Explosive power generated from trunk rotation plays a key role in various sports movements, including baseball, tennis, and golf. Miyanishi et al. (19) demonstrated that the large mechanical energy produced by the trunk was transferred to the distal throwing arm segment during the final phase of throwing motion. In other sports performance, the importance of trunk function has been recognized by examining EMG activities of the trunk musculature (14,22). However, an appropriate method for the evaluation of trunk rotation strength has not been established yet.
Some studies have examined isometric and isokinetic trunk rotation strength (4,12,13,17). Kumar et al. (17) reported that females produced 65% of the torque of their male counterparts. However, there have been few reports about measuring dynamic trunk rotation power using a medicine ball. Ikeda et al. (10) found that the contributing factors of S-MBT and FS-MBT performance, which are used as a means to measure trunk rotation power or a method of training for trunk rotation power, are different in males and females.
Several studies have examined exercise using the medicine ball (16,18,24,25). Whether traditional resistance training using barbells and machines, which have a deceleration phase, is appropriate for testing and developing power is open to discussion (27). However, it is possible for athletes to perform power training and power tests by using a medicine ball without a deceleration phase at the end of the concentric movement, just like a sports movement. Although Ikeda et al. (10) reported the relationship between the performance of the side medicine-ball throw and strength including the isometric trunk rotation strength, no study to date has focused on EMG activity during dynamic trunk rotation movement using the medicine ball.
With respect to the muscle activation of the trunk, Clark et al. (3) investigated the magnitude of EMG activity during various abdominal exercises. Kumar et al. (17) found that the pattern and magnitude of EMG in performing maximal isometric axial rotation were significantly different between males and females. They suggested that, although among the males the maximal peak EMG was recorded from the ipsilateral latissimus dorsi and contralateral external oblique muscles, among females the maximal output was recorded from the contralateral pectoralis muscles. Of note, they also expected that where rotation motion occurs in a nonisometric condition, a contraction of agonists and antagonists will be essential for executing the task. Pope et al. (21) reported that the largest EMG activity during isometric trunk rotation was recorded over external obliques, but considerable antagonistic activity was also present. However, the precise activity pattern and level of the trunk muscles and their antagonists during dynamic trunk rotation is far from clear in the literature.
Muscle fiber types are considered one of the main factors affecting the activation of muscles. Häggmark and Thorstensson (7) reported that mean fiber distribution range of type 1 in all abdominal muscles for normal subjects was 55 to 58%. This value is slightly high compared with the value (48-56%) for the vastus lateralis muscle (5,15,23). This difference of muscle fiber distribution can lead to different muscle activation patterns in the case of ballistic movement.
Concerning the stretching-shortening cycle (SSC) action, it has been stressed that elastic energy stored by stretching enhances muscle performance during the concentric contraction (1,2,8). However, Tauchi et al. (26) suggested that the shoulder extensor muscles are not eccentrically contracted but passively stretched during the shoulder flexion phase in the overhead throw. Regarding the SSC of the trunk, little is known about its effect.
In dynamic trunk rotation movement such as the side medicine-ball throw, if throwers stumble as a result of the medicine-ball mass, it is impossible to throw a fast ball. Throwers need to stabilize the trunk before the release point. Thus, there is a possibility that differences in performance are induced not only by strength but by a different strategy of trunk muscle activation. The purpose of this study was to compare the electromyogram (EMG) activity of the trunk musculature during the fast side medicine-ball throw (FS-MBT) between 2 groups having different abilities and to determine the phase and magnitude characteristics of the trunk muscles.
Experimental Approach to the Problem
This study was designed to investigate the relationships between performance of FS-MBT and EMG activities of the trunk muscles. Subjects were grouped based on the side medicine-ball throw performance in the first testing session. EMG activities of the trunk and kinematic data during FS-MBT were compared between the 5 best throwers and 5 worst throwers. At the time of the experiment, fitness level was high in all subjects. The experience of resistance training was almost the same.
