To the authors' knowledge, only 2 studies (8,22) have examined the mechanics of catchers throwing to second base. Fortenbaugh et al. (8) found significant kinematic differences between catchers and pitchers who were throwing an equal distance. Catchers had a significantly shorter stride length, open foot position, closed foot angle, and reduced pelvis-trunk separation angle at foot contact compared with pitchers. In addition, catchers also displayed excessive elbow flexion during arm cocking and less forward trunk tilt at ball release. Furthermore, the pitchers had significantly greater ball velocity than the catchers (8). In addition, it has been reported that catchers have lower humeral elevation than pitchers, speculating abbreviated mechanics and possible pathomechanics (22). Thus, based on these aforementioned differences, it is believed that catchers have a less efficient throwing motion.
Pelvic and trunk kinematics previously displayed by catchers (8,22) may ensue upper extremity pathomechanics and thus injury susceptibility. As defined by Kibler et al. (11), the lumbopelvic-hip complex includes the spine, hips and pelvis, proximal lower limbs, and abdominal area. Thus, the pelvis and trunk are included in the musculature of the lumbopelvic-hip complex and are responsible for pelvic stability while also assisting in the generation and transfer of energy from the lower extremity to the upper extremity (11). The stability of the pelvis is fundamentally supplied by the musculature of the lumbopelvic-hip complex, specifically through activation of the gluteal muscle group (18). The gluteal muscle group acts to stabilize the torso over a leg that is planted and allows for transference of power during any forward leg movements (18,23,24,25,26). Previous research in baseball pitchers suggests that the gluteus medius serves as both a pelvic stabilizer and a hip internal rotator, which may be related to a pitchers ability to control the rate of pelvis axial rotation (18).
Lumbopelvic-hip complex stability has been defined as the ability to maintain or resume trunk position after static and dynamic muscular contractions (29). Inability to control the musculature of the lumbopelvic-hip complex potentially leads to postural collapse and movement inefficiencies (17,26). It is the musculoskeletal core that is responsible for maintaining stability of the spine and pelvis and transferring energy from large segments of the body to smaller segments (11). The transfer of energy from the larger to smaller segments follows the premise of proximal-to-distal sequencing during dynamic movements, known as the summation of speed principle (3). It is understood that the distal segment explains the direction and speed of motion that contributes to the outcome of the skill (23,24).
It is evident that the lower extremities, pelvis and trunk, play a critical role in energy transfer and force development during dynamic activities, such as throwing (3,11,18). As the energy and forces are transmitted from the proximal lower extremity to the trunk, the pelvis functions as a conduit. As a result, the pelvic stabilizers, in particular the activation patterns of the gluteal muscle group, are imperative in understanding the throwing motion of catchers. A catcher's inability to control their proximal (lower extremity and lumbopelvic-hip complex) segments of the kinetic chain may predispose injurious pathomechanics.
Once energy has been generated and transferred from the proximal (lower extremities, pelvis, and trunk) segments, the energy must continue to be transferred distally to the shoulder, elbow, wrist, and hand for ball release (3). Therefore, the primary purpose of this study was to determine the relationship between gluteal muscle activation and pelvis and trunk kinematics when catchers throw to second base. In addition, as a secondary investigation, the effects of gender on muscle activation and lower extremity kinematics were also examined. It was hypothesized that significant relationships between gluteal muscle activation and pelvis and trunk kinematics would exist thus helping to better understand the role of the kinetic chain during catchers throwing to second base.
Experimental Approach to the Problem
The goal of this experiment was to determine the relationship between gluteal muscle activation and pelvis and trunk kinematics when baseball and softball catchers throw to second base. Both baseball and softball catchers were selected because the throwing motion between the two is similar (22). Catchers in both sports receive a pitch, in a squatted position, and quickly transfer the ball to their throwing hand to throw out a stealing runner. The participants received a pitch and were then required to make an accurate throw to second base as kinematic data were collected. The design implemented was a descriptive analysis, and Pearson's product moment correlation coefficients were also calculated to determine if relationships existed between gluteal muscle activation and torso kinematics. The dependent variables analyzed were muscle activations (gluteus medius and maximus) and pelvis and trunk kinematics. To address the secondary investigation, gender was the independent variable and a 1-way analysis of variance (ANOVA) was performed to determine if gender influenced muscle activation and trunk kinematics in catchers throwing to second base.
