Predictors of Throwing Velocity in Youth and Adolescent Pitchers

J Shoulder Elbow Surg (2015) -, 1-7

1 56 2 57 3 58 4 59 www.elsevier.com/locate/ymse 5 60 6 61 7 62 8 63 9 64 10 Predictors of throwing velocity in youth and 65 11 66 12 adolescent pitchers 67 13 68 14 69 15 Q2 Terrance Sgroi, DPT, MTCa, Peter N. Chalmers,MDb,*, Andrew J. Riff,MDb, 70 16 Matthew Lesniak, DPTa, Eli T. Sayegh,BSc, Markus A. Wimmer, PhDb, 71 17 b b b 72 18 Nikhil N. Verma,MD, Brian J. Cole, MD, MBA , Anthony A. Romeo,MD 73 19 74 20 75 Q1 a 21 Accelerated Rehabilitation Centers Ltd, Chicago, IL, USA 76 bDepartment of Orthopaedic Surgery, Rush University Medical Center, Chicago, IL, USA 22 c 77 23 College of Physicians and Surgeons, Columbia University, New York, NY, USA 78 24 79 25 80 Background: Shoulder and elbow injuries are a common cause of pain, dysfunction, and inability to play 26 in overhead throwers. Pitch velocity plays an integral part in the etiology of these injuries; however, the 81 27 demographic and biomechanical correlates with throwing velocity remain poorly understood. We hypoth- 82 28 esized that pitchers with higher velocity would have shared demographic and kinematic characteristics. 83 29 Methods: Normal preseason youth and adolescent pitchers underwent dual-orthogonal high-speed video 84 30 analysis while pitch velocity was collected with a radar gun. Demographic and pitching history data 85 31 were also collected. Kinematic data and observational mechanics were recorded. Multivariate regression 86 32 analysis was performed. 87 33 Results: A total of 420 pitchers were included, with a mean pitching velocity of 64 10 mph. After multi- 88 < 34 variate logistic regression analysis, the most important correlates with pitch velocity were age (P .001; 89 2 ¼ < 2 ¼ < 2 ¼ 35 R 0.658), height (P .001; R 0.076), separation of the hips and shoulders (P .001; R 0.027), 90 and stride length (P < .001; R2 ¼ 0.016); in combination, these 4 variables explained 78% of the variance 36 91 in pitch velocity. Each year of age was associated with a mean 1.5 mph increase in velocity; each inch in 37 height, with 1.2 mph; separation of the hips and shoulders, with 2.6 mph; and a 10% increase in stride 92 38 length, with 1.9 mph. 93 39 Conclusion: Pitch velocity is most strongly correlated with age, height, separation of the hips and shoul- 94 40 ders, and stride length. 95 41 Level of evidence: Basic Science Study, Kinesiology. 96 42 Ó 2015 Journal of Shoulder and Elbow Surgery Board of Trustees. 97 43 Keywords: Baseball; injury prevention; pitching; overhand throwing; motion analysis; ulnar collateral 98 44 ligament tear; superior labral anterior-posterior tear 99 45 100 46 101 47 102 48 103 This study was approved by the Institutional Review Board of Rush Overhead throwing places substantial forces and torques 49 104 University Medical Center: Protocol No. 13090101, Expedited Review on the shoulder and elbow, with forces regularly exceeding 50 105 Initial Approval Notiﬁcation. 390 N and torques regularly exceeding 1000 Nm in pro- 51 *Reprint requests: Peter N. Chalmers, MD, Department of Orthopaedic 106 fessional pitchers.13 These forces have been implicated in 52 Surgery, Rush University Medical Center, 1611 W Harrison St, Chicago, 5 107 IL 60612, USA. the pathogenesis of shoulder and elbow injuries, which are 53 24,25 108 E-mail address: [email protected] (P.N. Chalmers). common in baseball pitchers. For example, superior 54 109 55 1058-2746/$ - see front matter Ó 2015 Journal of Shoulder and Elbow Surgery Board of Trustees. 110 http://dx.doi.org/10.1016/j.jse.2015.02.015

