The overhead throwing motion has been described as sequential in terms of position, movement, and force production, through the kinetic chain in a proximal to distal pattern (20). The scapula is a critical component of the throwing kinetic chain. It is the link that provides for the transfer of velocity, energy, and forces to the upper extremity, through proper timing and coordination (15). Specifically, the scapula must coordinate the demands of humeral positioning and energy transfer while providing a stable platform for muscle attachments. To fulfill these roles, the scapula must rely on the stability of the pelvis.
During baseball pitching, the scapula must fully retract to allow for the cocking position. In addition to retraction, the scapula must also upwardly rotate and elevate when the humerus is abducted to 90° (18). This positioning of the scapula maximizes the subacromial space in which the rotator cuff tendons pass. An inability of the scapular stabilizing muscles to position the scapula appropriately can lead to subacromial impingement of the rotator cuff tendons causing pain and dysfunction in the shoulder. The muscles supporting the scapula must negotiate seemingly contradictory demands of movement to accommodate the humerus and providing a stable platform from which the muscles can pull. The scapula is stabilized by a force couple formed by the upper and lower trapezius, rhomboid, and serratus anterior muscles (15). Any inefficiency in scapular position will alter lines of pull for these muscles and may lead to injury.
During the dynamic movement of the baseball pitch, it is the movement of the proximal (lower extremity) segments that influence the shoulder and ultimately ball speed. Thus, it is the coordinated sequential movements of the kinetic chain that leads to maximal ball speed. In an attempt to transfer forces from the lower extremity to the upper extremity, the pelvis must possess stability for fluid force transfer, through the trunk, to the scapula and distally (17, 20). With the lower extremity generating much of the force and energy for baseball pitching, dynamic and efficient movement of the upper extremity is dependent on neuromuscular interaction with the lower extremity.
Understanding the neuromuscular interactions associated with pitching is critical for improving training and rehabilitation protocols for baseball pitchers. Currently, there have been studies examining muscle activation during the baseball pitch (5,11,13,26); however, no studies have examined both pelvic and scapula stabilizing muscles and their activations throughout the baseball pitch. Activations of pelvic and scapular stabilizing musculature are also important to identify to better comprehend the role of the kinetic chain during pitching. Therefore, it was the purpose of this study to examine muscle activation of a primary pelvic stabilizer, gluteus medius, and scapular stabilizers during baseball pitching. It was hypothesized that these muscles would be significantly active throughout the fastball pitch in youth baseball pitchers.
Experimental Approach to the Problem
The goal of the experiment was to determine the muscle activations of a pelvic stabilizer and selected scapular stabilizing muscles during a fastball pitch in youth baseball pitchers. The muscles examined were bilateral gluteus medius and throwing arm side lower trapezius, upper trapezius, and serratus anterior. An observational descriptive study was implemented. Descriptive statistics were used to determine muscle activations by examining normalized surface electromyographic (sEMG) data as a percent of the subject's maximum voluntary isometric contraction (%MVIC). Pearson's product correlations were performed to determine muscle activation relationships between the pelvic and scapular stabilizers during designated phases of the pitching cycle. The pitching motion was divided into 3 phases: (a) from foot contact to maximum shoulder external rotation, (b) from maximum shoulder external rotation to ball release, and (c) from ball release to maximum shoulder internal rotation (Figure 1).
Phases were examined vs. the traditional pitching events, as sEMG studies of pitching have focused on the phases leading to the events (5,25–27).
Twenty youth baseball players (age: 11.3 + 1.0 years; height: 152.4 + 9.0 cm; weight: 47.5 + 11.3 kg) participated. Coach recommendation, years of pitching experience, and freedom from injury within the past 6 months were the selection criteria. Coach recommendation was sought to ensure that the subjects were experienced competitive pitchers. Freedom from injury within the past 6 months was one of the criterion for selection; however, the subjects neither reported that they had suffered any injury nor reported any pain or stiffness in their upper or lower extremity after extensive throwing sessions within the past year. The Institutional Review Board of the University approved all testing protocols. Before data collection, all testing procedures were explained to each subject as well as parent(s)/legal guardian(s) and informed consent and subject assent was obtained. All subjects were tested during the fall baseball season and had not thrown that day before arrival for testing.
