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Research Article, J Athl Enhancement Vol: 3 Issue: 2

Electromyographic Activation Patterns during Handball Throwing By Experts and Novices

Elissavet N Rousanoglou1*, Konstantinos S Noutsos2, Ioannis A Bayios2 and Konstantinos D Boudolos1
1Sport Biomechanics Lab-Department of Sport Medicine and Biology of Exercise, National and Kapodistrian University of Athens, Greece
2Department of Sport Games, Faculty of Physical Education and Sports Science, National and Kapodistrian University of Athens, Greece
Corresponding author : Rousanoglou N Elissavet
Sport Biomechanics Lab-Department of Sport Medicine and Biology of Exercise, National and Kapodistrian University of Athens, Ethnikis Antistasis 41, Daphne, 172-37, Greece
Tel: 00302107276090
E-mail: [email protected]
Received: October 22, 2013 Accepted: January 29, 2014 Published: February 04, 2014
Citation: Rousanoglou EN, Noutsos KS, Bayios IA, Boudolos KD (2014) Electromyographic Activation Patterns during Handball Throwing By Experts and Novices. J Athl Enhancement 3:2. doi:10.4172/2324-9080.1000142

Abstract

Electromyographic Activation Patterns during Handball Throwing By Experts and Novices

Knowledge of the electromyographic (EMG) activation pattern during an athletic movement, as well as its difference between experts and novices, are helpful in providing appropriate technical instructions, strength training, and injury prevention protocols. This study aimed to compare the timing and intensity EMG activation pattern of experts and novices during the handball standing throw. Surface EMG recordings were taken for the trapezius, pectoralis major, triceps brachii, and biceps brachii muscles. In synchronization with EMG recordings, trials were video recorded to determine the timing of the throwing phases (tcocking, tacceleration, tfollow through). The throw with the greater ball velocity was selected for further analysis. The significance of group differences was examined with t-tests for independent samples. ANOVAs, for repeated measures, were applied for the differences among muscles and across throwing phases. The level of significance was set at p ≤ 0.05 for all analyses (SPSS version 21.0). The ball throwing velocity and throwing accuracy were significantly better in experts than novices (p ≤ 0.05). No significant group difference was found for the timing of throwing phases and the timing EMG activation (p>0.05). The experts showed increased intensity of EMG activation for the trapezius and the pectoralis major muscles during tcocking, with the group difference being reversed during tacceleration (p ≤ 0.05). The group invariance in the timing pattern of throwing phases and EMG activation possibly suggests that the throwing pattern is acquired early in the learning process. The differences in the intensity pattern of EMG activation probably highlight the insufficiency of the novices to optimally store elastic energy during tcocking. Thus, from the early stages of training, care should be focused on the achievement of an optimal tcocking.