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111 labral anterior-posterior tears are a common cause of completeness with participants. A standardized physical exami- 166 112 shoulder discomfort in pitchers and remain an unsolved nation was performed. Passive glenohumeral rotation was 167 113 measured by a goniometer with the subject supine and the scapula 168 problem, with rates of return to a preinjury level of play of 114 8,15,19,22 stabilized at neutral shoulder flexion, 90 shoulder abduction, and 169 22% to 60%. 115 The neuromuscular and biomechanical factors that 90 elbow flexion. Total arc of motion, glenohumeral internal 170 rotation deficit, and glenohumeral external rotation excess were 116 correlate with injury during overhead pitching have been 171 117 24,25,28 then calculated from these measurements. These measurements 172 previously studied, and velocity has been identified were performed in both upper extremities. 118 6,28 173 as a primary factor. However, the demographic and ki- All subjects then underwent video motion anal- 119 nematic factors that correlate with velocity remain only ysis.3,7,16,18,24,25,29-31,33,36-39 With use of high-definition orthog- 174 120 partially understood. Multiple prior kinematic and electro- onal video cameras from the frontal and lateral views, subjects 175 121 myographic analyses have been performed examining were filmed at 210 Hz while pitching from a regulation practice 176 122 normal youth, collegiate, and professional mound appropriate for the subject’s level of play. Throwing ve- 177 123 pitchers.1,2,6,10,12-14,17,21,32,36-39 These studies have focused locity was measured with a radar gun (JUGS Sports, Tualatin, OR, 178 124 on correlations between kinematic and kinetic fac- USA), which per the manufacturer has an accuracy of 0.5 mph. 179 125 tors.1,2,10,13,14,32,36-39 Very few studies have identified ki- Filming took place after a full warm-up and once subjects felt 180 126 nematic and demographic correlates with velocity. Those ready to pitch at 100% velocity. All subjects pitched fastballs from 181 the wind-up position over a regulation distance for their age at a 127 studies that have been performed were conducted on small 182 128 strike zone target appropriately positioned and sized for their age. 183 groups of pitchers and did not incorporate demographic For each pitcher, the single pitch most representative of the 129 4,11,14,26,27,34,35,40 184 factors, which limits their generalizability. pitcher’s best effort was recorded for analysis. 130 A better understanding of the demographic and kinematic 185 131 186 factors that correlate with velocity could provide therapists Data analysis 132 and pitching coaches with areas on which to focus in 187 133 188 training and pitcher development. A standardized protocol was used to extract kinematic data from 134 189 Our overarching goal was to perform a demographic and video footage using commercial software (Dartfish Inc., Alphar- 135 biomechanical analysis of the correlates with velocity in etta, GA, USA). Only those kinematic variables shown previously 190 136 overhead youth and adolescent pitchers. Our primary aim to correlate with kinetic variables, as identified a priori, were 191 137 with this study was to determine the demographic and recorded (Table I). Observational mechanics were recorded once 192 138 biomechanical factors that predict throwing velocity. We for each pitch, with 2 study authors performing the measurements. 193 139 hypothesized that pitchers with higher velocity would have These were assigned a binary yes vs. no as previously described.9 194 140 shared demographic and kinematic characteristics. These included whether the subject (1) led with the hips, (2) had 195 141 the hand on top of the ball during the stride phase, (3) had the arm 196 142 in the throwing position at front foot contact, (4) had closed 197 143 Methods shoulders at the hand-set position, (5) had a closed foot orientation 198 144 at front foot contact, (6) had separation of rotation in the hips and 199 shoulders, and (7) was in the fielding position at follow-through.9 145 This is a single-episode cross-sectional study. As many youth and 200 Separation of rotation in the hips and shoulders was defined as a adolescent overhand baseball pitchers as possible within our 146 binary yes in those pitchers in whom, during the cocking phase, a 201 geographic area were recruited, and no a priori power analysis was 147 period could be identified during which the pelvis rotated to face 202 conducted. All subjects were currently in preseason training and 148 home plate while the shoulders continued to face third base (for a 203 underwent a standardized evaluation. Exclusion criteria included 149 right-handed pitcher). Pitchers in whom no such period could be 204 