THE EFFECT OF HUMERAL RETROTORSION ON PITCH VELOCITY IN YOUTH BASEBALL PLAYERS

Abstract

Background:

Pitching velocity is a critical measure of performance, but it may also play a role in the development of injury. It has been proposed that increased humeral retrotorsion (HRT) may be an advantageous adaptation among throwers, resulting in increased throwing velocity. However, there is limited published data directly investigating this relationship.

Purpose / Hypothesis:

The purpose of this study was to examine the effects of HRT on pitching velocity in a group of youth baseball players. We hypothesized that there would be a positive association between pitching velocity and increased humeral retrotorsion.

Study Design:

Cross-sectional cohort study

Methods:

Demographic and physical variables that may correlate to pitching velocity (age, height, weight, glenohumeral external rotation (ER) range of motion, dominant arm humeral retrotorsion and shoulder internal rotation (IR) strength) were assessed. Univariate analysis using Pearson correlation coefficients examined the relationship of each variable to pitching velocity. Significant variables were retained and entered into a multivariable regression analysis.

Results:

All variables significantly correlated with pitching velocity (p<0.05) with the exception of ER (r = -0.169,p = 0.145). Multivariable regression model was significant and accounted for 81.7% of pitching velocity (R² = 0.817 F(5,70) = 62.59,p<0.001). Player age (B = 1.7,p < 0.001), height (B = 0.225,p = 0.001) and shoulder IR strength (B = 0.622, p < 0.001) significantly contributed to the model. After accounting for all other variables, HRT had a non-significant (B = 0.005,p = 0.884) and very small contribution to pitching velocity adding only .005mph per degree of HRT.

Conclusions:

Pitching velocity in youth baseball players is strongly influenced by age, height and IR strength. In opposition to the hypothesis, the degree of humeral retrotorsion did not have a significant effect on pitching velocity.

Level of Evidence:

Level 3

INTRODUCTION

Pitching velocity is one of the most coveted metrics in baseball and serves as the initial benchmark that most aspiring pitchers are judged upon. Unfortunately, this critical measure of performance may also be associated with an increased potential for shoulder or elbow injuries, particularly within youth baseball players.^1,2 Due to the relationship to both performance and injury, gaining a more detailed understanding of how various player-specific physical characteristics contribute to pitching velocity may be beneficial to not only optimize performance, but to help develop more individualized injury protection strategies. Several investigators have generated a long list of factors that contribute to pitching velocity including age, height, strength in the shoulder, core or lower extremities, and multiple kinetic, kinematic or temporal variables associated with throwing technique such as stride length, pelvic trunk separation and arm postion.^3-7 The position of maximum external rotation (MER) during late cocking phase of the throwing motion is of particular importance. Through a combination of thoracic extension, scapular retraction/posterior tilting and glenohumeral external rotation, the shoulder can reach a position of 165-180 ° degrees of combined humerothoracic rotation during throwing.⁶ This position is thought to be critical in generating high throwing velocities by improving elastic energy storage, optimizing the stretch-shortening cycle of the internal rotators and allowing for the generation of maximal arm acceleration prior to ball release.^6,8,9 In support of this theory, several investigations have found a positive association between ball velocity and a position of increased shoulder external rotation during pitching.^10-12

Humeral retrotorsion (HRT) refers to a twisting about the long axis of the humerus in which the humeral head is oriented in a more posterior medial direction.^9,13 It is well evidenced that baseball players have a higher degree of HRT within their dominant arm.^14-21 and increases in HRT have been shown to correlate with increased shoulder external rotation range of motion.¹⁷,^21-24 Due to this relationship, it is reasonable to conclude that increased HRT may be an advantageous attribute within pitchers, resulting in improved pitching velocity. While this hypothesis has been generally accepted as true, there is a limited amount of published research that directly investigates this relationship,⁹ and currently there are no studies within the youth population. Thus, the purpose of this study was to examine the contribution of HRT to pitching velocity within a group of youth baseball players. It was hypothesized that there would be a positive association in which increased HRT would be associated with increased pitching velocity.

METHODS

Participants

This was a cross-sectional study involving male baseball players aged 8-14 years-old, who were currently playing baseball. All subjects either currently identified as pitching or had previous pitching experience. Participants were recruited from local little leagues and private baseball academies. This cohort was a portion of participants from a larger study design.²² All protocols were approved by the Institutional Review Boards of all participating institutions. Written parental consent and child assent were obtained prior to data collection. Participants were excluded if they were female, participated in other repetitive overhead sports such as tennis, squash or swimming, if there was any current shoulder pathology that limited sports participation, had any history of humeral fracture, or any known systemic disorder that may result in joint hypermobility (e.g. Ehlers-Danlos syndrome).

