Comparison of lower limb muscle activity during the dynamic balance test between baseball pitchers and healthy young adults

Abstract

BACKGROUND:

Dynamic balance is an essential factor for efficient pitching by baseball pitchers.

OBJECTIVE:

To compare distances reached and lower-extremity muscle activity during the star excursion balance test (SEBT) in baseball pitchers and healthy young adults.

METHODS:

Nineteen baseball pitchers (BPG) and 20 healthy adults (HAG) were recruited. Surface EMG was used to measure the activity of vastus medialis (VM), vastus lateralis (VL), tibialis anterior, and lateral gastrocnemius.

RESULTS:

The BPG exhibited greater dynamic balance than in the HAG ( $p<$ 0.05) in the posteromedial (PM) and posterolateral (PL) directions. For the PM and PL directions, significantly greater muscle activity of VM and VL was found in the BPG than in the HAG ( $p<$ 0.05).

CONCLUSION:

SEBT performance is characterized by high-level VM and VL muscle activities. Neuromuscular control of knee extensors, such as the VM and VL of pitchers, might affect the dynamic balance measured by the SEBT.

Keywords

Baseball pitcher dynamic balance star excursion balance test electromyography lower extremity

1. Introduction

Balance is divided into static and dynamic. Static balance is required to establish and maintain a stable base of support (BOS) with minimal movement [1]. Dynamic balance, on the other hand, is accompanied by some level of motion around the BOS [1]. In other words, it is the ability to maintain the center of mass within the BOS during the movement. Although the dynamic balance measurement does not accurately reflect sport participation, it mimics the demands of physical activity more closely than does a static balance measurement [2].

Table 1
Physical characteristics of subjects

	Baseball pitcher ( $n=$ 19)	Healthy adult ( $n=$ 20)	$p$ value
Age (years)	19.56 $\pm$ 1.71	20.33 $\pm$ 0.49	0.101
Height (cm)	176.56 $\pm$ 5.91	175.07 $\pm$ 5.31	0.466
Weight (kg)	75.56 $\pm$ 10.99	72 $\pm$ 8.27	0.319
Leg length (cm)	87.63 $\pm$ 4.54	87.4 $\pm$ 4.37	0.889
Limb dominance	Right $=$ 17, left $=$ 2	Right $=$ 19, left $=$ 1

Values are presented as mean $\pm$ standard deviation.

The star excursion balance test (SEBT) was used to assess the athlete’s dynamic balance ability in various sports [3, 4, 5, 6, 7]. In this test, the individual is required to maintain a BOS while standing on a single leg and using the other leg to reach as far as possible. As distance reached increases, better postural control ability and neuromuscular-control systems are required [7, 8]. Previous studies have demonstrated that the SEBT is a reliable clinical test [9, 10]. SEBT scores are affected by various factors such as ROM, muscle strength, lower limb injury, and sports participation level [3, 11, 12, 13, 14, 15]. Studies of lower-extremity muscle activity during SEBT have shown that muscle activity is different in each direction [16]. Results of studies on factors affecting the SEBT provide useful information in selecting rehabilitation programs for specific injuries, which also helps clinicians decide when to integrate the SEBT into therapeutic exercise during rehabilitation.

The baseball pitching motion is a complex sequence of movements that uses the entire body. At the same time, it is the process of effectively transferring energy to the upper extremity that throws the ball [17, 18]. The lower extremity plays a role in decelerating the trunk and upper extremity after the ball release, as well as in preserving the energy and stabilizing the trunk while throwing the ball [19]. In addition, the lower-extremity movements contribute to the ball velocity, one of the success factors of the pitcher, and controls the dynamic balance during the pitching motion [20, 21, 22]. While one is throwing the ball, the dynamic balance is a key component in controlling the entire body [21]. Furthermore, the upper-extremity injury, such as the ulnar collateral ligament tear of baseball players, is related to decreased balance ability [17]. It has been suggested that the loss of initial momentum due to fatigue of the lower extremity places greater demands on the upper extremity to produce ball velocity and thus increases the probability of upper-extremity injury [19]. Therefore, it stands to reason that the dynamic balance of the lower extremities might be one of the important factors for successful pitching.

Lower extremity muscle activity during SEBT in healthy adults has been investigated [16]. It has also been reported that the dynamic balance ability measured by SEBT varies according to the baseball competition level [4, 23]. Based on previous studies, muscle activity during SEBT varies depending on the cohort. To the best of our knowledge, no studies have examined the muscle activity during the SEBT in baseball pitchers. Therefore, the purpose of this study was to compare distances reached and lower extremity muscle activity during SEBT in baseball pitchers and healthy young adults.