age <9 years; sidearm or ‘‘submarine’’ style pitching motion, as 150 identified were recorded as having a binary no for separation of 205 the kinematic data obtained were thought to be incomparable to rotation of the hips and shoulders. All analyses were performed in 151 the rest of the cohort; those who were not planning to pitch for 206 Excel X (Microsoft, Redmond, WA, USA) and SPSS 21 (IBM 152 their team that year; and those pitchers who did not think they 207 Inc., Armonk, NY, USA). An independent observer who was not 153 would be able to throw because of excess discomfort at the time of 208 aware of the study hypothesis entered all data. Continuous data 154 the evaluation. Pitchers who thought they were able to throw and 209 normality was evaluated with the Kolmogorov-Smirnov test. Ve- 155 who had been throwing in practice were included even if they had 210 locity was compared between discrete groups by Student t test or 156 a history of injury or current discomfort within their arm. Par- 211 Mann-Whitney U test as appropriate. Velocity was correlated with ticipants were unaware of the study hypothesis. In all cases, the 157 continuous variables by Pearson correlation coefficients. Because 212 dominant extremity was measured. 158 multiple comparisons were performed before regression, P values 213 159 underwent Bonferroni correction, and values <.00147 were 214 160 Data collection considered significant. Those variables that significantly corre- 215 161 lated with velocity or those variables in which there was a sig- 216 162 All pitchers completed a demographic survey, with the assistance nificant difference in velocity between groups were then entered 217 163 of their parents when possible. Data collected included age, into a multivariate stepwise regression model to determine the 218 164 height, and weight. Height and weight were used to calculate body most important correlates. Within this model, P values < .05 were 219 165 mass index (BMI). Surveys were administered in paper format in a considered significant. From this model, correlation coefficients 220 standardized fashion by 2 study authors and were reviewed for and R2 values, as an estimation of percentage of variance in injury

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221 276 222 Table I Demographic and kinematic correlates with throw velocity in miles per hour 277 223 Type Variable Univariate analysis Multivariate analysis 278 224 Correlation PR2 P Coeff. SE 279 225 Demographic Age 0.816 <.001 0.658 <.001 1.47 0.139 280 226 Height 0.792 <.001 0.076 <.001 1.191 0.227 281 227 Weight 0.732 <.001 282 228 BMI 0.495 <.001 0.003 .024 0.139 0.058 283 229 Physical examination ER-Dom 0.204 <.001 0.004 .006 0.05 0.022 284 230 IR-Dom 0.003 .949 285 231 Arc-Dom 0.183 <.001 286 232 GIRD 0.069 .166 287 233 GERE 0.013 .791 288 < 234 Wind-up Max. knee height (% Ht) 0.287 .001 0.004 .005 0.089 0.031 289 Kinematics at front Stride length (% Ht) 0.438 <.001 0.016 <.001 0.187 0.036 235 290 foot contact Elbow flexion 0.084 .09 236 Knee flexion 0.318 <.001 0.006 .001 0.083 0.022 291 237 Shoulder abduction 0.07 .156 292 238 Foot angle 0.196 <.001 0.004 .003 0.036 0.012 293 239 Kinematics at maximum Max. shoulder ER 0.132 .008 294 240 shoulder ER Max. shoulder abduction 0.117 .017 295 241 Lateral trunk tilt 0.152 .002 296 242 Kinematics at ball release Elbow flexion 0.107 .03 297 243 Forward trunk tilt 0.171 .001 0.002 .04 0.062 0.03 298 244 Knee flexion 0.07 .16 299 245 Shoulder abduction 0.086 .082 300 Lead hip flexion 0.266 <.001 246 301 Lateral trunk tilt 0.191 <.001 247 Observed mechanics Leads with hips NA .139 302 248 Hand on top of ball NA .002 303 249 Arm in throwing position at front NA .091 304 250 foot contact 305 251 Closed shoulders at hand separation NA .001 306 252 Foot closed NA .03 307 253 Hip and shoulder separation NA 0 0.027 <.001 2.621 0.511 308 254 Fielding position at follow-through NA .411 309 255 BMI, body mass index; ER, glenohumeral external rotation; Dom, dominant extremity; IR, glenohumeral internal rotation; Arc, glenohumeral rotational 310 256 arc; GIRD, glenohumeral internal rotation deficit; GERE, glenohumeral external rotation excess; Max, maximum; %Ht, values expressed as a percentage of 311 Coeff SE NA 257 subject height; , coefficient of correlation; , standard error; , not applicable. 