Forty-two baseball and softball catchers (14.74 ± 4.07 years; 161.85 ± 15.24 cm; 63.38 ± 19.98 kg) volunteered to participate. When analyzed by gender, 22 male baseball catchers (12.77 ± 3.48 years; 159.35 ± 18.97 cm; 57.21 ± 22.96 kg) and 20 female softball catchers (16.90 ± 3.60 years; 166.70 ± 8.57 cm; 70.16 ± 13.65 kg) participated. Subject inclusion criteria included coach recommendation, multiple years of catching experience, and freedom from injury. Subjects suffering an injury within the past 6 months, which required medical attention, were excluded from participation. Additional criterion included coaching staff recommendation and multiple years (up through the current season) of catching experience. Data collection sessions were scheduled at the beginning of the subject's competitive season. Subjects reported for testing before engaging in resistance training or any vigorous activity that day. Testing was conducted in a gym inside the University of Arkansas Health, Physical Education, Recreation, and Dance building. The University of Arkansas Institutional Review Board approved all testing protocols. Approved testing procedures were explained to each subject and their parent(s)/legal guardian(s), and informed consent and subject assent were obtained before testing began.
Subjects reported for testing before engaging in any vigorous activity for that day. Electromyographic (EMG) data were collected through a Noraxon Myopac 1400L 8-channel amplifier (Noraxon USA, INC, Scottsdale, AZ, 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. The surface EMG data were notch filtered at frequencies of 59.5 and 60.5 Hz (2,18,20). The identified locations for surface electrode placement were shaved, abraded, and cleaned using standard medical alcohol swabs. Subsequent to surface preparation, adhesive 3M Red-Dot bipolar, 6 cm Al/AgCl diameter disk shaped, surface electrodes (3M, St. Paul, MN, USA) were attached over the muscle bellies of the bilateral gluteus maximus and medius and positioned parallel to the muscle fibers using techniques previously described (1,2,18–20). The selected interelectrode distance was 25 mm (10,18–20). An additional electrode was placed on the anterior superior iliac spine to serve as a ground lead for the examined muscles.
After the application of surface electrodes, manual muscle testing (MMT) techniques by Kendall et al. (10) were used to determine the steady-state contraction. A certified athletic trainer (ATC), familiar with the techniques, performed all MMT to ensure reliability throughout testing. To test the gluteus maximus, the subject was prone with knee flexed to 90°. The subject was instructed to perform leg extension, with the knee flexed, while the ATC provided resistance at the upper leg in the direction of flexion (10). The gluteus medius was tested with the subject side lying with the underneath leg flexed at the hip and knee and pelvic rotated slightly forward. The ATC stabilized the pelvis and instructed the subject to abduct the hip with slight external rotation. The ATC then provided resistance to the lateral upper leg in the direction of adduction (10). Both tests were performed bilaterally. Three MMT, lasting 5 seconds, were performed for each muscle, and the first and last second of each contraction was removed (18–20). The MMT provided baseline data in which all surface EMG data were compared.
The MotionMonitor (Innovative Sports Training, Chicago, IL, USA) synched with electromagnetic tracking system (Flock of Birds Ascension Technologies, Inc., Burlington, VT, USA) was used to collect data. The electromagnetic tracking system has been validated for tracking humeral movements, producing trial-by trial interclass correlation coefficients for axial humerus rotation in both loaded and nonloaded condition in excess of 0.96 (12). With electromagnetic tracking systems, field distortion has been shown to be the cause of error in excess of 5° at a distance of 2 m from an extended range transmitter (4), but increases in instrumental sensitivity have reduced this error to near 10° before system calibration and 2° after system calibration (15,21). Thus, before data collection, the current system was calibrated using previously established techniques (4,14,21). After calibration, pilot data collected before subject testing revealed the magnitude of error for sensor position and orientation within the calibrated world axes system was less than 0.01 m and 3°, respectively.
Subjects had a series of 10 electromagnetic sensors (Flock of Birds Ascension Technologies, Inc.) attached at the following locations: (1) the medial aspect of the torso at C7; (2) medial aspect of the pelvis at S1; (3–4) bilateral distal/posterior aspect of the upper arm; (5–6) bilateral distal/posterior aspect of the forearm; (7–8) bilateral distal/posterior aspect of the lower leg; and (9–10) bilateral distal/posterior aspect of the upper leg (15,18,19,22). Sensors were affixed to the skin using double-sided tape, with the sensor cord pointed upward and then wrapped using flexible hypoallergenic athletic tape to ensure the sensors were secure throughout testing. An 11th sensor was attached to a wooden stylus and used to digitize the palpated position of the bony landmarks (15,18,19,22). An ATC trained with the electromagnetic system completed all subject digitization. Subjects were instructed to stand in anatomical neutral while selected bony landmarks were accurately digitized. A link segment model was developed through digitization of joint centers for the ankle, knee, hip, shoulder, T12-L1, and C7-T1. By virtue of the least squares method, the hip and shoulder joint centers were defined (13,14).
