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Journal of Back and Musculoskeletal Rehabilitation 28 (2015) 191–196 DOI 10.3233/BMR-140569 IOS Press
Intensive unilateral core training improves trunk stability without preference for trunk left or right rotation Yushin Kim, Jungjin Kim and BumChul Yoon∗ Department of Physical Therapy, College of Health Sciences, Korea University, Seoul, Korea
Abstract. BACKGROUND AND OBJECTIVES: It is possible to increase multi-directional trunk stability using co-activation. However, it is unclear whether there is a preference for left or right trunk rotation after intensive unilateral stability training. The aim of this study was to examine the directional preference in trunk rotational stability after unilateral core training. MATERIAL AND METHOD: This study was conducted on 16 female basketball players. For eight weeks, eight participants performed unilateral core training that focused on one side of the trunk. The remaining eight participants were not provided any additional training. To determine rotational trunk stability, all participants were requested to maintain an upright sitting posture against sudden, external, left or right rotational perturbations of the trunk. Angular displacement of the trunk was measured using a motion analyzer. RESULTS: At the end of the training period, the angular displacement in response to the perturbation was reduced for both rotational directions (left: −26%, right: −24%) in the trained group (p < 0.05). CONCLUSION: This study showed that trunk stability improved without particular directional preference in response to unilateral core training. This result adds to our understanding of the nature of trunk stability and multi-directional improvement. LEVEL OF EVIDENCE: Intervention study, Level 1b. Keywords: Postural control, trunk stability, core training, direction, perturbation
1. Introduction A wide range of sports and leisure activities demand trunk stability. Trunk stability is influenced by the stiffness of the trunk region that is regulated by co-contraction of the trunk muscles around the spine, rib cage and pelvis. In particular, the co-contraction of the abdominal muscles increases trunk stability, which in turn increases the overall trunk stiffness [1,2]. One interesting aspect of the abdominal muscles is that a contraction on one side can affect the contralateral side because the abdominal muscles on both sides are con∗ Corresponding author: BumChul Yoon, Department of Physical Therapy, College of Health Sciences, Korea University, Jeongneung 3-dong, Sungbuk-gu, Seoul 136-703, Korea. Tel.: +82 29402833; Fax: +82 29402839; E-mail: [email protected].
nected through passive structures [3,4]. In addition, previous research found that a symmetric activation of deep abdominal muscles occurred during asymmetric lifting and sudden perturbation tasks [5,6]. This finding suggested non-directional preference for either left or right side trunk stability. In addition, trunk stiffness during multidirectional trunk perturbations was shown to be symmetrical for the left and right side bandings [7]. Previous studies have demonstrated the effects of core training programs for trunk stability [8,9]. Carpes et al. found that a trunk strength and stability program is effective for improving body stability in women with back pain related to improved standing balance [10]. Another study concluded that core stability training is effective in improving lower trunk endurance performance [11]. These findings suggest that intensive train-
c 2015 – IOS Press and the authors. All rights reserved ISSN 1053-8127/15/$35.00
192
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
ing for trunk stability may be effective in improving trunk stability as well as the development of the muscular system. However, it has not been determined whether intensive training on one side of the body causes a trunk stability directional preference for left or right rotation. The purpose of this study was to examine the directional preference in trunk rotational stability after unilateral core training. Because previous research reported symmetrical bilateral trunk muscle activation and stiffness during left and right side trunk perturbations [7], we hypothesized that unilateral core training would improve trunk stability during both left and right rotations without directional preference. To measure trunk stability, we measured trunk angular displacement in a left or right rotational direction in response to unexpected sudden external perturbations of the trunk while participants were in a sitting position.