2. Methods

2.1 Subjects

Nineteen male pitchers from competitive baseball team in high-school and university and twenty healthy young males participated in this study. The subjects were assigned to the baseball pitcher group (BPG) and the healthy adult group (HAG). Table 1 shows the physical characteristics of BPG and HAG. The baseball pitchers were athletes trained on each baseball team, and the healthy young adults were not involved in any exercise programs. Subjects with pain, history of surgery, or musculoskeletal disorders that might affect the study were excluded. The subjects included in this study were those who understood and agreed on the procedure and purpose of the study. Written informed consent was obtained from all subjects and their parents before involvement. This study was conducted according to the protocol approved by the Institutional Review Board of Sunmoon University (SM-201706-037-2) and in accordance with the principles of the Declaration of Helsinki.

2.2 Maximal voluntary isometric contraction

To normalize and compare the EMG data, subjects performed a maximal voluntary isometric contraction (MVIC). Prior to the MVIC measurement, the warm-up was performed for 5 minutes using a cycle ergometer. Detailed MVIC test procedures were done according to the previous EMG [24, 25, 26, 27, 28]. For the MVIC of vastus medialis (VM) and vastus lateralis (VL), subjects were assessed with 60 ${}^{\circ}$ and 90 ${}^{\circ}$ flexion for knee and hip joints, respectively, in a sitting position using an isokinetic dynamometer (HUMAC NORM, CSMI Medical Solutions, Stoughton, MA). For tibialis anterior (TA), the subject was in a supine position, and resistance was applied to dorsiflexion and inversion of the foot. In the MVIC measurement of the lateral gastrocnemius (LG), each subject was given a resistance against the plantar flexion of the foot in the prone position. Three trials were recorded for 5 seconds, with a 1-minute rest between the trials. Of the 5 seconds the MVIC lasted, an average between 2 s and 4 s was calculated and the largest of the three trials was used for data analysis.

2.3 Star excursion balance test

The SEBT was used to measure the dynamic balance ability. We used three directions to simplify the experiment: anterior (ANT), posteromedial (PM), and posterolateral (PL). The three lines consist of each direction based on the stance leg. The stance leg in the BPG was the one on the same side as the arm used to throw the ball [17]. In the HAG, the dominant leg was set as the stance leg. The dominant leg was defined as the preferred leg when kicking the ball. Before performing the SEBT, subjects watched related videos to understand the test.

In the starting position, the subjects stood in the middle of the line on both legs and placed their hands on their waist or hips. We asked the subjects to reach as far as possible along the three lines with the reaching leg, touch the floor lightly, and return to the starting position while maintaining a single-leg stance with the stance leg. During the ANT and posterior (PM and PL) directions, the toe tip and heel of the stance leg were aligned with the beginning of the line, respectively [29]. A metronome with a rate of 60 beats/min was used to ensure consistent SEBT timing for each subject. Subjects were instructed to perform an eccentric phase for 2 seconds, corresponding from the starting position to the maximum reach, followed by a concentric phase for 2 seconds to return to the initial position. Subjects completed 4 practice trials to ensure familiarization followed by 3 test trials in each direction. There was a 2-min rest between each direction [24, 29]. The distances reached measured in the SEBT were normalized as a percentage of the leg length [30], which was measured from the anterior superior iliac spine to the medial malleolus while the subject was supine [31].

2.4 Electromyography

Surface EMG (Zerowire EMG, Aurion, Italy) at a frequency of 1000 Hz was used to measure the muscle activity during the SEBT. MyoResearch software (XP Master, version 1.07.1, Noraxon, Scottsdale, AZ, USA) was used to analyze raw data from EMG. The EMG software screen and subject were video recorded and used to identify the beginning of SEBT. The skin surface was shaved and wiped with ethyl alcohol to reduce the impedance before the electrode placement. The electrodes were attached to the following four muscles of the stance leg during the SEBT: VM, VL, TA, and LG. Electrodes were placed according to the recommendations of SENIAM (Surface Electromyography for the Non-Invasive Assessment of Muscles) [32, 33]. The electrode placement for the VM was 80% of the distance from the anterior superior iliac spine to the medial border of the patella. The VL electrode was placed at 66% on the line between the anterior superior iliac spine and the lateral border of the patella. TA and LG electrodes were placed at 33% of the distance from the tip of the fibula to the tip of the medial malleolus and the distance from the head of the fibula to the heel, respectively. The electrodes stayed in the same place for the duration of the test. Raw signals were filtered using a Butterworth band-pass (20–450 Hz). The raw signals for EMG during MVIC and SEBT in each muscle were filtered using a Butterworth band-pass (20–450 Hz). Then, the filtered data were full-wave rectified and smoothed with a 10-ms window. The average EMG value obtained during three SEBTs was normalized to each muscle’s MVIC (% MVIC).