Keywords: Standing throw; Cocking phase; Elastic energy storage; Timing pattern

Keywords

Standing throw; Cocking phase; Elastic energy storage; Timing pattern

Introduction

In throwing events, the shoulder and arm musculature is responsible for the propulsion of the object held in the hand as well as for the protection of the surrounding tissues. Electromyographic (EMG) recordings during athletic throwing movements allow for the determination of the timing and quantity of muscle activation, and they are helpful to trainers and therapists for providing athletes with appropriate technical instructions, strength training, injury prevention, and rehabilitation protocols [1]. In athletic throwing, the cocking phase is particularly important because, during this phase, the storage of elastic energy will be released later on in the acceleration phase [2,3]. During the cocking phase, the EMG activity of the trapezius and the pectoralis major muscles contributes to stabilize the scapula [4] and the ventral [5], respectively, for the achievement of the adequate torque generation for the subsequent humeral internal rotation [5,6]. Also, the biceps activation and deactivation EMG pattern is important for preventing and allowing the centrifugal forces to extend the elbow, thus facilitating the adequate storage of elastic energy during the cocking phase and the generation of a rapid elbow extension during the acceleration phase [3,7]. With optimizing throwing mechanics and preventing throwing injuries in mind, many studies have examined the EMG activation patterns in overhead throwing movements such as the baseball pitching [5,8-10] and the softball pitching [11,12]. However, there appears to be a lack of data regarding the EMG activation patterns during handball overhead throwing, despite the fact that team handball is among the top athletic games with a high injury risk [13].
The best time to try to prevent throwing injuries is at the beginning of the learning process when proper training can allow novices to develop good throwing mechanics [14]. Many studies have documented the greater throwing velocity and better throwing accuracy in experts when compared to novices [15-17]. However, the comparative research information regarding the EMG activation patterns in expert and novice throwing athletes is limited even for the widely studied movement of baseball pitching [8]. An expert baseball pitcher can throw the ball accurately and quickly, because he efficiently transfers energy to the ball due to selective muscle activation [8]. The reported EMG differences between expert and novice baseball pitchers are throwing-phase specific and associate to throwing velocity [8]. In specific, similar EMG activation patterns are reported for both the expert and novice baseball pitchers during the cocking and follow-through phases, while these diverged during the acceleration phase [8]. The EMG activation pattern provides insight into a muscle’s contribution either as stabilizer, accelerator, or decelerator during each particular phase of the throwing movement [5,8-10]. For novice throwers, their lower activation of the trapezius muscle during the cocking phase results in a less stabilized scapula [4]. Also, compared to experts, the novice throwers deviate from a “whip like” sequential motion in the transition from cocking to the acceleration phase [17]. At the end of the cocking phase, near the time of maximum shoulder external rotation, the shoulder and arm are susceptible to injury [18,19]. At this point of the throwing movement, the shoulder’s external rotators initiate a high eccentric muscle activity [13-15] to accomplish the rapid shoulder internal rotation (between 6000 and 7000 degrees-1) [20,21]. Manifesting the timing and intensity differences in the EMG activation between expert and novice handball players may be helpful for establishing optimal throwing mechanics early in the training process, thus enhancing throwing performance as well as reducing the potential injury risk. We hypothesized that the timing and intensity patterns of EMG activation would differ between experts and novices, with the differences being pronounced in the cocking phase. Thus, the purpose of this study was to compare the timing and intensity patterns of EMG activation during handball throwing between experts and novices.