2. Materials and methods 2.1. Participants Right-handed, middle school female basketball players with 1–3 years athletic experience participated in this study. All participants were from the same school and followed the same training program. The study was performed during the vacation period to avoid the effects of the match schedule. Players with a history of spinal surgery or those who had difficulty maintaining balance in a standing or sitting position were excluded. The participants were randomly assigned into a training or control group. The details of the participants are provided in Table 1. Based on angular displacement data from a previous study [12], the number of participants was decided using a power analysis program (G*Power 3, Institut für Experimentelle Psychologie, Germany), with the following as factors: effect size = 0.769, alpha error = 0.05, beta effort = 0.95, dropout rate = 0.1. The participants were minors, and therefore consent was obtained from their parents or guardians and coaches as well as the players themselves. Permission to conduct the study was obtained from the Korea University Institutional Review Board. 2.2. Measurements To assess trunk stabilization, an unexpected trunk perturbation was applied in a left or right rotational
direction. The apparatus used to produce this perturbation is shown in Fig. 1A. The participants were instructed to close their eyes, cross their arms on their chest, and sit on a bench tilted at 45◦ with their knees placed on a 30◦ inclined wedge. A Velcro-strap was fixed proximal to the femur to minimize movement of the lower extremities. A load of 8 kg was connected to the apparatus using a cable that passed through two electromagnets. Before the perturbation, the subjects were asked to maintain an upright sitting posture that required an effort to counteract the weight of the load. The current flow of the electromagnets was operated by a switch. When the current was turned off the two adhered electromagnets would separate, resulting in the apparatus moving the participant’s trunk in a left or right rotational direction. The load would suddenly be released without any warning. The direction of the perturbation was in the left or right rotational direction. To determine the direction, the load was connected either in the middle, left, or right side of the trunk at the level of T9–10 (Fig. 1B). Assessments were performed before and after the training periods. Three trials were conducted, and the resting time between trials was 10 to 15 s. 2.3. Training The unilateral core training group had a side bridge and quadruped exercise added to their regular training program while the control group continued with the regular program. The training was performed on the left side of the trunk, using a Swiss ball. The height of the Swiss ball was adjusted to the height of each participant’s knee. The side bridge and quadruped exercises were used, based on the evidence that these exercises can unilaterally activate the trunk muscles. Detailed explanations of the exercises were presented in a previous study [12]. For the quadruped exercise, the participants were initially asked to position themselves in a four-point stance, characterized by 90◦ flexion of the shoulders, hips, and knees. The left leg and contralateral arm were then straightened until both were parallel. For the side bride exercise, participants lay on the left side with the legs straight and elevated on the Swiss ball. The participants then elevated their pelvis until their whole body was straight and 45◦ to the floor. Each exercise consisted of 3 sets of 15 repetitions. The resting time between the sets was 1 min. The training lasted for 30 min and was performed twice a week for 8 weeks.
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Table 1 Demographic details of the participants Gender Age (yrs.) Height (cm) Weight (kg)
Unilateral core training group (n = 8) Female 14.75 ± 0.31 162.95 ± 2.68 55.11 ± 3.17
Control group (n = 8) Female 15.13 ± 0.44 161.63 ± 1.21 60.50 ± 2.63
Note: Values are expressed as mean ± standard deviation.
B
A
Fig. 1. (A) Equipment for sudden-release trunk perturbation. The subjects performed an isometric trunk contraction against a load of 8 kg, while seated on a bench. When the load was subsequently released, the movements of the markers were traced and quantified with a 3D motion capture system. (B) The direction of perturbation. The external load was attached to the upper trunk at an angle of 30 degrees from the midline. When the load was suddenly released, the perturbations were induced in the left or right direction, resulting in left or right trunk rotation.
2.4. Data analysis When the sudden trunk perturbation was induced, the angular displacement of the trunk was recorded using a three-dimensional motion analysis system (Motion Analysis Corporation System, Santa Rosa, CA, USA). Six motion analysis cameras were used that sampled at a rate of 60 Hz. The markers for capturing the motion were attached on both acromions, T1 spinous process, both anterior superior iliac spines, and S2 spinous process. To analyze the raw data, CORTEX 1.0 software (Motion Analysis Corporation, Santa Rosa, CA, USA) was used. The trunk segment was defined by the connection of two points: the center of both acromions and the spinous process of T1, and the center of both anterior superior iliac spines and the spinous process of S2 (Fig. 2). The reference position of angular displacement was determined as that immediately before perturbation. The angular displacement was calculated in the sagittal (S), frontal (F), and horizontal (H) planes at the point of maximal trunk displacement in response to the perturbation. The representative values for each left or right rotational angular displacement was calculated by root mean square as follows: Perturbed angle = (S − S0 )2 + (F − F0 )2 + (H − H0 )2 ,
where S0 , F0 , and H0 were the initial values before inducing trunk perturbation. 2.5. Statistics The angular displacements are presented as mean and standard deviation. The data were analyzed using the SPSS 12.0 (SPSS Inc., Chicago, IL, USA). To detect a change in angular displacements before and after the unilateral core training in each group, a 2-way repeated measures ANOVA was performed using the following factors: training (pre- and post-training) × perturbed directions (left and right rotations). Post hoc analysis was performed to compare before and after angular displacement in each direction. This involved a paired t-test with Bonferroni’s correction, set at p < 0.0167. The level of statistical significance was set at p < 0.05, except for the post hoc analysis.