312 For univariate analyses, because multiple comparisons were made, Bonferroni correction was performed and P values < .00147 were considered sig- 258 nificant. For multivariate analyses, only those variables found to be significant in univariate analyses were included, and thus the traditional P value of 313 259 .05 was used. P values identified as significant are marked in bold. R2 values > 0.01 are also marked in bold as these variables explained >1% of the 314 260 variance in velocity. 315 261 316 262 317 263 318 status explained by each variable, were determined. Only those 145.4 39.2 pounds, and mean BMI of 22.0 3.9. Mean 264 2 319 variables with R values > 0.01 are discussed. pitch velocity for the cohort was 64 10 mph. 265 320 On univariate correlation analyses, pitch velocity signif- 266 321 icantly correlated with the subject’s age, height, weight, 267 322 Results BMI, glenohumeral external rotation in the dominant 268 323 extremity, glenohumeral rotational arc in the dominant ex- 269 324 Of the 429 pitchers recruited, 9 were excluded because they tremity, glenohumeral external rotation in the nondominant 270 325 were no longer planning to pitch (3), threw with a sidearm extremity, and glenohumeral rotation arc in the nondominant 271 326 or submarine style (2), had too much pain to pitch (1), or extremity (P < .001 in all cases; Table I). On univariate an- 272 327 did not complete the demographic survey (3). A total of alyses of the kinematic analyses, pitch velocity significantly 273 328 420 subjects were included for a 98% inclusion rate. Our correlated with 7 of the 15 measured variables: maximal knee 274 329 cohort had a mean standard deviation age of 14.7 2.6 height during the wind-up as a percentage of subject height, 275 330 years, mean height of 67.5 5.3 inches, mean weight of stride length as a percentage of subject height at front foot

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331 386 332 387 333 388 334 389 335 390 336 391 337 392 338 393 339 394 340 395 FPO

341 = 396 342 397 343 398 344 399 Figure 1 Pitcher age signiﬁcantly correlates with pitch velocity

345 (P < .001; multivariate R2 ¼ 0.658). To simplify, mean velocity print & web 4C 400 346 for each year of age is shown. Figure 3 Clinical photograph demonstrating separation of 401 347 rotation within the hips and shoulders; while the pelvis has rotated 402 348 to face home plate (arrow), the shoulders still face third base. This 403 349 observed mechanical factor was signiﬁcantly correlated with pitch 404 2 ¼ < 350 velocity on multivariate analysis (R 0.027; P .001). 405 351 406 352 407 353 408 354 409 355 410 356 411 357 412 358 413 359 414 360 415 361 416 Figure 2 Pitcher height signiﬁcantly correlates with pitch ve- FPO

362 = 417 locity (P < .001; multivariate R2 ¼ 0.076). To simplify, the full 363 418 range of heights was divided into equally sized segments, and 364 mean velocity for each of these groups is displayed. 419 365 420

366 print & web 4C 421 367 contact, knee flexion at front foot contact, foot angle at front Figure 4 Clinical photograph demonstrating the measurement 422 368 foot contact, forward trunk tilt at ball release, lead hip flexion of stride length at the moment of front foot contact, which was 423 369 at ball release, and lateral trunk tilt at ball release (P <.001 in then normalized to the subject’s height. On multivariate analyses, 424 370 all cases; Table I). Among the observed mechanics, subjects stride length was significantly correlated with pitch velocity 425 (P < .001; multivariate R2 ¼ 0.016). 371 with a closed shoulder position at front foot strike had 426 372 significantly higher velocity than those with an open shoulder 427 373 position (P <.001; Table I). Those subjects with separation of pitch velocity. In multivariate analyses, each year of age was 428 374 the hips and shoulders had significantly higher pitch velocity associated with a 1.5 0.1 mph increase in velocity (Fig. 1). 