The subjects for this study consisted of 15 male competitive throwers and 15 male competitive baseball players. The subjects had performed resistance training at least once a week and the side medicine ball throw as training. All subjects were righthanded and injury free. The subjects' mean (±SD) age, height, and body mass were 19.4 ± 0.8 years, 1.75 ± 0.06 m, and 75.2 ± 12 kg, respectively. The study was approved by the Ethical Committee of the Osaka University of Health and Sports Sciences. The subjects were fully informed of the experimental purpose, procedures, and possible risks of the present study. The subjects signed informed consent forms prior to the study.
The testing consisted of 2 sessions separated by 2 days. During the first testing session, each subject performed 2 S-MBT using a 21.5-cm diameter, 4-kg medicine ball with left turn. Regarding the throwing technique of S-MBT and FS-MBT, S-MBT is to throw as far as possible, whereas FS-MBT is to throw as fast as possible emphasizing horizontal trunk rotation (10). Ikeda et al (10) revealed that the factors contributing to performance of S-MBT and FS-MBT were not different. Two trials were separated by approximately 5 minutes of recovery. The throw with the longest ball distance in S-MBT was selected as the result in the first testing session. Long throwers who were the top 5 and short throwers who were the worst 5 in S-MBT were decided by the results of S-MBT. These subjects' age, height, and body mass are provided in Table 1. Both groups had almost the same training experience (long throwers: 4.4 ± 0.55 years, short throwers: 4.4 ± 0.89 years) and did strength training at least once a week.
During the second session, bipolar surface electrodes were applied bilaterally to the pectoralis major, rectus abdominis, external oblique, and latissimus dorsi, with an interelectrode distance of 2 cm. A ground electrode was applied to the anterosuperior iliac spine. Electrode locations were determined according to Kumar et al. (17). The impedance between each electrode pair was measured to ensure resistance was below 5 kΩ. The EMG signals were telemetrically transmitted to a recorder (Synact Mt11, NEC, Japan) with a sampling frequency of 1 kHz. The recorded EMG signal was highpass filtered at a cutoff frequency of 10 Hz to achieve artifact cancellation. The filtered signal EMG signals were full-wave rectified and then lowpass filtered at 14 Hz.
During the second testing session, long throwers and short throwers performed FS-MBT to the left side using 2-, 4-, and 6-kg rubber medicine balls and an axial trunk rotation strength test (10,12) while recording the EMG activity. The trials of FS-MBT were recorded by 2 cameras (Sony Inc., Tokyo, Japan) to capture the subjects' movement and the flight path of the medicine ball at 60 fps (shutter speed 1000·s−1). A digitizing system (DKH Inc., Tokyo, Japan) was used to manually digitize the shoulders, the lower endpoints of the ribs, the trochanters major, and the medicine ball. The direct linear transformation method was used to obtain the 3-dimensional coordinates, and the coordinate values were filtered digitally with a Butterworth type fourth-order low-pass filter (cutoff frequency: 7 Hz). In FS-MBT, the subjects completed 3 successive trials with the same mass of medicine ball, and these trials were separated by approximately 1 minute of recovery. Another 3 successive trials with a different mass of medicine ball were performed after approximately 1 minute of recovery. The throw with the highest ball velocity in FS-MBT was selected as the result.
A 3-dimensional, 2-segment (upper trunk and lower trunk) model was used for analysis of the trunk movement in FS-MBT. The projected shoulder angle was defined as the angle between the shoulder line connecting the right shoulder and the left shoulder and x-axis, and the projected hip angle was the angle between the hip line connecting the right trochanter major and the left trochanter major and x-axis, as shown in Figure 1.
For an explanation of the results, FS-MBT was divided into 3 phases in terms of the angular velocity of the projected shoulder angle (av) between the line connecting the ball with the midpoint of the shoulder line and x-axis (Figure 1):
Early backward phase (EBP): 300-ms period preceding the peak negative (backward) av.