3. Results The 2-way repeated measures ANOVA found significant differences in angular displacement before and after the training in each of the two perturbed directions (p < 0.01). The post-hoc analysis showed that unilateral core training decreased the angular displace-
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Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
A Lateral View
B Superior View Fig. 2. Illustration of sensors attached to the trunk segments to obtain the trunk angular displacement data in 3D plane. The segment was defined by connection of the following points: the center of both acromions (L. and R. Shoulder) and spinous process at C7 and the center of both anterior superior iliac spine (L. and R. ASIS) and spinous process at S2. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/BMR-140569)
ment for the left and right rotations (p < 0.0167). After the training, the angular displacement for the left rotational perturbation was significantly reduced from 20.89◦ ± 4.90 to 15.38◦ ± 2.00, while the right rotational perturbation was significantly reduced from 20.54◦ ± 4.65 to 15.59◦ ± 2.02. When the control group was compared before and after 8 weeks of training, there were no statistically significant changes in the left or right angular displacements (p > 0.05). The angular displacement in the control group changed from 20.28◦ ± 4.22 to 18.70◦ ± 3.31 for the left rotational perturbation, and from 19.73◦ ± 4.13 to 18.05◦ ± 3.49 for the right rotational perturbation. Figure 3 shows the average change in the angular displacements following the two perturbations for both groups.
4. Discussion This study examined trunk angular displacements in response to sudden trunk perturbations before and after
unilateral core training. The results demonstrated that unilateral core training improved trunk stability in both the right and left rotational directions, as indicated by decreased angular displacement of the trunk. In addition, there were no significant changes in the control group, demonstrating the absence of a learning effect. This result shows that trunk stability was improved without a particular directional preference. Based on our results, we suggest that unilateral core training can bilaterally influence the neural network system. To maintain an upright sitting position against the perturbation, the participants had to resist left or right trunk rotation as soon as the perturbation was applied. In the case of the left rotational perturbation, the participants had to rotate their trunk to the right. This movement would require the activation of the left spinal erector, left external oblique, and right internal oblique muscles [13]. Based on electromyographic evidence, the quadruped and side bridge exercises were used to facilitate right trunk rotation. A previous electromyography study demonstrated that the quadruped
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
195
Direction of Trunk Perturbation
A
B
Fig. 3. Angular displacements of the unilateral core training group (A) and the control group (B) before (pre-training, black bar) and after training (post-training, gray bar). All values are presented as the mean and standard error. ∗ significant difference between the pre- and post-training values (p < 0.05).
exercise activates the left spinal erector, the left external oblique, and the right internal oblique muscles, and that overall, the side bridge exercise activates the left trunk muscles [14]. Therefore, we assume that the training program in the present study would enhance the ability to resist right rotational perturbations. In response to sudden perturbation, trunk stiffness is increased through a reflex contraction initiated by sensory input involving the muscle spindle, tendon, joint and ligament and is not related to voluntary muscle contraction initiated in the cerebral cortex [15, 16]. However, because the trunk is a multi-joint system, the central nervous system is limited in terms of the reflexes involved in regulating movement. Therefore, the central nervous system utilizes virtual trajectory of equilibrium, integrating the reflexes at a spinal level [17]. For the integration of the spinal reflex, we suggest that the bilateral net effects are regulated at a supraspinal level. Although there is no direct evidence, previous studies demonstrated that communication between muscles on both sides of the trunk occurs through a neural network [7,18,19]. Therefore, similarly to previous research, our findings suggest that trunk stiffness is developed bilaterally, despite intensive unilateral training. Intra-abdominal pressure is a factor that would increase trunk stability during rotational perturbations. There is a possibility that the unilateral training activated the internal oblique muscles bilaterally [14], thereby increasing intra-abdominal pressure. However, sudden trunk perturbation produces a low level coactivation of the abdominal and back muscles [20].