Methodology

Participants
A group of experts (n=13, age: 24.8 ± 2.7 years) and a group of novices (n=13, age: 21.8 ± 0.9 years) in handball throwing participated in this study. The group of experts consisted of the best scorers in the 1st division of the Handball National League with a training experience of 12.3 ± 3.0 years. The group of novices consisted of young men with a 4 month training experience in handball throwing and no experience in official competitive team handball. Their anthropometric characteristics are presented in Table 1 [22,23]. The group differences in anthropometric characteristics were examined with t-test for independent samples, which revealed that the experts had significantly greater body mass than the novices (t=3.72, p=0.03). All participants were free of medical problems or pain that could affect their performance and signed an informed consent form that described the testing procedure in detail. The work reported has been approved by the institutional review board and conforms to the principles outlined in the Declaration of Helsinki.
Table 1: Anthropometric characteristics of the participants determined by standardized procedures [22].
Procedures
The participants completed standing overhead throws from the 7 m penalty line using a standard handball for men (mass=0.44 kg, circumference=58.1 cm). A total of 5 successful trials were recorded for each participant with one minute rest between trials as in the studies of Oliver et al. [11] and Rojas et al. [12]. The front foot was in contact with the ground for the entire duration of the movement. A 15 min warm-up was allowed for each participant including general and shoulder-specific mobility exercises, as well as stretching exercises and familiarization trials. The speed-accuracy instruction was selected because it is similar to the competition requirements, where the player attempts to throw as fast and accurately as possible in order to score a goal. Furthermore, experts as well as novices achieve the greatest ball velocity [17,24] with the speed-accuracy instruction. The trial with the greatest ball velocity was selected for further analysis as in the studies of Oliver et al. [6] and Rojas et al. [12]. If the ball velocity was the same in two or more trials, the one with the best throwing accuracy was selected. To ensure that the selected trials did not differ significantly from the remaining trials, we applied univariate one-way repeated measures ANOVAs (5 levels of the trial repeated factor), followed by pairwise comparisons, in all 25 examined variables. The results of this statistical exploration did not reveal any significant difference among trials in any of the examined variables (level of significance was set at p ≤ 0.05, SPSS version 21.0).
Throwing performance was evaluated with ball throwing velocity and throwing accuracy. The ball throwing velocity was measured by an innovative electronic device, the technical characteristics of which have been described previously [25]. In short, the device consisted of a laser beam emitter and an electronic system of laser beam infrared detectors that were connected to a digital pulse counter. The device was positioned so that ball interrupted the laser beam at a distance of 1.5 m after the penalty line. The ball throwing velocity was calculated by multiplying the beam interruption time by the ball’s diameter, and it was expressed in meters per second (m.s-1). The throwing accuracy was measured by an innovative electronic device which has also been described previously [26]. It consisted of a Π-shaped tabloid surface that was attached firmly to the inner side of a handball goal post. The tabloid surface included a net of light-emitting diodes (LEDs) (target hit pointers) that were interwoven with a net of metal strips (hit point detectors). The hit point detectors transferred the coordinates of the actual hit point to the central unit with an accuracy of 1 mm. The throwing accuracy was defined by the difference between the coordinates of the target and the actual hit point. The player initiated his trial when the target-pointer lit up (randomly via an electronic programmer). If the ball did not hit the tabloid surface, the trial was rejected and additional trials were conducted until a total of 5 successful shots were achieved. Four expert and 12 novice participants (about ¼ and ¾ of the group, respectively) performed trials that did not hit the tabloid surface (6 and 25 trials, respectively, with an average of 15 ± 1.0 and 2.1 ± 1.2 trials per participant). The rejected trials corresponded to 7.2% and 25% of the total group trials (83 and 100 trials), for the experts and the novices, respectively.