The collection rate for these data describing the position and orientation of electromagnetic sensors was set at 144 Hz. Raw data were independently filtered along each global axis using a fourth-order Butterworth filter, with a cutoff frequency of 13.4 Hz. Raw data regarding sensor orientation and position were transformed to locally based coordinate systems for each of the respective body segments. Euler angle decomposition sequences were used to describe both the position and orientation of the torso relative to the global coordinate system (27,28). The use of these rotational sequences allowed the data to be described in a manner that most closely represented the clinical definitions for the movements reported (15).
Subjects were allotted an unlimited time to warm up and gain familiarity with the testing surface. The average warm-up time was 8–10 minutes. In addition to performing their regular precompetition warm-up, the subjects were required to perform test trials of throwing to second base from their squatting stance to assure that they landed properly on the force plate. All warm-up and test trials were conducted with the subject wearing full catchers gear consisting of a facemask, chest protector, shin guards, and turf shoes to best simulate a competitive environment. The subject's stride leg (nonthrowing arm side) was defined as the leg that steps toward the target of second base. The drive leg was the subject's throwing arm side leg that was used to propel the body toward second base. The catching surface was positioned so that the stride foot would land on top of a 40 × 60-cm Bertec force plate (Bertec Corp, Columbus, OH, USA) that was anchored into the floor. To best simulate a game experience, the subject received a pitch from a pitcher and then threw the ball to a position player on second base. The pitcher was instructed to place the pitch as close to the middle of the home plate as possible. Once the subject caught the pitch from their squat stance, they were instructed to throw to second base located, the regulation distance from home plate. Each subject was instructed to perform 5 accurate throws. Accurate was defined as a throw where the position player did not have to move off the base. For the current study, those data from the fastest throw to second base were selected for detailed analysis (18). A JUGS radar gun (OpticsPlanet, Inc., Northbrook, IL, USA), positioned at home plate and directed toward second base, was used to determine ball speed.
Data were analyzed using SPSS 19 for Windows (SPSS, Chicago, IL, USA). Descriptive statistics were calculated for all surface EMG and kinematic parameters for the fastest throw made by each subject. Surface EMG data were reported as a percentage of each individual's maximum voluntary isometric contraction (%MVIC), and kinematic data were reported in degrees and degrees per second. The kinematic variables analyzed were pelvis lateral flexion, pelvis rotation velocity, trunk flexion, trunk lateral flexion, trunk axial rotation velocity, and trunk axial rotation. The throwing motion was broken down into the events of foot contact, maximum external rotation (MER), ball release (BR), and maximum internal rotation (MIR) (Figure 1). Kinematic data were represented at each event, whereas surface EMG data represented the phases of throwing. Pearson's product moment correlation coefficients were then calculated to identify the relationships between gluteal activation and trunk and pelvis kinematics. A 1-way ANOVA was used to determine if kinematic differences were present between genders. Type I error rate was set a priori at p ≤ 0.05, and each phase of throwing was analyzed independently.
Gluteal surface EMG results are shown in Figures 2–5. Stride leg (nonthrowing arm side) gluteus maximus activity was greatest, at 71%MVIC, during the acceleration phase of throwing. In addition, stride leg gluteus medius activity reached a maximum value of 52%MVIC during deceleration. Muscle activity for the drive leg (throwing arm side) gluteus maximus was greatest during the acceleration phase. The drive leg gluteus medius activity reached a maximum value in the cocking phase of throwing, between foot contact and MER.
Pelvis and Trunk Kinematics
Kinematic data describing the pelvis and trunk are given in Table 1. The position of the pelvis was initially tilted toward the drive leg, but as the throwing motion progressed, the pelvis tilted toward the stride leg. The pelvis and trunk both reached maximum rotational velocity, at the point of MER of the shoulder, and then decreased the remainder of the motion. The magnitude of trunk rotational velocity was greater than that of the pelvis. The trunk displayed decreased flexion after foot contact and continued to laterally flex toward the stride leg.
