According to the German health interview and examination survey for children and adolescents (1- to 18-year-olds) (26), 15.2% of the participating children and adolescents had 1 accident during the 1-year examination period. Further, 60.4% of all monitored accidents were because of slips, trips, and falls, 20.6% because of collisions with objects or persons, and 8.3% because of traffic accidents. In young children (1- to 4-year-olds), the proportion of falls from elevated levels averaged 35.8%, from stairs 10.4%, and from playground equipment 7.7%. Another 32.4% of falls occurred on the ground level. It is of interest to note that a trend toward an increase in incidence rate of slips, trips, and falls has been observed in children and adolescents over the last 10 years (8). The reported injuries because of falls include soft tissue bruises (37.7%), sprains (14.4%), bone fractures (10.7%), and contusions (7.0%). These injuries may affect children's attitude toward physical activity (PA) in the future in terms of an increased restriction of PA (29). Furthermore, medical treatment of fall-related injuries lays a high financial burden on the public health care system. An Australian study revealed that the direct cost of falls in children to the health care system has been estimated to be >$130 million, of which $28 million was the cost of hospital inpatient care (32).
Children seem to be at greater risk to sustain falls than healthy young adults because their neuromuscular system is not fully developed, and many fundamental motor skills are still emerging (27,41,44). In fact, deficits in postural control and muscle strength have been observed in children compared to young healthy adults (3,24). Additionally, secular trends in balance and strength performance over the past 25-to 35 years were reported recently (33). These neuromuscular constraints represent major intrinsic fall-risk factors (39).
Thus, adequate intervention programs in Kindergarten, elementary school, or in sports clubs should be designed and administered, which have the potential to counter intrinsic fall risk factors (e.g., deficits in balance and strength) by reducing the risk of falling. Promoting these neuromuscular capacities could even motivate children to increase time spent in moderate-to-vigorous PA, which again could be beneficial for the children's overall health status (44).
It has been shown that balance training (BT) has the potential to promote postural control, strength of the lower extremities, and jumping performance in healthy young, middle-aged, and old subjects (14-16,21,22). Granacher et al. (14) scrutinized the effects of BT on postural sway, leg extensor strength, and jumping height in a cohort of adolescent high school students (age 19 ± 2 years). Four weeks of BT implemented in physical education (PE) induced significant decreases in postural sway, increases in jumping height, and rate of force development (RFD) of the leg extensors. Furthermore, Heitkamp et al. (21,22) investigated the impact of a 6-week BT in healthy active adults (age 32 ± 6 years) on static postural control and strength of the knee extensors and flexors. They observed that BT significantly improved 1-legged stance balance and isokinetic torque of the knee extensors and flexors. In addition, Granacher et al. (15) examined the effects of a 13-week BT on the ability to compensate for gait perturbations in elderly men (age 67 ± 5 years). Balance training resulted in a decrease in onset latency and an enhanced reflex activity in the prime mover compensating for the decelerating perturbation impulse. In another study, Granacher et al. (16) were able to show that 13 weeks of BT improved maximal and explosive force production capacity of the leg extensors in a cohort of healthy, elderly men (age 67 ± 5 years). Yet, there is no study that scrutinized the effects of BT in healthy prepubertal children on variables of postural control and strength performance. The only study that has been conducted in children investigated patients with hemiplegic cerebral palsy showing that BT improved the performance in stance parameters (30). Therefore, the objectives of this study were to examine the impact of standardized BT as described by Granacher et al. (14) and detraining on postural sway, force production of the plantar flexors, and jumping height in a not yet investigated population of healthy 6- to -7-year-old children. Based on previous studies investigating the effects of BT on postural control and strength in older age groups (14,15,21) and with reference to a recently published systematic review on the effects of BT on balance ability in healthy individuals (7), it is expected that balance and strength improve after 4 weeks of BT. Counteracting intrinsic fall risk factors like deficits in postural control and muscle strength may result in a reduced risk of falling in children. Thus, culminating costs in the public health care system could be reduced. The intervention was performed during regular PE classes to ensure compliance and to test the feasibility of integrating such a program in the regular school curriculum. Further, schools seem to provide an excellent opportunity for motor performance promotion as they access a large population of children and adolescents across broad ethnic and socioeconomic strata (36).
Experimental Approach to the Problem
To test our hypothesis, adaptations for BT were compared in a controlled longitudinal training study. The training period lasted 4 weeks to ensure adaptive processes in the postural control system. Training was followed by a 4-week detraining period to document the stability of training-induced changes. Improvements were verified by an analysis of postural control and static and dynamic force measurements. As a measure of quasi dynamic postural control, total displacement of the center of pressure (COP) was computed. We selected 2 dependent variables for static force measurements (maximal torque and RFD of the plantar flexors) and 1 for dynamic force measurements (countermovement jump [CMJ] height). Both, gains in balance control and force production developed during regular PE lessons are of vital importance for fall prevention and motor skill development.