Late backward phase (LBP): From the peak backward angular velocity to the transition point of change to the positive (forward) av from the negative av.
Forward phase (FP): From the transition point to the release point.
The definition of projected angle θ is shown in Figure 1. The velocity and acceleration of the medicine ball were calculated by differentiating the displacement data of analysis points with respect to time. The joint angular velocity and angular acceleration were calculated by differentiating the angular displacement with respect to time. The theoretical torque exerted by the subjects is calculated as follows (Figure 1): Theoretical torque = r2·aa·m
r: Temporary rotation radius. Length between midpoint of the shoulder line and the ball.
aa: Angular acceleration (calculated from the projected angle between the line connecting the ball with the midpoint of the shoulder line and the x-axis on the horizontal plane).
m: Mass of the medicine ball.
The theoretical torque was integrated with respect to time from the transition point to the peak positive av to calculate the angular impulse.
The forces exerted on the medicine ball in the x-axis (Force-x) and y-axis direction (Force-y), respectively, were calculated as follows:
F = m·a
a: Acceleration of medicine ball.
In this study, release was defined as the last frame in which subjects were touching the medicine ball.
Rectified EMG signals for each muscle were integrated with respect to time during the EBP, LBP, and FP and divided by time for each phase. These data were normalized by maximal voluntary contraction (MVC) to compare among individuals (aEMG). The values of MVC were obtained from several positions. The highest value was selected as the MVC value for each muscle. Peak EMG during the EBP, LBP, and FP was obtained from lowpass filtered data.
In the axial trunk rotation strength test, the subjects were asked to exert maximal isometric voluntary contraction in an upright standing position with the trunk position at 0 degrees in both the left and right directions 2 times each. Force exerted by rotation was transmitted to a strain gauge. The product of the force measured and lever arm length rendered the torque (12). The rest between trials was approximately 3 minutes.
Values for each item were given as mean ± standard deviation. Two repeated-measures ANOVA were used (1 for long throwers and 1 for short throwers) to investigate the difference between groups in EMG parameters. The Student's t-test was used to compare groups. Pearson product-moment correlation coefficients (r) were used to determine the relationships between FS-MBT and IMTRT. For FS-MBT, intraclass correlation coefficients were used to assess reliability. The level of statistical significance was set at p ≤ 0.05.
The mean throwing distance and standard deviation using the 4-kg medicine ball during the first testing session were 11.05 ± 1.93 m in S-MBT. This result was largely similar to a previous study (10). The mean throwing distances of the 5 long throwers and 5 short throwers and the throwing velocities using the 2-kg, 4-kg, and 6-kg medicine balls are shown in Table 2. Intraclass correlation coefficients were used to examine the relationship among 3 trials in FS-MBT. The range of correlations was 0.892 to 0.972 for FS-MBT velocity. With regard to the relationship between body dimensions and the FS-MBT velocity, FS-MBT velocity with 6 kg in long throwers was correlated positively with height (r = 0.987, p < 0.01), whereas no significant correlations were found for FS-MBT with 2 kg (r = −0.148, p = 0.812) and FS-MBT with 4 kg (r = 0.742, p = 0.151). In short throwers, no significant correlations were found between height and FS-MBT velocity (FS-MBT with 2 kg, r = −0.195, p = 0.753, FS-MBT with 4 kg, r = 0.031, p = 0.961, FS-MBT with 6 kg, r = −0.420, p = 0.482). As for body mass, no significant correlations were observed in long throwers and short throwers.