To identify the duration of throwing phases, all trials were recorded on video at 50Hz in synchronization with EMG data collection. The camera was positioned so that the entire throwing movement was clearly visible. The throwing phases determined from the video analysis (PEAK PERFORMANCE software, Version 8.2, Colorado Springs, USA) were the cocking phase (CP), which is a period of shoulder abduction and external rotation that begins as the ball is released from the non-dominant hand and ends when maximal external rotation of the shoulder is attained. The acceleration phase (AP) is defined from maximal shoulder abduction and external rotation until the ball leaves the fingers and includes the follow through phase (FTP), which is the final interval of arm motion from ball release until the arm is crossed in front of the body. The FTP could be further subdivided by the point of maximal humeral internal rotation into early and late subphases. The phase definitions used by Gowan et al. [8], however, were adopted for the comparison of results. The variables inserted for statistical analysis were the total throwing duration (ttotal) as well as the absolute and relative phase durations of the CP, AP, and FTP (tcocking, tacceleration and tfollow through, respectively) expressed in seconds (s) and as a percentage of ttotal (% ttotal), respectively.
The EMG activity of the trapezius (upper portion), the pectoralis major (sternal head), the triceps brachii (lateral head), and the biceps brachii (long head) was recorded by self-adhesive Silver-Silver Chloride surface electrodes [5,8-12]. The electrodes were placed along the longitudinal axis of the muscle belly, as estimated according to Basmajian and DeLuca [27] and DeLuca [28] with a 25 mm interelectrode distance. The ground electrode for all muscles was placed on the clavicle. The surface of the skin was shaved, treated with alcohol, and rubbed with fine sandpaper to reduce inter-electrode resistance before the electrodes were attached the skin. All EMG signals were visually monitored during the collection of data (with a sampling frequency at 1000 Hz, gain of 1000, signal to noise ratio of 90 dB, MP100A data acquisition unit of ΒIOPAC SYSTEMS, Inc. Santa Barbara, CA) to verify signal quality and were stored for offline processing (AcqKnowledge 3.7.3 software for Windows, BIOPAC SYSTEMS, Inc. Santa Barbara, CA). The raw signals were band-pass filtered (10-500 Hz, FIR Blackman 61db/octave) [29]. A 50 Hz notch filter was used to remove the power line interference according to the manual of the AcqKnowledge 3.7.3 software. After a full wave rectification, the linear envelope of the filtered EMG data was detected (i.e., low pass filter, cut off frequency at 6Hz, FIR Blackman 61db/ octave) [30]. For the determination of the onset of the EMG activity, a threshold value was set to the mean baseline activity plus 3 SD (mean baseline activity was averaged over 100 ms, between 400 and 300 ms prior to initiation of the throwing movement). The point at which the preceded baseline value had exceeded the threshold value for a critical duration (25 ms) was taken as the onset of EMG burst activity [31]. These criteria were verified in all cases by a visual examination of the individual records. From the detected linear envelopes, the EMGvariables determined for each muscle were the time to peak EMG activity (tEMG-peak) expressed as a percentage of the total movement duration (% ttotal) as a measure of the timing pattern and, the EMG integral (V.s) during ttotal, tcocking, tacceleration and tfollow through as a measure of activation intensity for each muscle (EMGtrapezius, EMGpectoralis, EMGtriceps and EMGbiceps). The normalization to the task under investigation, instead of an isometric maximum contraction, was selected [32,33]. Thus, for each one of the examined muscles, the EMG integral during each phase was expressed as a percentage of the EMG integral during the total movement duration (% EMGtotal).
Statistical analysis
To test the significance of the group differences (experts vs. novices), the t-test for independent samples was applied to the variables of ball throwing velocity, throwing accuracy, ttotal as well as absolute and relative tcocking, tacceleration and tfollow through. In all group comparisons, the homogeneity of variances was tested with the Levene’s statistic, and the results are reported for homogeneous variances unless stated otherwise. If a violation of the homogeneity assumption was detected, the contingent statistical protocol included the statistics for the “equal variances not assumed.” To test the differences of tEMG-peak among muscles, one-way repeated measures ANOVAs were applied separately in the experts and the novices (4 levels of the muscle repeated factor: trapezius, pectoralis major, triceps brachii, and biceps brachii muscles). Pairwise comparisons, with Bonferroni adjustment for multiple comparisons, were applied to determine the specific pairs of muscles, concluding that the tEMGpeak differed significantly. To test the significance of differences of the throwing phase durations and EMG activation across the throwing phases, one-way repeated measures ANOVAs were applied separately to the experts and the novices (3 levels of the phase repeated factor: CP, AP, and FTP). Pairwise comparisons, with Bonferroni adjustment for multiple comparisons, were applied to determine the specific pairs of throwing phases that differed significantly. In the one-way repeated measures of ANOVAs, the sphericity of variances was tested with the Mauchly’s test. The results are reported for non-violated sphericity of variances unless stated otherwise. If a violation of sphericity was detected, the contingent statistical protocol included the use of the Greenhouse-Geisser correction of the degrees of freedom. The level of statistical significance was set at p ≤ 0.05 (SPSS version 21.0).