Thirty grade 1 elementary school children with no significant anthropometrical differences in body mass, body height, body mass index, and Tanner stages participated in the study after experimental procedures were explained (see Table 1). None had any history of musculoskeletal, neurological, or orthopedic disorder that might have affected their ability to maintain balance. The dominant leg was determined according to the lateral preference inventory (6). Appropriate informed consent has been gained from their parents. None of the students had an athletic background and none had previously participated in systematic balance or resistance training. All students performed <6 hours of sports activities a week (including PE). Out of school sports activities were primarily conducted in sports clubs (e.g., soccer clubs). Local ethical permission was given by the Ethical Commission of the University of Basel and all experiments were conducted according to the latest version of the declaration of Helsinki. Informed consent was obtained from the children and their parents or guardians before the start of the study.
Participants were recruited from 2 different PE classes in the same elementary school. The 2 classes were randomly assigned to either the intervention class (INT) or the control class (CON). Before intervention, the authors of this study comprehensively instructed the regular PE teacher about the BT methodology. The intervention program was taught by the regular PE teacher and by an expert on BT to keep the student-to-teacher ratio small (2 teachers vs. 15 children). The INT conducted a BT program that proved to be effective in healthy young, middle-aged, and old subjects and that has been previously used (14,18,19). Briefly, BT lasted 4 weeks with 3 training sessions per week on unstable training devices (soft mats, ankle disks, balance boards, air cushions) during regular PE classes. Balance training was organized as a circuit-training with each teacher supervising 2-3 stages. Each BT session started with a 10-minute warm-up, followed by 45 minutes of BT, and finishing with a 5-minute cool down program. Balance exercises were performed in 2-legged and later on in training 1-legged stance with knees slightly bent (ca. 30°) and barefoot. Four sets of each exercise were performed with each set lasting 20 seconds with a 40-second rest in between. A longer rest of 2 minutes was allowed between different exercises to reduce fatigue. Exercise intensity was progressively increased by reducing the base of support, by using dynamic arm movements to perturb the center of gravity, and by reducing the sensory input (e.g., standing with eyes closed). Pictures were used to make instructions appropriate for children. Participants of the CON (see Table 1) attended their regular PE lessons (3 times a week) during the 4-week intervention period and were primarily taught in swimming. No specific balance exercises were performed during their PE lessons.
Apparatus and Testing Protocol
Pre, post, and follow-up measurements were conducted in our biomechanic laboratory. Test circumstances (e.g., room illumination, temperature, noise) were in accordance with recommendations for posturographic testing (28). Before testing, all subjects underwent a 5-minute warm-up consisting of submaximal plyometrics. Pre and post measurements included (a) testing of quasi-dynamic postural control on a balance platform, (b) the analysis of jumping height on a force platform, and (c) the assessment of maximal voluntary contraction of the plantar flexors under isometric condition on an isokinetic device. This testing sequence was applied to keep the effects of neuromuscular fatigue minimal.
Quasi-dynamic postural control was assessed by a balance platform (GKS 1000®, IMM, Mittweida, Germany). The platform is mounted to 4 springs and is free to move in the transversal, medio-lateral, and anterior-posterior directions (see Figure 1). The balance platform consists of 4 sensors measuring displacements of the COP in the medio-lateral and anterior-posterior directions. Data were acquired for 20 seconds at a sampling rate of 40 Hz (28). Total displacement of the COP was computed. During the measurement of dynamic postural control, participants stood bare footed, 2-legged (heels together, at an angle of 30° between the medial sides of the feet), and in an upright position for 20 seconds on a balance pad (Airex balance pad, Aalen-Ebnat, Germany), which covered the balance platform (9). Participants were asked to place hands on their hips and to look straight ahead, focusing on a figure (colored snowman) attached to the wall. Children were instructed to remain as stable as possible and to refrain from any voluntary movements during the trials. Before testing, children performed 2 practice trials on the balance platform. Thereafter, 1 test trial was conducted. If participants did not accomplish the whole sampling duration, they were allowed to repeat. Intraclass correlation (ICC) coefficients were calculated for total displacements of the COP (ICC = 0.93). This protocol has recently been described in detail elsewhere (37).
Participants performed maximal vertical CMJs while standing on a one dimensional force platform (Kistler® type 9290AD, Winterthur, Switzerland). The vertical ground reaction force was sampled at 500 Hz. During the CMJ, subjects stood in an upright position on the force platform and were instructed to begin the jump with a downward movement, which was immediately followed by a concentric upward movement, resulting in a maximal vertical jump. Subjects performed 3 CMJs with a resting period of 1 minute between jumps. The best trial in terms of maximal jumping height was taken for further data analysis. The ICC was calculated for CMJ height (ICC = 0.71). This protocol has recently been described in detail elsewhere (17).