The values of isometric maximal trunk rotation torque (IMTRT) are presented in Table 2. Although no difference was observed in IMTRT in the right turn (R) between long throwers and short throwers, the values of IMTRT in the left turn (L) in long throwers were greater than those recorded for short throwers. In this study, IMTRT (R) values were correlated positively with the value of FS-MBT 4 kg (r = 0.659, p < 0.05), except for FS-MBT 2 kg (r = 0.521, p = 0.122) and FS-MBT 6 kg (r = 0.471, p = 0.169), whereas IMTRT (L) values correlated positively with the value of FS-MBT 4 kg (r = 0.826, p < 0.01) and FS-MBT 6 kg (r = 0.737, p < 0.05), except for FS-MBT 2 kg (r = 0.599, p = 0.067) (Table 3).
A typical subject's patterns of the Force-x, Force-y, and projected shoulder and hip angle change over time are shown in Figure 2. The force exerted on the medicine ball in long throwers increased in both the x-direction and y-direction with increasing medicine-ball mass. However, the force exerted on the 6-kg medicine ball to the y-direction in short throwers was similar to the force for the 4-kg medicine ball. There was also a slight difference in Force-x. For all medicine-ball masses, the peak Force-x and Force-y of long throwers were higher (p < 0.05-0.01) than in short throwers (Figure 3). Regarding the angles, which are projected onto the horizontal plane, there was no significant difference in the maximum rotation angle of shoulder and hip in the backward phase between long throwers and short throwers. The means of the maximum rotation angle of the shoulder in long throwers were 133.8 ± 28.0 degrees, 128.1 ± 19.0 degrees, and 126.1 ± 30.6 degrees and in short throwers were 111.5 ± 21.1 degrees, 114.7 ± 21.2 degrees, and 115.3 ± 21.0 degrees for FS-MBT 2 kg, FS-MBT 4 kg, and FS-MBT 6 kg, respectively. The means of the maximum rotation angle of hip in long throwers were 58.7 ± 26.1 degrees, 50.7 ± 19.5 degrees, and 49.4 ± 27.3 degrees and in short throwers were 56.3 ± 16.4 degrees, 51.3 ± 25.2 degrees, and 47.3 ± 16.9 degrees for FS-MBT 2 kg, FS-MBT 4 kg, and FS-MBT 6 kg, respectively. There were no differences in the maximum rotation angle of the shoulder and hip. Compared with the mean value of the shoulder angle at the release point, the short thrower's angle using the 6-kg medicine ball was significantly different from that of long throwers (Figure 4). As for the angle of θ, there were no significant differences at the transition point (long throwers: 69.0 ± 10.4 degrees, 68.5 ± 8.9 degrees, and 73.4 ± 5.8 degrees; short throwers: 55.8 ± 13.1 degrees, 57.1 ± 10.8 degrees, and 64.5 ± 7.1 degrees for FS-MBT 2 kg, FS-MBT 4 kg, and FS-MBT 6 kg, respectively), but there was a significant difference at the release point between long throwers and short throwers (Table 4).
The peak angular velocity of the shoulder and the peak value of the theoretical torque and angular impulse did not differ between long throwers and short throwers (Table 4). As for the theoretical torque, no difference was observed in the average values of the temporary rotation radius during FP between long throwers (2 kg: 0.53 ± 0.05 m, 4 kg 0.52 ± 0.06 m, 6 kg 0.52 ± 0.04 m) and short throwers (2 kg: 0.51 ± 0.03 m, 4 kg 0.48 ± 0.01 m, 6 kg 0.47 ± 0.03 m) despite a significant difference in height. Regarding the angular impulse, there were significant differences between the 2 groups for FS-MBT 4 kg (long throwers: 16.47 ± 2.54, short throwers: 12.25 ± 1.39, p < 0.05) and FS-MBT 6 kg (long throwers: 21.84 ± 3.72, short throwers: 15.17 ± 3.05, p < 0.05) except for FS-MBT 2 kg (long throwers: 9.63 ± 1.76, short throwers: 7.73 ± 0.94, P = 0.075) if 1 subject is excepted from the long throwers. This subject rotated the trunk while shortening the temporary rotation radius and extended the elbow joint largely just before the release-that is, this subject performed the FS-MBT using a different throwing pattern compared with other long throwers.