Therefore, we suggest that in the current study, intraabdominal pressure had a minimal effect on changes in the angular displacements of the trunk. The results of this study revealed a clinical message linked with physical education and training methods for trunk stability. Intensive unilateral training would increase overall trunk stability against sudden perturbations. In line with our results, a previous study suggested that integration exercises that elicit greater both proximal and distal trunk muscle activity would be more effective in enhancing stability and maintaining mobility [21], and another study indicated that many different trunk muscles work harmoniously to stabilize the trunk as one unit during a static action of the trunk [22]. Therefore, unilateral training would be a suitable core stabilization method of the intervention as well. One of the limitations of the study was the relatively small sample size that reduced the power of the study. In addition, the study focused on trunk rotations because of the possible effect of fatigue on the results. Further research is required to identify the effect of unilateral core training on multi-directional trunk stability. In addition future research should also examine the effects of gender and age on directional preferences relating to trunk stability. Despite the limitations, we found that the unilateral core training improved trunk stability without directional preferences. This result contributes to the understanding of the characteristics of trunk stability.
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5. Conclusions In this study, we examined the effect of unilateral core training on the direction of rotational preferences relating to trunk stability. The results demonstrated that angular displacements decrease in response to unilateral training regardless of the direction of trunk rotation. This implies that intensive unilateral training improves both left and right rotational stability, possibly through a bilaterally connected neural network.
[8]
[9]
[10]
[11]
Acknowledgment
[12]
This study received financial support from a grant by Korea University.
[13]
Conflict of interest None of the authors has any conflict of interest to report.
[14]
[15]
[16]
References [1]
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McGill SM. Low back stability: from formal description to issues for performance and rehabilitation. Exerc Sport Sci Rev. 2001; 29(1): 26-31. Lee PJ, Rogers EL, Granata KP. Active trunk stiffness increases with co-contraction. J Electromyogr Kinesiol. 2006; 16(1): 51-57. Tesh KM, Dunn JS, Evans JH. The abdominal muscles and vertebral stability. Spine. 1987; 12(5): 501-508. Gardner-Morse MG, Stokes IAF. The effects of abdominal muscle coactivation on lumbar spine stability. Spine. 1998; 23(1): 86-91. Danneels LA, Vanderstraeten GG, Cambier DC, Witvrouw EE, Stevens VK, De Cuyper HJ. A functional subdivision of hip, abdominal, and back muscles during asymmetric lifting. Spine. 2001; 26(6): E114-E121. Beith ID, Harrison PJ. Stretch reflexes in human abdominal muscles. Exp Brain Res. 2004; 159(2): 206-213. Gardner-Morse MG, Stokes IAF. Trunk stiffness increases with steady-state effort. J Biomech. 2001; 34(4): 457-463.