429 375 than those without separation of the hips and shoulders Each inch in height was associated with a 1.2 0.2 mph 430 376 (P < .001; Table I). increase in velocity (Fig. 2). Separation of rotation within the 431 377 On multivariate regression analysis, those variables with hips and shoulders was associated with a 2.6 0.5 mph in- 432 378 R2 values > 0.01 (i.e., those variables that explained >1% of crease in velocity. Each increase in stride length by 10% of 433 379 the variance in pitch velocity) included age (Fig. 1), height the subject’s height was associated with a 1.9 0.4 mph 434 380 (Fig. 2), hip and shoulder separation (Fig. 3), and stride increase in velocity. 435 381 length as a percentage of the patient’s height (P <.001 in all 436 382 cases; Table I, Fig. 4). In combination, these 4 variables 437 383 explained 78% of the variance in pitch velocity within our Discussion 438 384 group; in total, all 11 variables with significant correlations 439 385 with velocity on multivariate analysis explained 81% of the Shoulder and elbow injuries are common among baseball 440 variance. Age alone accounted for 66% of the variance in pitchers,24,25 and operative treatment of these injuries does

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441 not predictably return players to painless pitching with Stride length may play a similar role. Each increase in 496 442 preinjury velocity and control.8,15,19,22 Whereas multiple stride length by 10% of the subject’s height was associated 497 443 prior kinematic analyses have been performed to under- with a 1.9 0.4 mph increase in velocity. Whereas multiple 498 444 stand the kinematic correlates with joint loads in kinematic factors, such as elbow flexion angle at various 499 445 pitchers,1,2,6,10,12-14,17,21,32,36-39 fewer have analyzed cor- points within the pitch and shoulder abduction angle within 500 446 relates with pitch velocity and none have incorporated de- various points within the pitch, have been associated with 501 447 mographic data.4,11,14,26,27,34,35,40 Our overarching goal increased elbow valgus torque and shoulder proximal 502 448 with this project was to perform a demographic and force,2,32,36-39 no previous studies have associated stride 503 449 biomechanical analysis of those factors that correlate with length with increased stress on the arm. However, stride 504 450 increased velocity in youth and adolescent pitchers using length is associated with increased velocity. As a result, 505 451 video motion analysis. pitching coaches could focus on stride length to improve a 506 452 The 4 factors independently associated with an increase in pitcher’s velocity. 507 453 velocity on multivariate regression analysis were age, height, Several previous studies have been conducted to corre- 508 454 hip and shoulder separation, and stride length as a percentage late factors identified in pitching motion analysis with pitch 509 455 of the patient’s height. In combination, these factors velocity. Other studies performing similar analyses have 510 456 explained 78% of pitch velocity variance. The covariance of correlated velocity with the kinematic variables shoulder 511 457 age and pitch velocity is likely due to multiple factors. Older external rotation,11,34,40 shoulder abduction,11,34 knee 512 458 pitchers are more likely to have learned proper pitching flexion,34 trunk tilt,34 elbow flexion,34 trunk-pelvis separa- 513 459 mechanics and are more likely to have the muscle develop- tion,26 pelvis orientation at maximal shoulder external 514 460 ment to allow higher velocity pitching. After correction for rotation,35 and stride length.4 Several of these variables 515 461 the remaining variables, each year of age was associated with were measured by our study and did not significantly 516 462 a 1.5 0.1 mph increase in velocity. correlate with pitch velocity. Multiple potential explana- 517 463 The correlation between the pitcher’s height and pitch tions exist for the differences between our results and those 518 464 velocity is likely due to the longer lever arm this allows of the previous studies,4,11,14,26,27,34,35,40 including differ- 519 465 subjects to use to transfer force onto the ball. Each inch in ences in the underlying population of patients (i.e., the 520 466 height was associated with a 1.2 0.2 mph increase