All subjects reported for testing before engaging in any vigorous activity for that day. Location of bilateral gluteus medius, throwing arm side lower trapezius, upper trapezius, and serratus anterior were identified through palpation of the muscle belly. Single differential electrodes (interelectrode distance: 10 mm) were attached over the muscle bellies and positioned parallel to the muscle fibers using previously published standardized methods (2,8,29). Gluteus medius muscle belly was identified as the proximal third of the distance from the iliac crest and greater trochanter. And, care was taken to place the electrodes anterior to the gluteus maximus to minimize cross talk (8). Lower trapezius electrode placement was obliquely upward and laterally between the spine of the scapula and vertebral border of the scapula and seventh thoracic spinous process (2). Upper trapezius placement was at the angle of the neck and shoulder, over the muscle belly, parallel with the muscle fibers (2). And, the serratus anterior electrode placement was vertically below the axilla, anterior to the latissimus dorsi, over the fourth to sixth ribs (2). Before electrode placement, all identified locations were shaved, abraded, and cleaned using standard medical alcohol swabs for electrode placement. An additional electrode was placed on the anterior superior iliac spine to serve as a ground lead for the examined muscles. The use of surface electrodes was chosen because they have been deemed to be a noninvasive technique that is able to reliably detect surface muscle activity (2).
Electromyographic data were collected using a Delsys Bagnoli-8-channel electromyography (EMG) system (Delsys, Inc., Natick, MA, USA). The signal was full-wave rectified and root mean squared at 100 milliseconds. Surface EMG data were sampled at a rate of 1,000 Hz. After the application of surface electrodes, manual muscle testing techniques by Kendall et al. (14) were used to determine steady-state contraction. A certified athletic trainer, trained in performing manual muscle tests (MMTs), conducted all MMTs. Three MMTs lasting 5 seconds were performed for each muscle, with the first and last second of each test removed to obtain steady-state results (29). The MMT provided baseline MVIC data to which all sEMG were normalized. Electromyographic data were collected through The MotionMonitor (Innovative Sports Training, Chicago, IL, USA) synched with electromagnetic tracking system (Flock of Birds Ascension Technologies, Inc., Burlington, VT, USA). Postprocessing analysis was performed through MATLAB (version 8.2.0; The MathWorks, Inc., Natick, MA, USA).
After the electrodes were attached and MMTs were performed, the subjects were given an unlimited time to perform their own specified precompetition warm-up routine. Average warm-up time was 10 minutes. Once the subjects deemed themselves prepared, they were instructed on the protocol. Subjects were instructed to throw maximal effort 4-seam fastballs for strikes over a regulation distance (46 ft; 14.02 m) to a catcher. A JUGS radar gun (OpticsPlanet, Inc., Northbrook, IL, USA) positioned in the direction of the throw determined ball speed. The fastest of the 4-seam fastballs for strikes was selected for analysis.
Data from each muscle were normalized and expressed as a percent contribution of the MVIC. Statistical analyses were performed using IBM SPSS Statistics 19 (Armonk, New York, USA). Descriptive statistics were expressed by mean and SDs. Pearson's product-moment correlation coefficients were calculated to identify relationships between bilateral gluteus medius and scapular stabilizing musculature activation during the 3 specified phases of pitching with a p value of 0.05 being significant.
Normalized muscle activations expressed as a %MVIC are summarized in Figure 2. Data revealed moderate (20–39% MVIC) to moderately strong (>40% MVIC) activation of the ipsilateral (stance leg/throwing arm side) gluteus medius throughout phases 2 and 3, whereas moderately strong activation of the upper trapezius and serratus anterior during phases 2 and 3 of the pitching motion. Additionally, Pearson's product-moment correlation revealed significant relationships between bilateral gluteus medius and the force couples about the scapula during all 3 phases of the pitching motion (Table 1).
The baseball pitching motion is a total kinetic chain activity that must efficiently use both the upper and lower extremity. In baseball pitching, pelvic and trunk kinematics influence the loads placed on the shoulder and elbow joints (1,9). Thus the dynamic movement of the upper extremity is dependent on the interaction of structural and functional components of the entire neuromuscular system. Specifically, for normal shoulder movement to occur, there must be stability at both the pelvis and scapula (15), as abnormalities in pelvic or hip motion can lead to kinetic chain alterations (22,32). Therefore, it is the lower extremity that provides a base in which energy can be transferred through the stable pelvis to the scapula and on to the shoulder and distal arm. The results of this study provide important data that improve the understanding of the muscular relationship between the pelvic and scapular stabilizers during the fastball baseball pitch.