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Sekendiz B, Cug M, Korkusuz F. Effects of swiss-ball core strength training on strength, endurance, flexibility, and balance in sedentary women. J Strength Cond Res. 2010; 24(11): 3032-3040. Jamison ST, McNeilan RJ, Young GS, Givens DL, Best TM, Chaudhari AM. Randomized controlled trial of the effects of a trunk stabilization program on trunk control and knee loading. Med Sci Sports Exerc. 2012; 44(10): 1924-1934. Carpes FP, Reinehr FB, Mota CB. Effects of a program for trunk strength and stability on pain, low back and pelvis kinematics, and body balance: A pilot study. J Bodyw Mov Ther. 2008; 12(1): 22-30. Aggarwal A, Kumar S, Kumar D. Effect of core stabilization training on the lower back endurance in recreationally active individuals. J Muscoskel Res. 2010; 13(4): 167-176. Kim Y, Son J, Yoon B. Intensive unilateral neuromuscular training on non-dominant side of low back improves balanced muscle response and spinal stability. Eur J Appl Physiol. 2012; 113(4): 997-1004. Toren A, Oberg K. Maximum isometric trunk muscle strength and activity at trunk axial rotation during sitting. Appl Ergon. 1999; 30(6): 515-525. Garcia-Vaquero MP, Moreside JM, Brontons-Gil E, PecoGonzalez N, Vera-Garcia FJ. Trunk muscle activation during stabilization exercises with single and double leg support. J Electromyogr Kinesiol. 2012; 22(3): 398-406. Ebenbichler GR, Oddsson LIE, Kollmitzer J, Erim Z. Sensory-motor control of the lower back: implications for rehabilitation. Med Sci Sports Exerc. 2001; 33(11): 1889-1898. Brown SHM, McGill SA. The intrinsic stiffness of the in vivo lumbar spine in response to quick releases: Implications for reflexive requirements. J Electromyogr Kinesiol. 2009; 19(5): 727-736. Hogan N. The mechanics of multi-joint posture and movement control. Biol Cybern. 1985; 52(5): 315-331. Thrasher TA, Sin VW, Masani K, Vette AH, Craven BC, Popovic MR. Responses of the trunk to multidirectional perturbations during unsupported sitting in normal adults. J Appl Biomech. 2010; 26(3): 332-340. Bjerkefors A, Carpenter MG, Thorstensson A. Dynamic trunk stability is improved in paraplegics following kayak ergometer training. Scand J Med Sci Sports. 2007; 17(6): 672-679. Radebold A, Cholewicki J, Panjabi MM, Patel TC. Muscle response pattern to sudden trunk loading in healthy individuals and in patients with chronic low back pain. Spine. 2000; 25(8): 947-954. Gottschall JS, Mills J, Hastings B. Integraion core exercises elicit greater muscle activation than isolation exercises. J Strength Cond Res. 2013; 27(3): 590-596. Colston M, Taylor T, Minnick A. Abdominal muscle training and core stabilization: The past, present, and future. Athlet Ther Today. 2005; 10(4): 6-12.
Journal of Back and Musculoskeletal Rehabilitation 28 (2015) 191–196 DOI 10.3233/BMR-140569 IOS Press
Intensive unilateral core training improves trunk stability without preference for trunk left or right rotation Yushin Kim, Jungjin Kim and BumChul Yoon∗ Department of Physical Therapy, College of Health Sciences, Korea University, Seoul, Korea
Abstract. BACKGROUND AND OBJECTIVES: It is possible to increase multi-directional trunk stability using co-activation. However, it is unclear whether there is a preference for left or right trunk rotation after intensive unilateral stability training. The aim of this study was to examine the directional preference in trunk rotational stability after unilateral core training. MATERIAL AND METHOD: This study was conducted on 16 female basketball players. For eight weeks, eight participants performed unilateral core training that focused on one side of the trunk. The remaining eight participants were not provided any additional training. To determine rotational trunk stability, all participants were requested to maintain an upright sitting posture against sudden, external, left or right rotational perturbations of the trunk. Angular displacement of the trunk was measured using a motion analyzer. RESULTS: At the end of the training period, the angular displacement in response to the perturbation was reduced for both rotational directions (left: −26%, right: −24%) in the trained group (p < 0.05). CONCLUSION: This study showed that trunk stability improved without particular directional preference in response to unilateral core training. This result adds to our understanding of the nature of trunk stability and multi-directional improvement. LEVEL OF EVIDENCE: Intervention study, Level 1b. Keywords: Postural control, trunk stability, core training, direction, perturbation
1. Introduction A wide range of sports and leisure activities demand trunk stability. Trunk stability is influenced by the stiffness of the trunk region that is regulated by co-contraction of the trunk muscles around the spine, rib cage and pelvis. In particular, the co-contraction of the abdominal muscles increases trunk stability, which in turn increases the overall trunk stiffness [1,2]. One interesting aspect of the abdominal muscles is that a contraction on one side can affect the contralateral side because the abdominal muscles on both sides are con∗ Corresponding author: BumChul Yoon, Department of Physical Therapy, College of Health Sciences, Korea University, Jeongneung 3-dong, Sungbuk-gu, Seoul 136-703, Korea. Tel.: +82 29402833; Fax: +82 29402839; E-mail: [email protected].
