then about those axes.

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pelvis and scapula orientation relative to the thorax (Table II) (Wu et al., 2005).

Results Statistics Statistical analyses were performed using IBM SPSS Statistics 22 (IBM Corp., Armonk, NY, USA). Descriptive statistics were expressed by means and standard error. Intra-class correlation coefficients (ICCs) were calculated to assess reliability of the kinematic variables between trials. Median ICC across all variables was strong (r = 0.959), with a strong positive minimum (r = 0.751) and strong positive maximum (r = 0.995). Pearson product moment correlation coefficients were calculated to identify relationships between dynamic hip internal and external range of motion and upper extremity kinematics as well as between dynamic and passive hip range of motion (Table III). Because comparisons between upper extremity kinematics were not of interest in this study, a separate correlation was conducted for each upper extremity kinematic variable (total of 6) comparing it to each hip’s range of motion across the four events in the pitch cycle. This allowed for eight comparisons to be made for each statistical test. In order to control for type I error based on the large number of statistical comparisons, the initial alpha level of P ≤ 0.05 was adjusted to P ≤ 0.006 using a Bonferroni adjustment. Correlation strengths were defined as weak ±0.20 to ±0.29, moderate ±0.30 to ±0.39 and strong ±0.40 to ±0.69. Descriptive statistics are shown in Table IV and Figure 2 for upper extremity kinematics and hip range of motion. For stance hip rotation and scapular upward rotation, a strong relationship was found at maximum shoulder external rotation (r = 0.531, P = 0.002) and at ball release (r = 0.536, P = 0.002). No statistically significant correlations were found between dynamic hip rotational range of motion and passive hip range of motion.

Discussion The results of the current study upheld the first hypothesis that dynamic hip range of motion would exhibit significant relationships with upper extremity kinematics during pitching. As the wind-up is completed, the pitcher began to stride towards the batter and elevated the throwing arm humerus. As the humerus was elevated to 93° at foot contact, which is slightly higher than the optimal position of 90°, the scapula was in a position of upward rotation and elevation. Scapular upward rotation during humeral elevation maintains the integrity of the glenohumeral joint and provides sufficient subacromial space, helping to reduce the risk of shoulder impingement injury as the throwing arm is accelerated forward (Myers et al., 2005). Stance hip extension and external rotation initiated the stride towards home plate as external rotation of the stride hip prepared the stride leg and foot for foot contact. Previous research states that adequate stride hip external rotation allowed for optimal foot position at foot contact, which is crucial for the most effective rotation of the hips, pelvis and trunk as the pitcher progressed into maximal shoulder external rotation (Dillman, Fleisig, & Andrews, 1993;

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Table III. Pearson product moment correlation coefficients (P value) between hip rotation and shoulder and scapular kinematics. Stride hip rotation Shoulder plane of elevation

FC

FC −0.038 (0.838)

MER

MER

BR

0.154 (0.408) −0.086 (0.646)

MER

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0.222 (0.230)

MER

−0.351 (0.053)

−0.290 (0.114)

0.151 (0.419)

0.120 (0.520)

0.163 (0.381)

−0.054 (0.773)

0.339 (0.062)

0.340 (0.061) 0.375 (0.038)

−0.272 (0.139)

MER

−0.318 (0.081)

0.083 (0.658)

0.531 (0.002)* 0.396 (0.028)

MIR

0.031 (0.869)

BR

0.536 (0.002)* 0.403 (0.025)

−0.184 (0.322)

MER

−0.351 (0.053)

−0.150 (0.422)

0.296 (0.106)

BR

FC

0.176 (343) 0.417 (0.020)

MIR

Scapula anterior/posterior tilt

0.345 (0.057)

−0.226 (0.221)

0.004 (0.984)

BR

FC

0.331 (0.069)

0.096 (0.607)

MER

Scapula upward/downward rotation

0.034 (854)

0.337 (0.063)

MIR FC

0.192 (0.300)

−0.036 (0.849)

BR

Scapula protraction/retraction

MIR

0.227 (0.220)