The time-related change of EMG in each muscle and force exerted on the 6-kg medicine ball for a typical subject are shown in Figure 5. EMG activity in the left external oblique was very high in long throwers but that of short throwers was low. One of the features of the FS-MBT is very high activity of the left pectoralis major. Activity of the right external oblique agonist was observed during only FP in FS-MBT for both groups.
The peak EMG and the aEMG of each muscle are shown in Figure 6. As for peak EMG, the left pectoralis major and left external oblique exhibited high values in the LBP and FP for both groups. The peak values of left external oblique in EBP and right rectus abdominis in FP for long throwers were significantly higher than those for short throwers. Regarding the aEMG, the left pectoralis major and left external oblique exhibited high activation through the LBP and FP. The aEMG of the left external oblique in FP and right rectus abdominis in FP for long throwers showed significantly higher activation than for short throwers. The ANOVA for EMG magnitude value revealed no difference between medicine-ball masses.
In this study, the EMG activation of the trunk musculature and kinematics during FS-MBT were compared between long throwers and short throwers to examine the cause of performance difference. The 2 groups, which were divided by the distance of side medicine-ball throw in the first testing session, also showed significantly different values of ball velocity during FS-MBT (Table 2). The values of long throwers during FS-MBT were much higher than in a previous study (10). Regarding the relationship between FS-MBT velocity and body dimensions, FS-MBT velocity with 6 kg in long throwers was correlated positively with height (r = 0.987, p < 0.01). In short throwers, no significant correlations were found between FS-MBT velocity and body dimensions. This result showed that height may have a major influence on FS-MBT velocity when highly trained athletes throw a heavy medicine ball.
In this study the correlation coefficients between IMTRT and throwing velocity were examined. The results of the correlation analysis showed significant correlations between IMTRT (L) and FS-MBT 4 kg and FS-MBT 6 kg except for FS-MBT 2 kg. This result is approximately consistent with Ikeda et al. (10). It is interesting to note that IMTRT (R) was significantly correlated to FS-MBT 4 kg. This suggests that the trunk rotation strength to the right may play a major role in dynamic trunk rotation to the left.
Regarding the medicine-ball mass, the decrease in performance for short throwers caused by medicine-ball mass in FS-MBT was slightly greater than that of long throwers. Short thrower/long thrower ratios in terms of the average velocity in FS-MBT were 88%, 84%, and 80% at 2 kg, 4 kg, and 6 kg, respectively. The medicine-ball mass was not reflected in EMG activities of the muscles studied. Activation of the agonists and antagonists in FS-MBT was not different among the 3 medicine-ball masses (Figure 6). Therefore, it appears that EMG activity does not change significantly provided that the subjects performed FS-MBT with utmost effort.
As for the angle of shoulder and hip rotation at release (Figure 4), the peak value of angular velocity, theoretical rotation torque, and angular impulse (Table 4), there were no significant differences between long throwers and short throwers except for the angle of shoulder rotation in FS-MBT 6 kg. However, there was a significant difference in the angle of θ at the release point, although no significant differences were observed at the transition point (Table 4). With respect to angular impulse, there were significant differences between the 2 groups. Based on these results, it seems that long throwers maintained a large angle of θ to directly transmit the torque generated by trunk rotation to the medicine ball.
Regarding the force exerted on the medicine ball, the peak values of Force-x and Force-y among the long throwers were significantly higher than those of short throwers (Figure 3). In this study, it was found that Force-x, which is the throwing direction, and Force-y, which is the backward direction to the subjects, increased rapidly before release and that the velocities of the medicine ball were significantly correlated with the peak values of Force-x and Force-y (r = 0.711-0.798). Additionally, the changes in pattern of Force-x and Force-y for typical subjects broadly corresponded with the changes in patterns of EMG activity of the right external oblique and right pectoral major (Figure 5). Considering these results, it is suggested that the velocity of the medicine ball was generated mostly by the right external oblique and right pectoral major.