nected through passive structures [3,4]. In addition, previous research found that a symmetric activation of deep abdominal muscles occurred during asymmetric lifting and sudden perturbation tasks [5,6]. This finding suggested non-directional preference for either left or right side trunk stability. In addition, trunk stiffness during multidirectional trunk perturbations was shown to be symmetrical for the left and right side bandings [7]. Previous studies have demonstrated the effects of core training programs for trunk stability [8,9]. Carpes et al. found that a trunk strength and stability program is effective for improving body stability in women with back pain related to improved standing balance [10]. Another study concluded that core stability training is effective in improving lower trunk endurance performance [11]. These findings suggest that intensive train-
c 2015 – IOS Press and the authors. All rights reserved ISSN 1053-8127/15/$35.00
192
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
ing for trunk stability may be effective in improving trunk stability as well as the development of the muscular system. However, it has not been determined whether intensive training on one side of the body causes a trunk stability directional preference for left or right rotation. The purpose of this study was to examine the directional preference in trunk rotational stability after unilateral core training. Because previous research reported symmetrical bilateral trunk muscle activation and stiffness during left and right side trunk perturbations [7], we hypothesized that unilateral core training would improve trunk stability during both left and right rotations without directional preference. To measure trunk stability, we measured trunk angular displacement in a left or right rotational direction in response to unexpected sudden external perturbations of the trunk while participants were in a sitting position.
2. Materials and methods 2.1. Participants Right-handed, middle school female basketball players with 1–3 years athletic experience participated in this study. All participants were from the same school and followed the same training program. The study was performed during the vacation period to avoid the effects of the match schedule. Players with a history of spinal surgery or those who had difficulty maintaining balance in a standing or sitting position were excluded. The participants were randomly assigned into a training or control group. The details of the participants are provided in Table 1. Based on angular displacement data from a previous study [12], the number of participants was decided using a power analysis program (G*Power 3, Institut für Experimentelle Psychologie, Germany), with the following as factors: effect size = 0.769, alpha error = 0.05, beta effort = 0.95, dropout rate = 0.1. The participants were minors, and therefore consent was obtained from their parents or guardians and coaches as well as the players themselves. Permission to conduct the study was obtained from the Korea University Institutional Review Board. 2.2. Measurements To assess trunk stabilization, an unexpected trunk perturbation was applied in a left or right rotational
direction. The apparatus used to produce this perturbation is shown in Fig. 1A. The participants were instructed to close their eyes, cross their arms on their chest, and sit on a bench tilted at 45◦ with their knees placed on a 30◦ inclined wedge. A Velcro-strap was fixed proximal to the femur to minimize movement of the lower extremities. A load of 8 kg was connected to the apparatus using a cable that passed through two electromagnets. Before the perturbation, the subjects were asked to maintain an upright sitting posture that required an effort to counteract the weight of the load. The current flow of the electromagnets was operated by a switch. When the current was turned off the two adhered electromagnets would separate, resulting in the apparatus moving the participant’s trunk in a left or right rotational direction. The load would suddenly be released without any warning. The direction of the perturbation was in the left or right rotational direction. To determine the direction, the load was connected either in the middle, left, or right side of the trunk at the level of T9–10 (Fig. 1B). Assessments were performed before and after the training periods. Three trials were conducted, and the resting time between trials was 10 to 15 s. 2.3. Training The unilateral core training group had a side bridge and quadruped exercise added to their regular training program while the control group continued with the regular program. The training was performed on the left side of the trunk, using a Swiss ball. The height of the Swiss ball was adjusted to the height of each participant’s knee. The side bridge and quadruped exercises were used, based on the evidence that these exercises can unilaterally activate the trunk muscles. Detailed explanations of the exercises were presented in a previous study [12]. For the quadruped exercise, the participants were initially asked to position themselves in a four-point stance, characterized by 90◦ flexion of the shoulders, hips, and knees. The left leg and contralateral arm were then straightened until both were parallel. For the side bride exercise, participants lay on the left side with the legs straight and elevated on the Swiss ball. The participants then elevated their pelvis until their whole body was straight and 45◦ to the floor. Each exercise consisted of 3 sets of 15 repetitions. The resting time between the sets was 1 min. The training lasted for 30 min and was performed twice a week for 8 weeks.