Results

Throwing performance
The ball throwing velocity (Experts: 23.9 ± 2.1 m/s, Novices: 16.7 ± 1.7 m/s) and throwing accuracy (Experts: 19.5 ± 7.5 cm deviation from target, Novices: 39.3 ± 13.2 cm deviation from target) were both significantly better in the experts (p ≤ 0.05). The exact statistics of the group comparisons are thoroughly presented in Table 2.
Table 2: Statistics for the t-tests applied to compare experts and novices in throwing performance, duration of throwing phases, timing of EMG activation and intensity of EMG activation. The statistics for the Levene's test for equality of variances are also presented.
Duration of throwing phases
No significant differences were found between the experts and the novices in ttotal (p>0.05) as well as in absolute and relative duration values of tcocking, tacceleration and tfollow through (p ≤ 0.05) (Figure 1). The assumption of homog eneity of variances was violated for ttotal and absolute tcocking (p ≤ 0.05). The exact statistics of the group comparisons are thoroughly presented in Table 2. The comparison across phases yielded statistical significance for the absolute and relative phase durations in both he experts (p ≤ 0.05) and the novices (p≤0.05) (Figure 1). The significant pairwise comparisons showed that, in both groups, the absolute and relative tcocking was significantly longer than tacceleration (p ≤ 0.05) and tfollow through (p ≤ 0.05). Also in both groups, the absolute and relative tacceleration were significantly shorter than the respective tfollow through (p ≤ 0.05) (Figure 1). The sphericity assumption was violated for both the absolute and relative phase durations in the experts (p ≤ 0.05) and the novices (p ≤ 0.05). The exact statistics of the throwing phase comparisons are thoroughly presented in Tables 3 and 4.
Figure 1: Mean (SD) of the duration of the total throwing movement (ttotal) and of the cocking (CP), acceleration (AP) and follow through (FTP) phases, expressed in seconds (s) (Top) as well as a percentage of the total duration (% ttotal) (Center) and of the time to peak EMG activation (tEMG-peak) for the trapezius (Trap), pectoralis major (Pec), triceps brachii (Tric) and biceps brachii (Bic) muscles expressed as percentage of ttotal (% ttotal) (Bottom), for the experts (black bars) and the novices (gray bars). ns: non significant differences between the experts and the novices (p>0.05), † and ††: significantly different than AP and FTP, respectively (p ≤ 0.05), ×: significant difference between the specific muscle and all others for the particular group (p † 0.05).
Figure 2: Mean (SD) of the EMG activation during the cocking (CP), acceleration (AP) and follow through (FTP) phases, expressed as percentage of the EMG integral during the total throwing duration (% EMGtotal) for the trapezius (solid bar), the pectoralis major (dotted bar), the triceps brachii (diagonal lines bar) and the biceps brachii (vertical lines bar) muscles. * Significant difference between the experts (black) and the novices (grey) (p ≤ 0.05). †, ††, and †††: significantly different than AP, FTP and CP, respectively (p ≤ 0.05).
Table 3: Statistics of the univariate ANOVAs for the comparison across muscles and across throwing phases. The statistics for the Mauchly's test of sphericity are also presented. The statistics for the pairwise comparisons following up the ANOVAs are presented in Table 4.
Table 4: Statistics for pairwise comparisons among muscles and throwing phases following up the univariate Anovas presented in Table 3.
Timing of EMG activation
The timing of peak EMG activation (tEMG-peak) did not differ between experts and novices for any of the examined muscles (p>0.05) (Figure 1). The exact statistics of the group comparisons are thoroughly presented in Table 2. In both the experts and the novices, the timing of peak EMG activation differed significantly across muscles (p≤0.05). The significant pairwise muscle comparisons showed that, in the experts the trapezius muscle peaked its intensity of earlier than all other muscles (p ≤ 0.05) (Figure 1) and that, in the novices, the triceps brachii muscle peaked its intensity later than all other muscles (p ≤ 0.05) (Figure 1). The sphericity assumption was violated in the novices (p ≤ 0.05). The exact statistics of the throwing phase comparisons are thoroughly presented in Tables 3 and 4.
Intensity of EMG activation
The EMGtrapezius and EMGpectoralis differed significantly between the experts and the novices in both tcocking and tacceleration (p ≤ 0.05) (Figure 2). In tfollow through, no significant group difference was found for neither EMGtrapezius nor EMGpectoralis (p>0.05) (Figure 2). In specific, the EMGtrapezius and EMGpectoralis were significantly greater in the experts during tcocking, while the reverse was found during tacceleration (Figure 2). The EMGtriceps and EMGbiceps had no significant group difference during tcocking, tacceleration and tfollow through (p>0.05) (Figure 2). The exact statistics of the group comparisons are thoroughly presented in Table 2. In both groups, the intensity of EMG activation differed significantly among throwing phases for the EMGtrapezius (p ≤ 0.05) and the EMGbiceps (p ≤ 0.05). The significant pairwise phase comparisons showed that, in both the experts and the novices, the EMGtrapezius during tcocking was greater than during tacceleration (p ≤ 0.05) and tfollow through (p ≤ 0.05) (Figure 2). The EMGbiceps during tcocking was significantly greater than tacceleration only in the experts (p ≤ 0.05) and than tfollow through only in the novices (p ≤ 0.05) (Figure 2). The throwing phase comparison did not yield significant differences in the experts for either EMGpectoralis or EMGtriceps (p>0.05), while, in the novices, it yielded statistical significance for both EMGpectoralis and EMGtriceps (p ≤ 0.05) (Figure 2). The significant pairwise phase comparisons showed that, in the novices, the EMGpectoralis during tacceleration was greater than tcocking and that, the EMGtriceps during tfollow through was greater than tcocking (p ≤ 0.05) and tacceleration (p ≤ 0.05) (Figure 2). The sphericity assumption was violated for EMGbiceps in both groups (p ≤ 0.05). The exact statistics of the throwing phase comparisons are thoroughly presented in Tables 3 and 4.