2.5 Statistical analysis

All data were reported as mean and standard deviation. We used independent t-tests to compare distances reached (% leg length) and lower extremity muscle activity (% MVIC) during SEBT between the two groups. SPSS (Statistical Package for Social Sciences) version 22.0 (IBM Corp., Armonk, NY, USA) was used for statistical analysis. The statistical significance level was set at 0.05.

3. Results

The results of the SEBT of the two groups are presented in Table 2. The BPG exhibited greater dynamic balance in the PL and PM directions than in the HAG ( $p=$ 0.011, ES $=$ 0.85 and $p=$ 0.002, ES $=$ 1.05, respectively). However, no significant difference in ANT distance reached was found between the two groups ( $p>$ 0.05).

Table 2
SEBT performance difference between baseball pitcher and healthy adult groups

SEBT	Distance reached (% leg length)		$p$ value	ES
	Baseball pitcher	Healthy adult
Anterior	76.16 $\pm$ 5.55	75.15 $\pm$ 4.77	0.546	0.20
Posteromedial	105.42 $\pm$ 5.22	99.65 $\pm$ 5.73	0.002 ${}^{**}$	1.05
Posterolateral	102.16 $\pm$ 6.01	97.25 $\pm$ 5.48	0.011 ${}^{*}$	0.85

Values are presented as mean $\pm$ standard deviation. SEBT: star excursion balance test. ${}^{*}p<$ 0.05, ${}^{**}p<$ 0.01.

Table 3

Comparison of the lower extremity muscle activity (% MVIC) during the SEBT between baseball pitcher and healthy adult groups

	Muscle activity (% MVIC)		$p$ value	ES
	Baseball pitcher	Healthy adult
SEBT anterior
Vastus medialis	87.34 $\pm$ 11.99	85.39 $\pm$ 17.88	0.693	0.13
Vastus lateralis	83.17 $\pm$ 14.94	80.51 $\pm$ 17.29	0.611	0.16
Tibialis anterior	40.84 $\pm$ 9.06	42.05 $\pm$ 11.67	0.721	0.12
Leteral gastrocnemius	35.96 $\pm$ 7.65	32.05 $\pm$ 10.78	0.201	0.42
SEBT posteromedial
Vastus medialis	80.98 $\pm$ 13.38	70.44 $\pm$ 15.77	0.031 ${}^{*}$	0.72
Vastus lateralis	76.88 $\pm$ 12.57	67.29 $\pm$ 14.77	0.036 ${}^{*}$	0.70
Tibialis anterior	42.14 $\pm$ 8.31	43.77 $\pm$ 11.77	0.621	0.16
Leteral gastrocnemius	32.91 $\pm$ 10.83	28.39 $\pm$ 10.21	0.188	0.43
SEBT posterolateral
Vastus medialis	77.85 $\pm$ 11.86	68.11 $\pm$ 16.25	0.039 ${}^{*}$	0.68
Vastus lateralis	71.88 $\pm$ 11.67	63.24 $\pm$ 13.87	0.042 ${}^{*}$	0.67
Tibialis anterior	48.86 $\pm$ 9.54	49.78 $\pm$ 11.43	0.787	0.09
Leteral gastrocnemius	29.65 $\pm$ 11.36	26.23 $\pm$ 11.88	0.363	0.29

Values are presented as mean $\pm$ standard deviation. SEBT: star excursion balance test. ${}^{*}p<$ 0.05.

The lower-extremity muscle activity (VM, VL, TA, and LG) during the SEBT is shown in Table 3. During the PM direction of SEBT, the VM and VL muscle activity of BPG was significantly higher compared to the HAG ( $p=$ 0.031, ES $=$ 0.72 and $p=$ 0.036, ES $=$ 0.70, respectively). Also, the muscle activity of VM and VL during the PL direction was significantly higher in BPG than in HAG ( $p=$ 0.039, ES $=$ 0.68 and $p=$ 0.042, ES $=$ 0.67, respectively). However, no significant differences were observed between the two groups in TA and LG muscle activity during PM and PL directions ( $p>$ 0.05). During the ANT direction, no significant difference in lower extremity muscle activity was found between the two groups ( $p>$ 0.05).