MIR FC

BR

0.182 (0.327)

BR

Shoulder rotation

MER

0.039 (0.835)

MIR FC

FC 0.021 (0.909)

0.009 (0.962)

BR

Shoulder elevation

Stance hip rotation MIR

−0.139 (0.457)

MIR

0.386 (0.032)

−0.180 (0.333)

−0.259 (0.159)

−0.013 (0.946)

−0.327 (0.073)

−0.204 (0.271)

Notes: FC, foot contact; MER, maximum shoulder external rotation; BR, ball release; MIR, maximum shoulder internal rotation. *Correlation is significant at the Bonferroni adjusted alpha level P ≤ 0.006 level (two-tailed).

Table IV. Descriptive statistics of kinematic variables (mean and ±standard error of the mean). FC Hip Stance IR (+)/ER (–) Stride IR (+)/ER (–) Scapula Protraction (–)/retraction (+) Upward rotation Anterior (–)/posterior (+) tilt Humerus Plane of elevation Elevation External rotation

MER

BR

MIR

5.81° ± 2.44°

−8.17° ± 2.85°

−7.82° ± 2.66°

−7.70° ± 2.44°

−0.50° ± 2.64°

6.44° ± 2.64°

7.05° ± 2.57°

7.39° ± 2.71°

18.84° ± 4.28° 27.29° ± 4.68° 0.00° ± 2.69°

14.98° ± 2.30° 27.41° ± 4.42° 11.58° ± 2.42°

4.80° ± 1.99° 21.48° ± 3.54° 0.03° ± 2.06°

−17.12° ± 2.99° 17.65° ± 2.96° −7.92° ± 2.15°

−25.66° ± 4.84° 92.64° ± 3.80° −40.93° ± 4.70°

2.49° ± 6.65° 88.95° ± 3.10° −86.04° ± 5.84°

2.77° ± 3.28° 81.96° ± 3.51° −55.44° ± 6.06°

24.46° ± 3.66° 85.47° ± 2.95° −4.42° ± 6.00°

Notes: Hip: stance, throwing arm side; stride, non-throwing arm side; FC, foot contact; MER, max shoulder external rotation; BR, ball release; MIR, max shoulder internal rotation. Plane of elevation: 0° – abduction; 90° – forward flexion.

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Figure 2. (a) Graph of descriptive statistics for passive hip internal and external rotational range of motion. (b) Graph of descriptive statistics for dynamic hip internal and external rotational range of motion at foot contact (FC), max shoulder external rotation (MER), ball release (BR) and max shoulder internal rotation (MIR).

Wilk et al., 2000). External rotation deficits in the stride leg may cause alterations in foot placement at foot contact. It is known that deviations greater than 25° towards the target can negatively affect proximal to distal energy transference, so decreased force production in the hips and trunk can occur due to a more closed pelvis position and the pitcher having to throw across his body (Kibler et al., 2013; Laudner et al., 2010, 2010; Wilk et al., 2000). Too much stride hip external rotation would have the foot in a more open position at foot contact and thus would result in premature pelvis and trunk rotations (Dillman et al., 1993; Wilk et al., 2000). The pitchers in the current study displayed comparable passive stance hip external rotation to professional pitchers from previous literature (Laudner et al., 2010; Robb et al., 2010; Sauers et al., 2014), suggesting that the current pitchers do possess the capability for adequate foot landing position. However, the current study revealed a neutral stance hip position at foot contact, only 0.50° of external rotation. It is known that stride hip external rotation affects foot position and that stance hip range of motion affects pelvis rotation, so a neutral stride hip position and an internally rotated stance hip at foot contact possibly indicate premature pelvis rotation due to poor neuromuscular control in youth pitchers.