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Table 1 Demographic details of the participants Gender Age (yrs.) Height (cm) Weight (kg)
Unilateral core training group (n = 8) Female 14.75 ± 0.31 162.95 ± 2.68 55.11 ± 3.17
Control group (n = 8) Female 15.13 ± 0.44 161.63 ± 1.21 60.50 ± 2.63
Note: Values are expressed as mean ± standard deviation.
B
A
Fig. 1. (A) Equipment for sudden-release trunk perturbation. The subjects performed an isometric trunk contraction against a load of 8 kg, while seated on a bench. When the load was subsequently released, the movements of the markers were traced and quantified with a 3D motion capture system. (B) The direction of perturbation. The external load was attached to the upper trunk at an angle of 30 degrees from the midline. When the load was suddenly released, the perturbations were induced in the left or right direction, resulting in left or right trunk rotation.
2.4. Data analysis When the sudden trunk perturbation was induced, the angular displacement of the trunk was recorded using a three-dimensional motion analysis system (Motion Analysis Corporation System, Santa Rosa, CA, USA). Six motion analysis cameras were used that sampled at a rate of 60 Hz. The markers for capturing the motion were attached on both acromions, T1 spinous process, both anterior superior iliac spines, and S2 spinous process. To analyze the raw data, CORTEX 1.0 software (Motion Analysis Corporation, Santa Rosa, CA, USA) was used. The trunk segment was defined by the connection of two points: the center of both acromions and the spinous process of T1, and the center of both anterior superior iliac spines and the spinous process of S2 (Fig. 2). The reference position of angular displacement was determined as that immediately before perturbation. The angular displacement was calculated in the sagittal (S), frontal (F), and horizontal (H) planes at the point of maximal trunk displacement in response to the perturbation. The representative values for each left or right rotational angular displacement was calculated by root mean square as follows: Perturbed angle = (S − S0 )2 + (F − F0 )2 + (H − H0 )2 ,
where S0 , F0 , and H0 were the initial values before inducing trunk perturbation. 2.5. Statistics The angular displacements are presented as mean and standard deviation. The data were analyzed using the SPSS 12.0 (SPSS Inc., Chicago, IL, USA). To detect a change in angular displacements before and after the unilateral core training in each group, a 2-way repeated measures ANOVA was performed using the following factors: training (pre- and post-training) × perturbed directions (left and right rotations). Post hoc analysis was performed to compare before and after angular displacement in each direction. This involved a paired t-test with Bonferroni’s correction, set at p < 0.0167. The level of statistical significance was set at p < 0.05, except for the post hoc analysis.
3. Results The 2-way repeated measures ANOVA found significant differences in angular displacement before and after the training in each of the two perturbed directions (p < 0.01). The post-hoc analysis showed that unilateral core training decreased the angular displace-
194
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
A Lateral View
B Superior View Fig. 2. Illustration of sensors attached to the trunk segments to obtain the trunk angular displacement data in 3D plane. The segment was defined by connection of the following points: the center of both acromions (L. and R. Shoulder) and spinous process at C7 and the center of both anterior superior iliac spine (L. and R. ASIS) and spinous process at S2. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/BMR-140569)
ment for the left and right rotations (p < 0.0167). After the training, the angular displacement for the left rotational perturbation was significantly reduced from 20.89◦ ± 4.90 to 15.38◦ ± 2.00, while the right rotational perturbation was significantly reduced from 20.54◦ ± 4.65 to 15.59◦ ± 2.02. When the control group was compared before and after 8 weeks of training, there were no statistically significant changes in the left or right angular displacements (p > 0.05). The angular displacement in the control group changed from 20.28◦ ± 4.22 to 18.70◦ ± 3.31 for the left rotational perturbation, and from 19.73◦ ± 4.13 to 18.05◦ ± 3.49 for the right rotational perturbation. Figure 3 shows the average change in the angular displacements following the two perturbations for both groups.