Pearson's product moment correlation coefficients were calculated to determine if any relationships existed between stride leg gluteal activity (Table 2), drive leg gluteal activity (Table 3), and both pelvis and trunk kinematics during throwing from the stance. No statistically significant relationships were observed between the drive leg gluteus medius and both pelvis and trunk kinematics. A moderate negative correlation between stride leg gluteus maximus activity and pelvis axial rotation at foot contact (r = −0.31, r2 = 0.10, p = 0.05) was observed. Drive leg gluteus maximus activity and trunk flexion had a moderate positive relationship at foot contact (r = 0.33, r2 = 0.11, p = 0.04). Correlation coefficients were also calculated to determine the relationships between pelvis and trunk kinematics. Trunk axial rotation and pelvis lateral flexion displayed a significant, moderate negative correlation (r = −0.34, r2 = 0.12, p = 0.03) at foot contact. These data are presented in Table 4.
Few significant kinematic differences and no differences in gluteal muscle activation were observed between genders. Trunk lateral tilt at MER was significantly more tilted to the nonthrowing side (p < 0.001) in men (−13.17°) compared with women (1.65°). Trunk lateral tilt at MIR was also significantly more tilted to the nonthrowing side (p = 0.002) in men (−29.98°) compared with women (−16.40°). At MIR, men had significantly greater (p = 0.046) pelvis lateral flexion to the nonthrowing side (−16.22°) compared with female catchers (−11.38°).
Pelvic stability is supplied through the lumbopelvic-hip complex. The gluteal muscle group is directly responsible for the stability of the pelvis (11,16). The inability of the gluteal muscle group to stabilize the lumbopelvic-hip complex leads to kinematic alterations in the functioning of the entire kinetic chain. The first phase of the throwing motion of a squatting catcher begins when the catcher receives the pitch and strides out to foot contact. During this phase, the catcher generates energy to propel the stride leg toward the target (second base). The gluteus maximus functions as a hip extensor and external rotator, whereas the gluteus medius acts as a lateral stabilizer during open chain component of stepping and assists in maintaining a level pelvis during the closed chain component of loading/weight transfer. Moderate activation levels, as defined by DiGiovine et al. (5) and Escamilla and Andrews (6), were reached in the stride and drive leg gluteal muscles during this initial phase of throwing and indicates that the stride leg is beginning to move into extension and external rotation as energy from the drive leg is transferred. Additionally, a negative correlation between stride leg gluteus maximus activity and pelvis axial rotation at foot contact was observed. Drive leg gluteus maximus activation and trunk flexion had a positive relationship at foot contact indicating that the gluteals acted to stabilize the pelvis and form an anchor point for the trunk flexors. Trunk axial rotation and pelvis lateral flexion displayed a significant negative correlation at foot contact indicating that as the trunk rotation reached a maximum, pelvic lateral flexion reached a minimum.
The second phase of throwing, known as the cocking phase, occurs after foot contact and ends with maximum shoulder external rotation (7). Rotation of the throwing side pelvis and trunk toward the target (second base) occurred during this phase. The greater shoulder external rotation a catcher can achieve, the greater internal rotation range of motion is available to generate forward velocity of the humerus (7).
In a study examining the same muscle activation and kinematic variables in high school baseball pitchers, Oliver and Keeley (18) observed drive leg gluteus maximus activity that was in excess of 100%MVIC. Although the activation values in the present study were not as great as those previously observed in baseball pitchers, the pattern of activation was similar. Both studies had peak activation values at shoulder maximal external rotation of the shoulder and activation decreased as the throw progressed. The gluteus medius also appeared to control pelvic lateral flexion and acted as an internal rotator of the hip. The stride leg gluteus medius activity continually increased during the throwing motion. This pattern of activation is expected because after foot contact, weight is beginning to be transferred from the drive leg to the stride leg. For proper energy transfer, the summation of speed principle states, each segment should start its motion at the instant of greatest speed of the preceding segment and reach a maximum speed greater than that of its predecessor (3,23,24). Both pelvis and trunk axial rotation velocity peaked at shoulder maximal external rotation and then steadily declined throughout the completion of the throw. Proximal to distal sequencing also allows for the more distal segment to reach its maximum when the more proximal segment is at a minimum (3). The results of the current study would imply that the trunk and pelvis act as one unit because both peak simultaneously. The gluteal muscle group works to maintain the pelvis to trunk position so that the energy, speed, and force from the lower extremity can be translated to the scapula. During this phase, the gluteal muscles had peak activation because maintaining lumbopelvic stability is still needed as the catcher continues to be positioned in a double leg support. Energy will then continue to be transferred up the kinetic chain to the shoulder, which should exhibit greater angular velocity than the pelvis and trunk unit.