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After foot contact, the pelvis and trunk should begin approaching their maximum rotational velocity as the throwing arm continues to elevate and externally rotate. At maximal shoulder external rotation, the arm was fully cocked and the scapula was placed in the optimal position of retraction, upward rotation and posterior tilt. In this position, the anterior muscles of the shoulder were eccentrically loaded in preparation for arm acceleration (Kibler, 1998). Our results indicate that at maximal shoulder external rotation as the humerus elevated, the scapula upwardly rotated to 27° and the stance hip externally rotated to 8° while the pitcher pivoted around the stance leg to face home plate. As the pitcher’s arm was then accelerated forward, the scapula rotated downwards from 27° to 21° at ball release accompanying a decrease in humeral elevation from 89° to 82°. Meanwhile, the stance hip remained externally rotated 8° at ball release as the pelvis had already rotated towards the batter. The strong significant relationships found between stance hip rotation and scapula upward rotation at maximum shoulder external rotation and at ball release indicate that as stance hip external rotation decreases so too does scapular upward rotation. It is postulated that the altered timing of pelvis rotation negatively affects the functional relationship between the muscles connecting the pelvis to the shoulder complex and could be evidenced by the decrease in humeral elevation and scapular upward rotation during arm acceleration. Altered timing of pelvis rotation may also diminish energy generation in the large proximal segments of the kinetic chain. Without adequate contribution from the lower extremities, shoulder pathomechanics can develop as the upper extremity must make up for the proximal energy loss in order to impart the needed velocity on the baseball (Laudner et al., 2010). Increasing the responsibility of the shoulder to generate energy alters the functional relationship within the musculature, resulting in subpar positioning of the scapula, thus increasing the risk for upper extremity injury (Kibler, 1998; Kibler et al., 2013). The second aim of this study was to compare dynamic and passive hip range of motion, and the hypothesis that a correlation would exist between the two range of motion measures was rejected. While the passive hip range of motion data were comparable to existing literature (Laudner et al., 2010; Robb et al., 2010; Sauers et al., 2014), the dynamic range of motion reported in this study was not. An explanation of this nonconformity in dynamic hip range of motion could be that youth pitchers lack the neuromuscular control and strength to move dynamically through the full hip range of motion during pitching. It is also possible that pitching does not necessitate hip movement to the end range of motion. Therefore, it appears that hip range of motion measured passively in the seated position may not accurately reflect the dynamic range of motion of the hips through the progression of the pitch cycle. However, the interpretation of this nonconformity is in need of further investigation. A major limitation of the current study is the exploratory nature of the data. The goal of this study was to highlight and explain any potential relationships that exist between hip range of motion and shoulder kinematics in a population of young baseball pitchers. Further investigation into each relationship is needed before any clinical implications or

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conclusions can be made. An additional limitation is that hip range of motion was measured passively and in the seated position. While seated passive hip range of motion is a commonly accepted measuring protocol in the literature (Laudner et al., 2010; Sauers et al., 2014; Scher et al., 2010), active-prone hip range of motion measures may be better suited for baseball pitchers. The prone position more closely resembles the pelviship orientation during pitching. In addition, active range of motion measures may more closely correlate to the degree of dynamic hip motion a pitcher moves through because they are forced to rely solely on their own musculature for hip rotation.

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Conclusion The current study illustrates a piece of the functional relationship between two major upper and lower extremity segments in the body. It has long been accepted that this relationship exists, and this study narrows the focus to the hip and scapula. Although the correlations found between the hip range of motion and shoulder complex kinematics do not imply causation, they do highlight the notion that in youth baseball pitchers as hip range of motion decreases so too do the optimal mechanics of the shoulder. The results of this study have important implications for young pitchers as it appears that focusing on lower body flexibility, specifically hip flexibility, can benefit their performance on the mound. Additionally, it can be concluded that passive range of motion measures may not translate to dynamic motion of the hip during pitching in youth, but further investigation is needed. Future research should continue to investigate this important link between two key components of the kinetic chain, the hips and the shoulder. Youth pitchers should continue to be a demographic of primary focus as they are at increased risk of having pathomechanics that could ultimately lead to injury and to a shortened career.

Acknowledgements There was no financial support or technical assistance provided on this project.

Disclosure statement No potential conflict of interest was reported by the authors.

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