in evaluation of youth and adolescent pitchers in this study, 521 467 velocity. Previous biomechanical analyses have normalized whereas other studies have largely analyzed elite collegiate 522 468 force and torque for subject height, as taller subjects are and professional pitchers), differences in the methods of 523 469 known to be able to exert more force and torque through the data collection (i.e., the use of video motion analysis 524 470 upper extremity because of the longer lever arm.1,9 Sub- instead of a markered motion analysis), differences in 525 471 sequent kinetic analyses of the pitching motion should sample size (i.e., the use of 420 pitchers instead of the 526 472 normalize for subject height. much smaller sample sizes of previous studies), and dif- 527 473 Two kinematic factors correlated with pitch velocity: hip ferences in data analysis (i.e., the analysis of 1 pitch per 528 474 and shoulder separation and stride length. In combination, subject instead of multiple pitches per subject as indepen- 529 475 these 2 factors explain 4.3% of the variance in pitch ve- dent variables). One variable identified in our study and 530 476 locity, suggesting that a pitcher with a short stride length also identified in multiple prior studies is proper timing of 531 477 and without hip and shoulder separation would be able to pelvic and trunk rotation, allowing optimal summation of 532 478 add 4.3% to the velocity by improving these aspects of the speed.14,26,35 533 479 mechanics (i.e., a 70 mph pitcher could increase to 73 Our study has several limitations. One limitation is the 534 480 mph). Adding separation of the hips and shoulders alone use of a video motion analysis system instead of a tradi- 535 481 added an average 2.6 0.5 mph. The importance of hip tional markered motion analysis system. Video motion 536 482 and shoulder separation to pitch velocity relates to the analysis has been widely used for this purpose and is a 537 483 ‘‘summation of speed’’ principle,1 that is, the greatest well-accepted method.3,7,16,18,24,25,29-31,33,36-39 However, 538 484 transfer of force occurs when the subsequent segment be- the authors have not performed any validation or reliability 539 485 gins rotating at the moment at which the prior segment studies with this methodology and are not aware of any 540 486 reaches maximal angular velocity; therefore, proximal within the literature. An additional limitation is the use of a 541 487 trunk rotation ideally begins at the moment of maximal single-episode study design. As a result, whereas the fac- 542 488 angular velocity of the pelvis, which explains the critical tors identified in this study correlate with velocity, alter- 543 489 importance of the core musculature for high-velocity ation of these factors would not necessarily improve 544 490 pitching. This factor has been previously associated with velocity. Correlation does not imply causation. These fac- 545 491 improved pitch efficiency (i.e., lower humeral rotational tors, in particular stride length and hip and shoulder sepa- 546 492 torque and elbow valgus torque per velocity9) and thus ration, could be the result of increased velocity instead of 547 493 could represent an avenue by which pitching coaches could the cause. In addition, many other unmeasured factors, such 548 494 improve velocity by improving mechanics. This factor, as strength, could also influence velocity. Without a pro- 549 495 dubbed the X-factor, has also been associated with spective longitudinal study to observe pitchers who expe- 550 increased club speed in golf.20,23 rience improvements in velocity, this limitation will remain.