The acceleration phase of throwing is initiated when the shoulder reaches MER and ends after the release of the ball. The catcher's weight is shifted onto the stride leg (single support), and the gluteus medius attempts to maintain neutral pelvis alignment. As the catcher's weight shifted from the drive leg to the stride leg, drive leg gluteus medius activity peaked at MER and then decreased as the throwing motion continued. This is analogous to the role of the gluteus medius during gait preventing a positive Trendelenburg's sign. Stride leg gluteus medius activity had a positive relationship with trunk axial rotation at ball release and a negative correlation at MIR The correlations indicate that the gluteus medius of the stride leg assisted in the rate of trunk rotation at ball release and then possibly shifted roles from an accelerator to a stabilizer during the remainder of the motion. The gluteus medius activity is indicative of attempting to stabilize the pelvis to accelerate the trunk. In the study by Oliver and Keeley (18), a similar correlation in pitchers was observed; however, it was found that this occurred at MER for the pitchers (r = 0.794, p ≤ 0.05). The differences between pitchers and catchers may be indicative of catchers not fully using trunk rotation at MER. The lack of relationship between trunk rotation and stride leg gluteus medius activity in catchers may result in altered forces being transmitted distally to the shoulder, elbow, and wrist. Previous research has found that youth pitchers have a propensity for initiating trunk rotation early, which can result in an increase in the horizontal abduction angle of the shoulder (9). Thus, the inability to control the rate of trunk rotation throughout the throwing motion may result in increased the forces acting about the upper extremity and potentially predispose catchers to shoulder injury. In addition, it has also been reported shoulder moments in younger catchers reaching a maximum average of −28.16 N in younger catchers and −96.41 N in an older group of catchers throwing to second base (22). The same study also revealed elbow moment of 8.80 N in younger catchers and 25.56 N in older catchers. The potential increase of these already high moments about the shoulder and elbow due to decreased control of trunk rotation in catchers may lead to an increased risk of injury at the weaker segments of the kinetic chain. Maintaining stability during follow through is needed for the catcher to remain in an upright posture and in order for this to occur the gluteal muscles must continue to fire.
Although this study provides valuable insight into the throwing mechanics of catchers, it is important to note that limitations are present. The participants ranged from 9 to 23 years old, and because of this wide age range, it is assumed that the older participants were more experienced catchers. The difference in experience may account for the large SE observed for some of the variables analyzed. Although it is ideal to have a more homogeneous sample sizeto study, it is often difficult to recruit a large sample of catchers in the relatively small geographic area in which this study was conducted. Many baseball and softball teams only have a few catchers, which further impedes the ability to recruit additional participants. Additionally, the dynamic muscle activations were normalized based on static MMT. Thus, normalizing dynamic activity to static test introduces some variability. However, it also should be noted that the standard protocols for EMG methodology were implemented in attempt to eliminate variability.
Further research is needed to quantify the kinematics and associated kinetics about the shoulder and elbow during the throwing motion of catchers. By understanding the kinematics and kinetics associated with the throwing motion of catchers, future studies may identify potential pathomechanics that lead to injury. Future research should also examine injury statistics in catchers in attempt to better understand the role mechanics play in injury.
The results of this study provide evidence of the function of the gluteal muscles on the control of the pelvis and torso during the throwing motion. The gluteal muscles in both legs exhibited moderate-to-high activity during the throw to second base. The drive leg gluteal muscles function to generate energy to propel the catcher forward as they stride out toward second base. After contact of the stride leg foot, energy from the drive leg is gradually transferred to the stride leg. Strength and appropriate timing of stride leg gluteal muscle activation is critical as the catcher moves into a position of single leg support. The inability to properly support the body during single leg stance will alter the forces that are transferred up the kinetic chain and ultimately lead to increased forces acting on the smaller more distal segments. These increased forces may lead to increase in the risk of injury in catchers. As the gluteal muscles play a direct role in maintaining the stability of the pelvis, catchers should incorporate strengthening of the entire lumbopelvic-hip complex into their training regimen. The superman, flying squirrel, abdominal bridge, and the single-leg abdominal bridge exercises have previously been found to activate the musculature of the lumbopelvic-hip complex (20). These exercises are body weight isometric exercises that can easily be implemented in the daily training regimen of both baseball and softball catchers.
The authors report no conflict of interest. In addition, the results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
electromyography; kinetic chain; lower extremity; motion analysis