Discussion

The purpose of this study was to compare the timing and intensity of EMG activation during handball throwing between experts and novices. To our knowledge, this is the first study regarding the EMG activity during a handball throw. The main findings of the study were a) the group invariance in the timing of EMG activation, b) the significantly greater intensity of EMGtrapezius and EMGpectoralis by the experts during tcocking which was reversed to greater intensity by the novices in the subsequent phase of tacceleration and c) the significantly greater EMGbiceps during tcocking than tacceleration only by the experts. Overall, these differences may be associated with a more efficient storage of elastic energy by the experts during tcocking.
The anthropometric characteristics of both experts and novices were within the previously reported range, for body mass [15-17,34], body height [15-17], arm length [35], forearm length [35], and hand width [34,35], while the hand length of the novices was rather smaller [34,35]. The significantly greater body mass of the experts was expected since body mass constitutes a well-established physical advantage, distinguishing expert handball players competing in senior levels [15-17]. As with previous studies that examined the standing handball throw [15-17], we found significantly greater ball throwing velocity and better throwing accuracy in the experts than the novices. The experts in our study achieved ball throwing velocity comparable to some studies [15,16] and higher than others [17]. Their training experience was shorter and longer, respectively, than the players of Gorostiaga et al. [15] and Laffaye et al. [16], while van den Tillaar and Ettema [17] do not report the training experience of their expert participants. Our novices achieved lower ball throwing velocity than some previous studies [15,16]. Conversely, their ball throwing velocity was higher than other studies [17]. Their throwing training experience is much less than the studies of Gorostiaga et al. [15] and Laffaye et al. [16], yet more than van den Tillaar and Ettema [17]. Therefore, the level of our expert players is comparable to international level players [15-17], while our novices could be considered to have a lower skill level when compared to some of the previous studies [15,16], but of similar level with other studies [17].
The two groups demonstrate a similar temporal organization of the movement, despite their significantly different performance. This is expressed by the 30% higher ball throwing velocity and the 50% better throwing accuracy in the experts compared to the novices. The similarity of temporal organization is evidenced by the absence of significant differences between experts and novices, not only in the durations of throwing phases, but also in the timing of EMG activation. The only exception was the significantly earlier tEMG-peak of the triceps brachii by the novices. The finding of an overall similar pattern of temporal organization in both the experts and the novices appears in agreement with van den Tillaar and Ettemma [17] and Wagner et al. [34]. Although they did not include EMG recordings in their studies, van den Tillaar and Ettemma [17] and Wagner et al. [34] reported an absence of significant differences between novices and experts in the timing of body segment kinematics during the handball standing and the jump throws, respectively. The findings of the temporal pattern similarity, despite the level of expertise, suggest that the motor pattern is probably acquired early on in the learning process and maintains as a generalized pattern that governs mainly the temporal structure of the upcoming action [17]. Thus, the significantly greater throwing velocities and better throwing accuracy of the experts do not appear to associate with the timing pattern of muscle activation.
The previously published information regarding the differences of EMG activation between experts and novices during overhead throwing appears limited in the study of Gowan et al. [8], who also found a high inter-individual variability. Our findings are in agreement with the overall presence of significant EMG differences between the expert and the amateur baseball pitchers reported by Gowan et al. [8], who had included in their study the trapezius, pectoralis major, and biceps brachii muscles but not the triceps brachii muscles. Gowan et al. [8] reported significantly more activation of the pectoralis major by the experts and of the biceps brachii by the amateurs throughout the throwing movement, a selective deactivation of the biceps brachii by the experts during tacceleration, but no significant group differences for the trapezius muscle. The disagreements with our findings may be associated with the fact that Gowan et al. [8] did not record the intensity of EMG activation for all muscles in all their expert (n=7) and amateur (n=6) participants (n=4, 0, and 7 experts and n=1, 5, and 5 amateurs for the trapezius, pectoralis major, and biceps brachii muscles, respectively). Gowan et al. [8] suggested that their findings show that the selective use of certain muscles determines the quality of a pitcher. They state that the average testing pitching velocities were considerably lower than those occurred in actual game situations. The lower throwing velocity of the participants in the study of Gowan et al. [8], when compared to the ball throwing velocities during actual game situations, may also explain the disagreements between their study and ours, since the throwing performance of our expert participants was at an international level [17].