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551 One additional limitation is the strong covariance of height with arthroscopic fixation using a bioabsorbable tack. Arthroscopy 606 552 and age. The multivariate regression model corrects for this 2006;22:136-42. http://dx.doi.org/10.1016/j.arthro.2005.11.002 607 553 limitation, and although the whole model remains valid, 9. Davis JT, Limpisvasti O, Fluhme D, Mohr KJ, Yocum LA, 608 554 Elattrache NS, et al. The effect of pitching biomechanics on the upper 609 interpretation of these factors as independent correlates can extremity in youth and adolescent baseball pitchers. Am J Sports Med 555 be more difficult. 2009;37:1484-91. http://dx.doi.org/10.1177/0363546509340226 610 556 10. Dillman CJ, Fleisig GS, Andrews JR. Biomechanics of pitching with 611 557 emphasis upon shoulder kinematics. J Orthop Sports Phys Ther 1993; 612 18:402-8. 558 Conclusion 613 559 11. Escamilla R, Fleisig G, Barrentine S, Andrews J, Moorman C III. 614 Kinematic and kinetic comparisons between American and Korean 560 Pitch velocity is most strongly correlated with age, professional baseball pitchers. Sports Biomech 2002;1:213-28. http:// 615 561 height, separation of the hips and shoulders, and stride dx.doi.org/10.1080/14763140208522798 616 562 length. These factors have implications with regard to 12. Fleisig G, Chu Y,Weber A, Andrews JR. Variability in baseball pitching 617 biomechanics among various levels of competition. Sports Biomech 563 the etiology of injury in youth pitchers, the rehabilitation 618 564 2009;8:10-21. http://dx.doi.org/10.1080/14763140802629958 619 of these injuries, and the improvement in pitching 13. Fleisig GS, Andrews JR, Dillman CJ, Escamilla RF. Kinetics of 565 performance. baseball pitching with implications about injury mechanisms. Am J 620 566 Sports Med 1995;23:233-9. 621 567 14. Fortenbaugh D, Fleisig GS, Andrews JR. Baseball pitching biome- 622 568 chanics in relation to injury risk and performance. Sports Health 2009; 623 569 1:314-20. http://dx.doi.org/10.1177/1941738109338546 624 Acknowledgment 15. Friel NA, Karas V, Slabaugh MA, Cole BJ. Outcomes of type II su- 570 perior labrum, anterior to posterior (SLAP) repair: prospective eval- 625 571 The authors wish to acknowledge Lisa Nold and the uation at a minimum two-year follow-up. J Shoulder Elbow Surg 626 572 athletic trainers at Accelerated Rehabilitation Centers 2010;19:859-67. http://dx.doi.org/10.1016/j.jse.2010.03.004 627 573 Ltd. for assisting with subject recruitment and filming. 16. Glousman RE, Barron J, Jobe FW, Perry J, Pink M. An electromyo- 628 574 graphic analysis of the elbow in normal and injured pitchers with 629 medial collateral ligament insufficiency. Am J Sports Med 1992;20: 575 311-7. 630 576 17. Gowan ID, Jobe FW, Tibone JE, Perry J, Moynes DR. 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Sports Biomech 2013;12:108-20. http://dx.doi.org/10. 641 587 1080/14763141.2012.728244 642 588 1. Aguinaldo AL, Buttermore J, Chambers H. Effects of upper trunk 21. Keeley DW, Hackett T, Keirns M, Sabick MB, Torry MR. A biome- 643 589 rotation on shoulder joint torque among baseball pitchers of various chanical analysis of youth pitching mechanics. J Pediatr Orthop 2008; 644 levels. J Appl Biomech 2007;23:42-51. 28:452-9. http://dx.doi.org/10.1097/BPO.0b013e31816d7258 590 2. Aguinaldo AL, Chambers H. Correlation of throwing mechanics with 22. Kim SH, Ha KI, Kim SH, Choi HJ. Results of arthroscopic treatment 645 591 elbow valgus load in adult baseball pitchers. Am J Sports Med 2009; of superior labral lesions. J Bone Joint Surg Am 2002;84:981-5. 646 592 37:2043-8. http://dx.doi.org/10.1177/0363546509336721 23. Kwon YH, Han KH, Como C, Lee S, Singhal K. Validity of the X- 647 593 3. Anz AW, Bushnell BD, Griffin LP, Noonan TJ, Torry MR, factor computation methods and relationship between the X-factor 648 594 Hawkins RJ. Correlation of torque and elbow injury in professional parameters and clubhead velocity in skilled golfers. Sports Biomech 649 baseball pitchers. Am J Sports Med 2010;38:1368-74. http://dx.doi. 2013;12:231-46. http://dx.doi.org/10.1080/14763141.2013.771896 595 org/10.1177/0363546510363402 24. Lyman S, Fleisig GS, Andrews JR, Osinski ED. Effect of pitch type, 650 596 4. Aruin AS. Simple lower extremity two-joint synergy. Percept Mot pitch count, and pitching mechanics on risk of elbow and shoulder pain 651 597 Skills 2001;92:563-8. in youth baseball pitchers. Am J Sports Med 2002;30:463-8. 652 598 5. Burkhart S. Shoulder injuries in overhead athletes: the ‘‘dead arm’’ 25. Lyman SL, Fleisig GS, Waterbor JW, Funkhouser EM, Pulley L, 653 599 revisited. Clin Sports Med 2000;19:125-58. Andrews JR, et al. 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