Data Collection

All participants completed a demographic survey with the assistance of their parent or guardian. Data collected included age, arm dominance (defined as arm used to throw a ball), baseball participation volume (months per year), number of teams playing on and general information on other sports participation.

Following survey completion, a standardized physical examination was performed on each subject, beginning with assessment of height and weight. Prior to initiating the upper extremity examination, a coin flip was utilized to determine which side of the body was measured first. The examiners were unaware of the participants’ arm dominance at the time of measurement.

Passive glenohumeral external rotation (GER) range of motion (ROM) at 90 ° of abduction was assessed in supine utilizing previously described validated methods.^25-27 The scapula was stabilized and the shoulder passively externally rotated until resistance was felt by the examiner and motion no longer occurred at the glenohumeral joint. A digital inclinometer was then firmly placed along the ulnar aspect of the forearm to determine degree of rotation relative to vertical. The average of two trials was utilized for the final value. This measurement has been shown to have excellent intra and interrater reliability with ICC values >0.90 and an SEM of 1.5 ° to 2.6 °.^25,28,29

Remaining in the supine position with the shoulder abducted to 90 ° and in neutral rotation, shoulder internal rotation (IR) strength was measured utilizing the peak force, recorded as the average of two trials during a make test, measured with a hand-held dynamometer (MicroFET Hogan Industries, Draper, UT) applied just proximal to the ulnar styloid. The validity and reliability of hand-held dynamometry in the assessment of upper extremity strength has been previously documented with ICC values ranging from 0.79-0.97.^30-32

Humeral retrotorsion (HRT) was assessed in the same body position utilizing indirect ultrasonographic techniques described and validated by Myers et al.³³ A 5-13MHz linear array ultrasound transducer (GE LOGIQe, General Electric, Milwaukee, Wisconsin, USA) was placed on the anterior aspect of the shoulder, perpendicular to the long axis of the humerus and aligned level to the treatment table with a bubble level. A second examiner then rotated the humerus so that the bicipital groove could be visualized directly with the apexes of the greater and lesser tubercles parallel to the horizontal plane. This examiner then placed a digital inclinometer firmly against the ulna, and the forearm inclination angle relative to horizontal was recorded. This measurement procedure represents the relative difference between the proximal segment (humeral head) and distal segment (epidondylar axis), thus giving an indirect measurement of the degree of longitudinal twisting within the humerus (humeral retrotorsion). The average of three measurements was utilized for data analysis. The examiners underwent specific training for this measurement and an independent reliability study was conducted by the examiner team, yielding excellent intraclass correlation with ICC coefficients of 0.91-0.98 and a SEM of 1.8 °. This degree of accuracy in the measurements is consistent with other reliability studies utilizing this technique.³⁴

Maximum pitching velocity was assessed with a Stalker Sport II (Stalker Radar, Plano, Tx) radar gun which can measure pitch speeds from 5-150 mph and has an established accuracy of ±3%. After a sufficient amount of warm-up throwing, participants were positioned 46 feet from a stationary target and asked to perform three maximum velocity throws. Each player threw from flat ground and pitched from the stretch position. The average of three throws was utilized for analysis.

Statistical Analysis

All statistical analyses were conducted with the Statistical Package for Social Sciences version 22.0 (SPSS Inc) and Stata version 12.0. Data were assessed for normality by use of the Kolmogorov-Smirnov test. Descriptive statistics were calculated for all variables of interest. To assess the relationship of each variable to pitching velocity, univariate analysis using Pearson correlation coefficients was performed. In order to more fully understand this relationship, the significant variables from the univariate analysis were retained and entered into a multivariable regression analysis. An a-priori significance level of 0.05 was utilized for all analyses.

Results

A total of 85 participants were recruited, and 76 were included in the analysis. Data screening revealed one subject as an outlier and thus was removed from further analysis. In addition, eight participants were excluded due to missing data (missing pitching velocity n = 1, missing height/weight n = 6, missing strength n = 1). The included participants had a mean age of 11.4 ± 1.4 and a mean pitching velocity of 50.7mph ± 7.8. Complete subject demographics and summary data of measured variables are presented in Table 1.

Table 1.

Demographic Characteristics.