4. Discussion

The main results indicate that the BPG had a higher dynamic balance in the PM and PL directions during the SEBT than did the HAG. In addition, muscle activity of knee extensors (VM and VL) while reaching in the PM and PL directions of SEBT showed a significant difference between the two groups.

SEBT was reported to differ based on the competition level of baseball players. Professional baseball players had higher PM and PL distances than did collegiate and high-school players, whereas high-school players had greater ANT distances [4]. In addition, previous studies have shown that balance or neuromuscular training significantly improved posterior, PM, and PL distances [5, 7, 34]. It has been also reported that patients with ACL injury had shorter distances in the PM and PL than did the healthy control group [5, 35]. Muscle strength and kinematic deficits (hip joint sagittal- frontal-, and transverse plane and knee joint sagittal plane) of the lower extremity were found to affect these directions [5, 35]. The results of these previous studies indicate that different factors affect each direction of the SEBT. In the ANT direction, it is primarily a sagittal plane movement, whereas the PM and PL are movements in the complex plane that include medial-lateral movements as well as anterior-posterior movements [12]. According to previous studies investigating factors affecting the SEBT, the ANT direction is affected by a reduced dorsiflexion range of motion, injury history, and plantar cutaneous sensation, whereas the PM and PL directions are influenced by hip extensor, quadriceps, and ankle evertor strength [4, 12, 34, 35, 36, 37]. The results of our study also confirmed that the BPG participating in baseball training exhibited better dynamic balance in the PL and PM directions of SEBT than did the HAG who did not participate in any training. However, there was no significant difference between the two groups in the ANT. Thus, our results indirectly support the evidence that the factors affecting each direction of the SEBT are different.

The pitchers who participated in this study were expected to have better dynamic balance because they repeatedly participated in the training for pitching. The ability to detect joint-position changes and focus on biomechanical signals, such as joint acceleration, has been suggested to be improved through sports participation [3]. In addition, training experiences improve neuromuscular coordination and strength, which can contribute to improved balance [3]. The stance leg of the BPG in this study was defined as being on the same side as the arm that throws the ball, which corresponds to the pivot leg. During the pitch, VM activity of the pivot leg at the phase of maximum stride leg knee height to stride foot contact (SFC) was reported to be high (68% of their respective MVIC) [19]. Furthermore, at the phase defined as from SFC to ball release, lower-extremity muscle activity was found to exceed 100% [19]. Therefore, they concluded that the muscles of the lower extremities were highly activated during the pitch [19]. In addition, high ball-velocity pitchers produce greater momentum in the lower extremities, including knee extensions, and consequently the hip and knee functions play an important role for faster ball velocity [20, 22]. To conclude, baseball pitchers showed better dynamic balance in the PM and PL directions of the SEBT, perhaps because of the increased muscle activity of knee extensors by participation in pitching training.

In our study, although there was no comprehensive investigation of factors contributing to SEBT distances in the BPG and the HAG, we compared lower-extremity muscle activity during each direction of the SEBT in the two groups. Doing a successful SEBT requires an increased knee flexion angle of the stance leg [14]. Knee extensors must contract eccentrically to control flexion of the knee joint and consequently require high levels of muscle activity. Indeed, several studies have highlighted the importance of knee extensors such as VM and VL during the SEBT performance [16, 25]. The trunk tends to flex to reach in the PM and PL directions, which requires the eccentric activity of the hamstrings [16]. Lockie et al. [15] divided subjects into better and lesser groups based on the SEBT distances and compared the muscle strength of the knee between the two groups. Their findings indicate that the better group had greater knee extensors, but the hamstrings showed no difference. They suggested the players could maintain stability by means of the co-contraction of the extensors and flexors of the knee during the SEBT and consequently reported that the strong muscle strength of the knee extensors could contribute to the difference in posterior directions (P, PL, PM) [15]. In our study, during PM and PL directions, the BPG had significantly higher muscle activity in VM and VL than did the HAG. Unfortunately, the hamstring activity was not measured in this study.