Phase 1 of the baseball pitch occurs from foot contact to maximum shoulder external rotation. Throughout phase 1, the ipsilateral leg must have adequate hip internal rotation to properly position the pelvis (33). It is the role of the gluteus medius to perform both hip abduction and internal rotation. Additionally, the gluteus medius must create an abduction moment for the pelvis to remain level during single leg support. It has been reported that maximum internal rotation of the stance leg occurs during this phase (4). In this study, the ipsilateral gluteus medius displayed low-to-moderate activation (17 + 23% MVIC) throughout phase 1. These findings of gluteus medius activation are in agreement with Plummer and Oliver (30) who examined gluteus medius activation of catchers throwing down to second base, as well as studies examining baseball (26) and softball pitching (28). Additionally, during phase 1, the scapula must effectively retract to allow for the shoulder to be in a position of approximately 90° of abduction and maximum external rotation (15). Just as retraction is needed to position the humerus in maximum external rotation, upward rotation allows for adequate subacromial space when the humerus is in the position of 90° of abduction. Upward rotation of the scapula is necessary for efficient pitching mechanics. It has been reported that reduced scapular upward rotation may result in shoulder pathomechanics, reduced lower-extremity energy transfer to the upper extremity, decreased muscular function, and subsequent risk of injury (15,20). It is the actions of the lower trapezius and serratus anterior that upwardly rotate the scapula; however, this study revealed low activation of the lower and upper trapezius with the serratus anterior low to approaching moderate activity (16 + 6% MVIC) during phase 1. This finding supports the notion that the participants were experienced in this motion and that minimal activation is required to achieve the scapular position. This finding further suggests that the required scapular stabilization demand is not high during this phase.
During phase 2, the acceleration phase, the pitcher moves from maximum shoulder external rotation to ball release. The abduction moment of the ipsilateral hip must progress from maintaining a level pelvis to moving into abduction of the hip, and assist in the forward momentum of the body toward the target. This study revealed strong ipsilateral gluteus medius activation (67 + 17% MVIC), which was again in agreement with the literature examining gluteus medius activation during throwing (26,28,30). In addition, the scapular stabilizers of the upper trapezius and serratus anterior were moderately strong (45 + 22% MVIC) and moderate (32 + 13% MVIC), respectively. The scapula moves from a position of retraction to protraction laterally and then anteriorly on the thoracic wall. These movements of the scapula are necessary to help dissipate the forces about the shoulder (15). The increased activation in both muscle groups suggests that this portion of the throw is more demanding than phase 1. Furthermore, the increased activity in the upper trapezius and serratus anterior suggests that these muscles have transitioned into more of a stabilizing role and not a positional role.
Phase 3 of the pitching motion encompasses the event of ball release to maximum shoulder internal rotation. Stance hip internal rotation occurs again as the stance hip has to internally rotate and extend, as the pelvis and trunk assist in energy dissipation and follow through (24). As the stance hip is internally rotating, the stride hip must externally rotate in an attempt to keep the stride foot in line with home plate (12,22). These hip functions are evident by the activations presented in this study as the ipsilateral (stance hip) gluteus medius exhibited moderate to moderately strong activation (32 + 9% MVIC), whereas stride leg gluteus medius only revealed low activation (6 + 1% MVIC). The scapular stabilizing muscles exhibited similar activations as presented in phase 2. The muscle activations presented by the scapular stabilizing musculature display the need for adequate scapular protraction if there is going to be efficient acceleration of the shoulder into internal rotation during phase 3.
This study exhibited significant relationships between bilateral gluteus medius and scapular stabilizing muscle activations during phase 2 of the pitching motion. Proper stability of the pelvis is critical for adequate energy transfer from the lower to upper extremity during baseball pitching (17). This was evident with the reported strong relationship between ipsilateral gluteus medius and scapular stabilizers, specifically, the lower trapezius (r = 0.52, p = 0.03) and serratus anterior (r = 0.68, p = 0.00), during phase 1. Similarly, phase 2 data revealed strong relationships between ipsilateral gluteus medius and scapular stabilizing musculature, displaying concomitant activity of both contralateral and ipsilateral gluteus medius and all 3 scapular stabilizing muscles. Furthermore, phase 3 displayed strong relationships between contralateral gluteus medius and lower trapezius (r = 0.68, p = 0.00) as well as between ipsilateral gluteus medius and upper trapezius (r = 0.65, p = 0.00).