Variable	Mean ± SD
Age (years)	11.4 ± 1.4
Height (cm)	150.4 ± 12.8
Weight (kg)	42.4 ± 12.8
ER ROM (°)	125.4 ± 9.3
IR Strength (kg)	9.3 ± 2.3
HRT (°)	75.7 ± 11.5

External Rotation (ER), Range of Motion (ROM), Internal Rotation (IR), Humeral Retrotorsion (HRT)

The data were screened and met the assumptions of normality, linearity, and homoscedasticity. Univariate analysis demonstrated that all variables were significantly correlated (p < 0.05) with pitching velocity, with the exception of GER (r = –0.0169, p = 0.145). (Table 2) A standard multivariable regression model was significant and accounted for 81.7% of pitching velocity (R² = 0.817 F(5,70) = 62.59, p<0.001). Tolerance values were assessed and indicated that there were no collinearity issues among the variables. Player age (B = 1.7, p < 0.001), height (B = 0.225, p = 0.001) and shoulder IR strength (B = 0.622, p < 0.001) significantly contributed to the model. After accounting for all other variables, HRT had a non-significant (B = 0.005, p = 0.884) and very small contribution to pitching velocity, adding only 0.005mph per degree of HRT. (Table 3) As HRT is an evolving characteristic within the youth population, a secondary regression analysis was performed to determine if there was an interaction between age and HRT on pitching velocity. Results demonstrated there was a significant interaction effect (p = 0.001), indicating the effect of HRT on pitch velocity changes as a child ages. Further analysis was conducted in which the participants were categorized into three different age categories based upon chronological age, consistent with previous investigations: Youth Players aged 8-10.5 (n = 25, mean age 9.7 ± 0.6), Pre-Teen 10.5-12.5 years (n = 31; 11.5 ± 0.6) and Early Teen 12.5-14.9 years (n = 21; 13.2 ± 0.4). Age-stratified scatter plot analysis demonstrated a positive relationship between pitching velocity and increasing HRT in youngest group, while a negative relationship was present within the oldest cohort. (Figure 1).

Table 2.

Results of Pearson correlation coefficient analysis demonstrating the variables association with pitching velocity.

Variable	Mean ± SD	Correlation	P Value
Age (years)*	11.4 ± 1.4	0.792	<0.001
Height (cm)*	150.4 ± 12.8	0.814	<0.001
Weight (kg)*	42.4 ± 12.8	0.695	<.0001
ER ROM (°)	125.4 ± 9.3	-0.169	0.145
IR Strength (kg)*	9.3 ± 2.3	0.805	<0.001
HRT (°)*	75.7 ± 11.5	-0.264	0.021

^*Statistically significant p<0.05

External Rotation (ER), Range of Motion (ROM), Internal Rotation (IR), Humeral Retrotorsion (HRT)

Table 3.

Results of multivariable regression analysis.

Variable	B (95%CI)	p- Value
Age (years)*	1.7 (0.86-2.56)	<0.001
Height (cm)*	0.23 (0.10-0.35)	0.001
Weight (kg)	-0.43 (-0.16-0.07)	0.453
IR strength (kg)*	0.62 (0.39-0.86)	<0.001
HRT (°)	0.005 (-0.07-0.08)	0.884
Pseudo R2	0.817

^*Statistically significant p<0.05

Internal Rotation (IR), Humeral Retrotorsion (HRT)

Figure 1.

Age stratified scatter plot of age humeral retrotorsion (HRT) (x-axis) and pitch velocity (y-axis) demonstrating a varible of effect with age. Within youth players, increasing HRT was associated with increasing pitch velocity, while in early-teen players increasing HRT was associated with decreased pitch velocity.

DISCUSSION

The results of this study indicate that pitching velocity within youth baseball players is strongly influenced by age, height and internal rotation strength. The multivariable regression model demonstrated that player age was most highly associated with increased velocity, with an addition of 1.7mph for every one-year increase in age.

While a previous study by Roach et al⁹ found a significant positive correlation (r = 0.44, p = 0.03) between HRT and pitching velocity within collegiate baseball, this result was not replicated within the current sample. Within the current study, HRT had a small, negative correlation to pitching velocity and when analyzed within the context of the regression model, only had a minimal, non-significant effect on pitching velocity, adding only 0.005mph for every 1 ° increase in HRT. As the proposed influence of HRT on pitching velocity relies upon a series of events, in which increased HRT creates a shift in glenohumeral motion favoring external rotation, it is important to analyze all aspects of the contributing factors. Within this sample, the correlation between HRT and glenohumeral external rotation was low (r = .29, p = 0.010) and thus HRT would not exert a strong influence on external rotation motion, which may explain why there was not a significant relationship between HRT and pitching velocity. While most studies do show a positive correlation between HRT and glenohumeral external rotation, these correlations are inconsistent and vary in magnitude, from as low as r = .030 to as high as r = 0.86.³⁵ This fact, combined with the results of the present study, suggest that further research is necessary to more accurately understand this relationship and the cascade of events proposed to allow HRT to exert an influence on pitching velocity.