There was no significant difference between the two groups in the activity of VM and VL while doing the ANT direction. Interestingly, however, higher levels of activation in the ANT than in the PM and PL were observed in both groups. A study examining the lower-extremity muscle activity during the SEBT showed that the VM had the highest activity during the ANT [16]. The VM activity in the ANT obtained in this study was similar to the high level of muscle activity in the previous study. Thus, reaching in the ANT requires high levels of muscle activity of knee extensors. However, there was no difference between the BPG and the HAG in distance reached as well as muscle activity in this direction. The distance reached in each direction of the SEBT depends on different factors, and the ANT is well known to be influenced by weight-bearing dorsiflexion [13, 38, 39]. Unfortunately, in our study, these factors were not measured. Further research is needed to investigate other factors that may affect the ANT direction in baseball pitchers.

The previous study has reported a high levels of activation in the muscles around the ankle during the pitching motion in the university baseball team [19]. Therefore, we expected a significant difference in the activation level of the muscles around the ankle between the BPG and the HAG during the SEBT in our study. However, there was no significant difference in TA and LG muscle activity between the two groups during any directions of the SEBT. Kageyama et al. [22] reported that high ball-velocity pitchers have significantly greater hip abduction, hip internal rotation, and knee extension joint torques than do low ball-velocity pitchers. It has also been suggested that adolescent baseball pitchers cannot generate the torques at the hip and knee joint of the pivot leg than collegiate baseball pitchers, which affects transfer the energy to the arms and trunk [40]. However, their results were not significantly different between the groups in the torque generation of the ankle joint [22, 40]. Thus, although the role of the ankle joint in the pitching motion is important, the contribution of hip and knee functions may be greater in high competition level or high ball-velocity pitchers. For this reason, there may not be a significant difference between the two groups in TA and LG activity. Although only the activation of TA and LG was measured in our study, previous studies suggested the importance of neuromuscular control of ankle invertors and evertors in the dynamic balance measured by the SEBT [12, 24]. Therefore, further research is needed to investigate the effects of these muscles.

A few limitations of our study should be taken into consideration. First, the pitchers involved in this study were only high-school and college pitchers. Lower-extremity muscle activity measurement during the SEBT in amateur or professional athletes may be different from the results of this study. Second, the SEBT consists of two phases, namely, down (eccentric) and up (concentric) phases [25]. However, in our study, the overall muscle activity during the SEBT was measured without distinguishing the phases. Third, hip-joint muscles contribute not only to pitching motion but also to the SEBT [14, 22, 26, 37, 38, 41]. However, these factors have not been investigated in this study. Finally, the subcutaneous fat may have affected the EMG signal. It is possible that the BPG in this study had a higher amplitude measured than the HAG due to the training state. Future research should investigate the factors that influence the dynamic balance and develop training to improve the balance ability of baseball pitchers.

5. Conclusion

Our results suggest that BPG had a better dynamic balance in the PM and PL direction of the SEBT than HAG. They also indicate that reaching in these directions is characterized by high-level muscle activities of VM and VL. Therefore, neuromuscular control of knee extensors may affect their dynamic balance ability measured by the SEBT. Moreover, it is cosidered that our results could help practitioners establish the reference value of the distance reached and lower extremity muscle activity during SEBT in baseball pitchers.

Author contributions

CONCEPTION: Jeongwoo Jeon.

PERFORMANCE OF WORK: Jeongwoo Jeon and Jiyeon Lee.

INTERPRETATION OR ANALYSIS OF DATA: Jeongwoo Jeon, Jiyeon Lee, Jaeho Yu and Jinseop Kim.

PREPARATION OF THE MANUSCRIPT: Jeongwoo Jeon and Dongyeop Lee.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Dongyeop Lee, Jiheon Hong, Jaeho Yu and Jinseop Kim.

SUPERVISION: Dongyeop Lee and Jiheon Hong.

Ethical considerations

This study was conducted according to the protocol approved on August 2, 2017, by the Institutional Review Board of Sunmoon University (SM-201706-037-2). Written informed consent was obtained from all subjects and their parents before participation.

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

The authors declare no conflict of interest.

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Comparison of lower limb muscle activity during the dynamic balance test between baseball pitchers and healthy young adults

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

Table 1 Physical characteristics of subjects

2.1 Subjects

2.2 Maximal voluntary isometric contraction

2.3 Star excursion balance test

2.4 Electromyography

2.5 Statistical analysis

3. Results

Table 2 SEBT performance difference between baseball pitcher and healthy adult groups

5. Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Physical characteristics of subjects

Table 2
SEBT performance difference between baseball pitcher and healthy adult groups