In our study, the significant group differences regarding the intensity pattern of EMG activation were evident in the EMGtrapezius, EMGpectoralis and EMGbiceps. In specific, during tcocking, both EMGtrapezius and EMGpectoralis were greater in the experts than the novices, with the group difference being reversed during tacceleration. The EMGbiceps was greater during tcocking than tacceleration but the difference was statistically significant only in the experts. Overall, our findings possibly show the better efficiency of the experts to store elastic energy during the cocking phase. The cocking phase is important for the storage of elastic energy that will be released later on during tacceleration to generate much of the power needed for rapid humeral rotation during throwing [2,3]. During tcocking, the trapezius and the pectoralis major contribute as the main stabilizing factors of the scapula [4] and the ventral [5], respectively, for the achievement of the adequate torque generation for the subsequent humeral internal rotation [5,6]. The humeral internal rotation during tacceleration is favored by an increased external rotation, during tcocking, due to the greater storage of elastic energy that will be subsequently released [2]. The disadvantageous cocking performance of the novices could be supported by the lower activation of their trapezius muscle, which possibly reflects a lower effort to stabilize the scapula [4]. The increase of the pectoralis muscle’s activation in the novices during tacceleration may also imply a disadvantageous cocking performance expressed as an effort to horizontally flex the shoulder (“pushing” the ball) rather than a “whip like motion.” The “whip like motion” sequence appears as the most effective strategy in reaching high throwing speeds [36] and an essential difference between experts and novices [17]. Finally, the presence of significantly greater EMGbiceps during tcocking rather than tacceleration in the experts may further reflect the insufficient cocking by the novices with regard to the elbow flexion.
During tcocking, the centrifugal forces generated by the shoulder external rotation try to swing the forearm's mass away from the body by extending the elbow [7]. Contraction of the biceps during this initial shoulder rotation prevents elbow extension, while the deactivation of the biceps activity shortly before the maximal external rotation allows the centrifugal force to contribute to rapid elbow extension [7]. An optimally flexed elbow during tcocking enables passive inertial forces to externally counter rotate the arm by stretching the short, parallel tendons, ligaments, and elastic components of muscles that cross the shoulder, potentially storing elastic energy in the large aggregate cross-sectional area of these structures [3]. Furthermore, when the biceps deactivate and elbow extension begins, the arm’s moment of inertia is reduced, allowing the stretched elements to recoil, releasing energy, and helping to power the extremely rapid internal rotation of the humerus [3]. Thus, the biceps brachii, rather than the triceps brachii, pattern of activation is perhaps more important for the generation of elbow extension velocity [3,7]. Both the experts and novices had the same EMGbiceps as well as EMGtriceps throughout the throwing movement. However, the greater EMGbiceps during tcocking in the experts, when compared to the novices, possibly reflects the technical advantage of the experts in effectively using the biceps to maintain the elbow flexion and facilitate the storage of elastic energy.
Handball players perform thousands of throws throughout their athletic career, which increases their risk of sustaining an overuse injury [18,19] due to the demands for rapid shoulder rotation [20,21]. The knowledge of muscle activation during a handball throw, as well its differences between experts and novices, provides a premise for optimal instruction regarding throwing mechanics, appropriate strength training, and injury prevention protocols early in the learning process [37,38]. Athletic trainers should incorporate strengthening exercises that mimic the timing of muscle activation during handball throwing. In addition, they ought to focus on the phases with the highest muscle activity with particular attention paid to the cocking phase when the storage of elastic energy is achieved. Thus, together with a typical shoulder strength training program, the strengthening of the trapezius and biceps brachii should be carefully designed due to their increased activation by the experts that may reflect their enhanced ability to store elastic energy. Furthermore, adequate biceps strengthening will allow for counteracting the centrifugal forces that extend the elbow during the cocking phase and the attainment of an optimal flexed elbow. This would facilitate the optimal stretching of the components that cross the shoulder and elastic energy storage. Finally, technical training exercises should focus on replicating the performance of a “whip like motion,” rather than a “pushing the ball motion”, since the latter is more likely to demand an unnecessary high activation of the pectoralis major muscle in the transition from the cocking into acceleration phase.

Conclusion

In conclusion, the first main finding of this study was the invariance of the temporal EMG activation pattern between the experts and the novices, which possibly suggests that the throwing motor pattern is acquired early on in the learning process. The second main finding was the experts’ significantly greater EMGtrapezius and EMGpectoralis during tcocking and the subsequent reversal of these differences during tacceleration, together with the greater EMGbiceps during tcocking than tacceleration only by the experts. The group differences regarding the intensity of EMG activation most likely highlight the insufficiency of the novices to optimally store elastic energy during tcocking. Thus, from early stages of the throwing learning process, care should be focused on the preparation of all elements, physical as well as technical, that will allow the achievement of an optimal cocking phase. The results of this study should be interpreted with regard to the particular type of handball throw examined, since the different spatio-temporal constraints of other common types of handball throw, such as the 3 step throw and the jump throw, may induce variations in the timing and intensity of EMG activation.

Acknowledgments

The authors thank Paraskevi (Evi) Nioti for her assistance in preparation of the EMG data files.

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