As Kibler and Livingston (16) have documented, the lower extremity, pelvis, and trunk provide the stable base for efficient arm movement and energy transfer. However, for efficiency of coordination between the lower and upper extremity, the spine must have appropriate movement in both the sagittal and transverse plane for any energy transfer (21). The strong relationships between the gluteus medius and scapular stabilizing musculature presented in this study are in support of previous works (16,17,21–24) indicating the importance of pelvic and scapular stability in pitching. This study revealed that throughout the pitching motion, gluteal activation was in agreement with other throwing studies (26,28,30), and it demonstrated a relationship between gluteus medius activity and scapular muscle activity.
A key component in pitching is hip motion, as the kinetic energy produced by pelvic and hip mechanics has a strong impact on ball velocity (24). The relationships presented in this study are in support of the fact that decreases in hip abduction have been seen in 49% of athletes with arthroscopically diagnosed posterior superior labral tears (3). Furthermore, it has been reported that pitchers have decreased hip abduction strength and internal rotation in their ipsilateral stance leg compared with position players (22,24). These alterations in motion may lead to breakdowns in the kinetic chain and increase the risk of upper- and lower-extremity injuries in baseball pitchers.
The function of the scapula in baseball pitching is optimized when it possesses efficient positioning through balanced force couples (7). It is a general consensus that the lower and upper trapezius along with the serratus anterior act as the primary force couple that provides dynamic stability to the scapula (6,10,15,18). The serratus anterior functions to pull the scapula lateral around the thoracic cavity, whereas the lower trapezius acts to stabilize and upwardly rotate the scapula at the same time the upper trapezius produces an upward rotation force complementing the serratus anterior (7). Thus, it is the lower trapezius that tries to maintain vertical and horizontal equilibrium thereby contributing to the dynamic stability of the scapula. It has been reported that often the stabilization function of the lower trapezius is slower to activate than the upper trapezius (7), so the stimulation of the lower trapezius and gluteus medius may assist in the stability of the scapula. The gluteal and scapular stabilizing musculature relationship reiterates the need for optimal activation of the gluteus medius to stabilize the pelvis in an attempt to allow for efficient scapula stabilization.
Alterations in shoulder mechanics are often found in overhead athletes with scapular instability and are primary factors associated with injury (19,34). Thus, it is important that the entire kinetic chain is examined, especially the relationship between proximal muscle activations and the more distal muscles about the scapula. Furthermore, gluteus medius, upper trapezius, lower trapezius, and serratus anterior activity and their coordinated efforts should be addressed during training and rehabilitation protocols. Altered muscle activation of the scapular stabilizing muscles is most frequently observed in the serratus anterior and lower trapezius muscles (15). Inhibition of these muscles decreases their ability to exert force and stabilizes the scapula during dynamic movements. If these muscles are inhibited and the pelvic stabilizer is ineffective in controlling the pelvis during pitching, then the ability to transfer energy to the upper extremity may be further compromised. One my postulate that to compensate for the inhibited muscle activation, the rhomboids and upper trapezius increase activation in an attempt to provide stability to the scapula during force transfer to the shoulder.
Although this study provides insight into pelvic and scapular stabilizing muscle activations during the fastball baseball pitch, some important limitations should be noted. A homogenous sample of youth pitchers was chosen for analysis; however, this sample might not be representative of the muscular activations in more experienced baseball pitchers. More experienced pitchers likely have more refined mechanics than the subjects examined in this study. However, although the pitchers who have been throwing longer may have more experience than the pitchers examined in this study, they may also have developed altered scapular mechanics over time. Further research is needed to determine whether similar muscle activation patterns exist in more experienced baseball pitchers. Furthermore, the relationship between scapular stabilizing muscles and upper-extremity kinetics during pitching should also be investigated.
The repetitive nature of the overhead pitching motion may contribute to increased scapular instability and scapular musculature inhibition in baseball pitchers. In addition, the inability of both the pelvic and scapular stabilizing musculature to function in an efficient manner may further contribute to the injury susceptibility in baseball pitchers. Therefore, training and rehabilitation programs should consider focusing on gluteus medius activation in both dynamic and stabilizing activities. Additionally, focusing on upper-extremity posture and activation of the lower trapezius and serratus anterior are key scapular stabilization. As is has been reported that shoulder exercises should be initiated with feet on the ground involving hip extension and pelvic control (31). Therefore, it is suggested that simultaneously focusing on pelvis stability with gluteus medius activation and scapular stability will offer greater specificity of training given the findings of this study.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
electromyography; lower trapezius; serratus anterior; shoulder; upper trapezius