4. Discussion This study examined trunk angular displacements in response to sudden trunk perturbations before and after
unilateral core training. The results demonstrated that unilateral core training improved trunk stability in both the right and left rotational directions, as indicated by decreased angular displacement of the trunk. In addition, there were no significant changes in the control group, demonstrating the absence of a learning effect. This result shows that trunk stability was improved without a particular directional preference. Based on our results, we suggest that unilateral core training can bilaterally influence the neural network system. To maintain an upright sitting position against the perturbation, the participants had to resist left or right trunk rotation as soon as the perturbation was applied. In the case of the left rotational perturbation, the participants had to rotate their trunk to the right. This movement would require the activation of the left spinal erector, left external oblique, and right internal oblique muscles [13]. Based on electromyographic evidence, the quadruped and side bridge exercises were used to facilitate right trunk rotation. A previous electromyography study demonstrated that the quadruped
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
195
Direction of Trunk Perturbation
A
B
Fig. 3. Angular displacements of the unilateral core training group (A) and the control group (B) before (pre-training, black bar) and after training (post-training, gray bar). All values are presented as the mean and standard error. ∗ significant difference between the pre- and post-training values (p < 0.05).
exercise activates the left spinal erector, the left external oblique, and the right internal oblique muscles, and that overall, the side bridge exercise activates the left trunk muscles [14]. Therefore, we assume that the training program in the present study would enhance the ability to resist right rotational perturbations. In response to sudden perturbation, trunk stiffness is increased through a reflex contraction initiated by sensory input involving the muscle spindle, tendon, joint and ligament and is not related to voluntary muscle contraction initiated in the cerebral cortex [15, 16]. However, because the trunk is a multi-joint system, the central nervous system is limited in terms of the reflexes involved in regulating movement. Therefore, the central nervous system utilizes virtual trajectory of equilibrium, integrating the reflexes at a spinal level [17]. For the integration of the spinal reflex, we suggest that the bilateral net effects are regulated at a supraspinal level. Although there is no direct evidence, previous studies demonstrated that communication between muscles on both sides of the trunk occurs through a neural network [7,18,19]. Therefore, similarly to previous research, our findings suggest that trunk stiffness is developed bilaterally, despite intensive unilateral training. Intra-abdominal pressure is a factor that would increase trunk stability during rotational perturbations. There is a possibility that the unilateral training activated the internal oblique muscles bilaterally [14], thereby increasing intra-abdominal pressure. However, sudden trunk perturbation produces a low level coactivation of the abdominal and back muscles [20].
Therefore, we suggest that in the current study, intraabdominal pressure had a minimal effect on changes in the angular displacements of the trunk. The results of this study revealed a clinical message linked with physical education and training methods for trunk stability. Intensive unilateral training would increase overall trunk stability against sudden perturbations. In line with our results, a previous study suggested that integration exercises that elicit greater both proximal and distal trunk muscle activity would be more effective in enhancing stability and maintaining mobility [21], and another study indicated that many different trunk muscles work harmoniously to stabilize the trunk as one unit during a static action of the trunk [22]. Therefore, unilateral training would be a suitable core stabilization method of the intervention as well. One of the limitations of the study was the relatively small sample size that reduced the power of the study. In addition, the study focused on trunk rotations because of the possible effect of fatigue on the results. Further research is required to identify the effect of unilateral core training on multi-directional trunk stability. In addition future research should also examine the effects of gender and age on directional preferences relating to trunk stability. Despite the limitations, we found that the unilateral core training improved trunk stability without directional preferences. This result contributes to the understanding of the characteristics of trunk stability.
Y. Kim et al. / Intensive unilateral core training improves trunk stability without preference for trunk rotation
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5. Conclusions In this study, we examined the effect of unilateral core training on the direction of rotational preferences relating to trunk stability. The results demonstrated that angular displacements decrease in response to unilateral training regardless of the direction of trunk rotation. This implies that intensive unilateral training improves both left and right rotational stability, possibly through a bilaterally connected neural network.
[8]
[9]
[10]
[11]
Acknowledgment
[12]
This study received financial support from a grant by Korea University.
[13]
Conflict of interest None of the authors has any conflict of interest to report.
[14]
[15]
[16]
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