A secondary analysis was performed in order to determine if the effect of HRT on pitching velocity varied with the age of the subject. This analysis was done due to the understanding that the angle of HRT is a developing characteristic that changes along with skeletal maturation. At birth, the humeral head is in marked retrotorsion and undergoes a process of derotation (less retrotorsion) during the pediatric and adolescent years, with no further changes occurring after skeletal maturity.^13,36 As illustrated within Figure 1, higher degrees of HRT in youth players (mean age 9.7 years) are associated with increased pitching velocity, while higher degrees of HRT within the early-teen group (mean age 13.2 years) are associated with decreased pitching velocity. Interestingly, there was minimal to no effect of HRT on pitching velocity noted within the pre-teen group (mean age 11.5 years). To the authors knowledge, this is the first study to illustrate an age-dependent relationship between HRT and pitching velocity. While the reason for this variable association is not clear, alterations in muscle flexibility, coordination, humeral retrotorsion and pitching characteristics that occur during this period of childhood development may help place these findings in context. It is important to consider that these athletes are in a period of significant body change in terms of growth and puberty. The mean age of height take-off (i.e. onset of growth acceleration) and peak height velocity in males is 11 and 13.5 years-old, respectively.³⁷ Previous research demonstrates that youth athletes may undergo regressions in motor coordination and neuromuscular control during early adolescence.³⁸ These regressions are hypothesized to be associated with alterations that occur as a result of the musculoskeletal system adapting to physical changes associated with body growth and ongoing sensory/motor development. Within a baseball-specific context, these neuromotor adaptations would be occurring during a period of life in which adaptations in HRT are also continuing to occur.²² It is possible that the inverse relationship between HRT and pitching velocity noted within the early-teen group may be due to an intermittent lack of motor adaptation occurring as a result of longitudinal limb growth and changing degree of HRT. These factors could limit the transfer of energy along the kinetic chain through altered timing of pelvic, trunk and shoulder contributions leading to altered mechanics. Similarly, it is plausible that the changing degree of HRT during adolescence could alter plyometric muscle capabilities by creating a period of time in which the athlete is less efficient at utilizing the stretch-shortening properties to accelerate the arm during the cocking/acceleration phases of throwing. This lack of efficiency in movement and motor coordination during older adolescence may help explain the dissimilar association of HRT to pitching velocity found between our older and younger cohorts. Further research is required in order to see if this association continues to hold true in other populations and to more fully understand this relationship.

The findings from this study may also help inform injury prevention efforts within this vulnerable population. Previous research has indicated that higher pitching velocity, higher side-to-side asymmetry in HRT, and age-related changes in body composition (i.e. increased height) are all risk factors for throwing related injuries among adolescents.^39-41 Within this study, younger players with greater HRT had greater pitching velocity, which would elevate their baseline level of injury risk. Clinicians may want to consider obtaining a baseline measure of HRT for all youth athletes who are heavily involved in baseball in order to understand this additional risk factor and be able to assess for changes over time. Taken together, these results support the need for future, longitudinal prospective studies within both the skeletally mature and skeletally immature populations, in order to more fully elucidate the details of how HRT may affect injury risk and pitching performance.

This study is not without limitations. Assessment of pitching biomechanics was not conducted and therefore this study is unable to account for any kinetic or kinematic variables that may have contributed to pitching velocity. Within the secondary analysis, chronological age was utilized as a grouping variable and there was no direct measure of physiological maturity. As normal proximal humeral torsion development occurs as a result of skeletal maturation, this may have impacted the degree of HRT within the participants included in this study. Future studies should consider the use of more precise measures of physiological maturity (Tanner staging or skeletal age via plain radiographs) in order to more precisely define skeletal maturity. Additionally, the participants did not perform a standardized warm-up prior to range of motion assessment, which may have impacted maximal pitching velocity, shoulder rotation or strength measurements. However, the impact of this was limited as all data was collected during practice or tournament scenarios, in which all participants had undergone a team specific warm-up prior to measurement and would have achieved a similar level of tissue preconditioning prior to assessment. Finally, this study was cross-sectional in nature and thus, a true cause and effect relationship cannot be determined. Future longitudinal studies should seek to include measures of HRT, shoulder range of motion and pitching velocity in order to gain a better understanding of this complex relationship.

CONCLUSION

In summary, these results indicate that pitching velocity is strongly influenced by age, height and internal rotation strength. Among the entire sample, HRT did not exert any influence on pitching velocity, which may be due to the limited correlation between HRT and shoulder external rotation range of motion. Upon secondary analysis within specific age cohorts, the effect of HRT on pitching velocity appears to vary with age, and future longitudinal studies should aim to further clarify this relationship.