The examination of EMG activity in each muscle group in long throwers and short throwers revealed that the major difference between long throwers and short throwers is EMG activity of the left external oblique (Figure 5 and Figure 6B). The peak EMG of the left external oblique in EBP in long throwers during FS-MBT 2 kg and FS-MBT 4 kg was significantly different from that in short throwers (Figure 6A). From these results, it is found that long throwers activate their left external oblique to perform the left turn in FS-MBT more strongly than do short throwers. In the FP, there is a significant difference of aEMG of left external oblique between long throwers and short throwers. As for EMG activity of trunk rotation, Kumar et al. (17) suggested that where trunk rotation occurs, antagonists and stabilizers will be essential for executing the task at hand for maintaining the stability and safety of the joint. Shaffer et al. (22) reported that there were no significant differences between bilateral abdominal obliques during baseball batting for professional baseball players. In dynamic trunk rotation such as the side medicine-ball throw or baseball batting, it is possible that stabilizing the trunk is an important factor for effective use of mechanical energy, especially when using a heavy weight. Based on this result, it was revealed that long throwers activated the left external oblique for enhancing stability of the trunk. This result provides a suggestive hint for athletes' strength training programs.
A high level of EMG activity of the pectoralis major has been reported previously during the golf swing (11,20). It is reported that the role of activity of the pectoralis major is to adduct the arm along with internal rotation to provide the power for the swing (20). In FS-MBT, the pectoralis major is more active than any of the other muscles tested. Therefore, the adduction strength of the arms probably plays a major role during FS-MBT.
If we think of FS-MBT as an SSC exercise of trunk rotation, the EMG activity in the right external oblique, which is the agonist muscle for FS-MBT in left rotation, is very low during the EBP and the LBP. The aEMG of the right external oblique during the EBP were only 3%, 3%, and 5% in long throwers and 6%, 8%, and 7% in short throwers for FS-MBT 2 kg, FS-MBT 4 kg, and FS-MBT 6 kg, respectively (Figure 6). This indicates that the agonist muscle stretches passively rather than contracting eccentrically during the EBP. Although previous studies suggested that the increased EMG activities during prestretching enhance muscle stiffness (6,9), this mechanism of the lower limb does not necessarily apply to trunk rotation movement. That is, the right external oblique used in FS-MBT does not store or reuse elastic energy. This result is identical to that of a study that investigated the effect of the SSC in overhead throwing (26). Although there are various factors that lead to different muscle activation compared with leg muscles, it is likely that the slightly higher value (55-58%) of Type 1 fibers in all abdominal muscles (7) compared with leg muscles induce the different muscle activation during SSC exercise.
In conclusion, the results of this study suggest that high-level EMG activity of the left external oblique during FP is important for generating high ball velocity in FS-MBT-that is, stabilizing the trunk is an important factor for rotation movement. It is essential for athletes and coaches who perform or direct trunk rotation training to know this fact. As for the effect of SSC in trunk rotation movement, the mechanism of the lower limb does not affect the trunk rotation muscles.
Medicine-ball throw is used for testing or training in various sports activities. The purpose of this study was to compare the EMG activity of the trunk musculature for long throwers and short throwers during the side medicine-ball throw. Although previous studies have confirmed the activation of the trunk musculature in the trunk rotation, there has been no study of the differences of trunk musculature activity for different throwing abilities during the dynamic trunk rotation. The results of this study revealed that long throwers activate their left external oblique more strongly to perform the left turn in FS-MBT compared to short throwers. This muscle activation observed in long throwers is similar to that in sports performance (22). Although further investigation is required to elucidate why the same muscle activation was not observed in short throwers, it is revealed that there are differences not only in strength but also in muscle activation of the trunk.
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Keywords:© 2009 National Strength and Conditioning Association
trunk rotation; explosive power; EMG