Maximum isometric plantar flexion strength was measured on an isokinetic system (Isomed 2000®, D & R Ferstl GmbH, Hemau, Germany). The maximum error of the torque sensor was <0.2%. Participants lay supine on the seat of the isokinetic device, with hip and knee angles in neutral position (180°) and the ankle angle at 100°. Straps attached to the isokinetic system firmly fixed the shoulders, the waist, the thigh, the shank, and the foot. In addition, participants were asked to cross their arms in front of their chest. Thus, evasive movements of the upper and lower body were not possible. The exact position of each participant was documented and saved so that it was identical in pre, post, and follow-up tests. Testing was performed with the dominant leg. The dominant leg was determined according to the lateral preference inventory (6). Before the testing started, participants warmed up by doing 3-5 submaximal isometric actions in the isokinetic system to get accustomed to the testing procedure. Thereafter, each subject performed 3-4 plantar flexor exercises with maximal voluntary effort. For each trial, subjects were thoroughly instructed to act as forcefully and as fast as possible and to avoid forced respiration. The torque signal was sampled at 200 Hz. A digital fourth-order recursive Butterworth low-pass filter, with a cut-off frequency of 50 Hz filtered the torque signal. During offline analysis, the best trial in terms of maximal torque was selected and used for further data analyses. Maximal torque was defined as the maximal voluntary torque value of the torque-time curve, determined under isometric condition. Rate of force development was defined as the mean slope of the torque-time curve between 20 and 80% of the individual maximal torque. These parameters were chosen to gain comparable data to previously conducted studies (19,20,43). The ICC coefficient was calculated for maximal torque (ICC = 0.79) and RFD (ICC = 0.83) of the plantar flexors. This protocol has recently been described in detail elsewhere (20).
Data are presented as group mean ± SDs. A multivariate analysis of variance) was used to detect differences between study groups (INT, CON) in all baseline variables. Balance and strength parameters were analyzed in separate 2 (Groups: INT, CON) × 3 (Tests: pre, post, follow-up) analysis of variance with repeated measures on test.
In addition, the classification of effect sizes (f) was determined by calculating partial η2p. The effect size is a measure of the effectiveness of a treatment, and it helps to determine whether a statistically significant difference is a difference of practical concern. f-values = 0.10 indicate small, f-values = 0.25 medium, and f-values = 0.40 large effects (5). An a priori power analysis (11) with an assumed type 1 error of 0.05 and a type 2 error rate of 0.10 (90% statistical power) was conducted for postural sway (14) and revealed that 15 participants per group would be sufficient for finding a statistically significant interaction effect. The significance level was set at p ≤ 0.05. All analyses were performed using Statistical Package for Social Sciences version 16.0.
Mean and SDs for all variables are presented in Table 2. There were no statistically significant differences in baseline values between the experimental groups.
Figure 2A illustrates slightly reduced COP displacements in the INT from pre, to post, to follow-up testing. However, the analysis failed to indicate main effects of test (F(2,116) = 1.45, p > 0.05, η2 = 0.05, f = 0.23) and group (F(1,28) = 0.18, p > 0.05, η2 = 0.01, f = 0.10). In addition, the analysis failed to detect a Group × Test interaction for COP displacements (F(2,116) = 0.68, p> 0.05, η2 = 0.02, f = 0.14).
Figure 2B demonstrates slight increases in CMJ height in the INT from pre, to post, to follow-up testing. Yet, the analysis did not detect main effects of test (F(2,116) = 1.86, p> 0.05, η2 = 0.06, f = 0.25), and group (F(1,28) = 0.49, p> 0.05, η2 = 0.02, f = 0.14). In addition, no significant Group × Test interaction were found for CMJ height F(2,116) = 1.75, p> 0.05, partial η2 = 0.06, f = 0.25).
Plantar Flexor Strength
Figure 2C illustrates slight increases in maximal torque of the plantar flexors in the INT from pre, to post, to follow-up testing. The analysis indicated main effects of test (F(2,116) = 7.33, p< 0.01, η2 = 0.21, f = 0.52) but not of group (F(1,28) = 0.13, p > 0.05, η2 = 0.01, f = 0.10). Furthermore, the analysis failed to detect a Group × Test interaction for maximal torque of the plantar flexors (F(2,116) = 0.73, p> 0.05, η2 = 0.03, f = 0.18).
Figure 2D demonstrates slight increases in RFD of the plantar flexors in the INT from pre to posttesting. However, the analysis failed to indicate main effects of test (F(2,116) = 0.39, p> 0.05, η2 = 0.01, f = 0.10) and group (F(1,28) = 2.97, p> 0.05, η2 = 0.10, f = 0.33). In addition, the analysis failed to detect a Group × Test interaction for RFD of the plantar flexors (F(2,116) = 0.09, p > 0.05, η2 = 0.003, f = 0.05).
This is, to the authors' knowledge, the first study that investigated the impact of BT on postural control and strength performance implemented in regular PE lessons in healthy 6- to 7-year-old children. Four weeks of intense BT resulted in tendencies however not statistically significant improvements in postural sway, force production of the plantar flexors, and jumping height. The tendency toward an improvement in COP displacements can also be manifested in the INT group in the observed reduction in SD of COP displacements from pre to posttests, whereas an increase was found in the CON group. This heterogeneity in between-subject variability in postural sway is generally reduced with age and with the child's improved stability because of training (38). In addition, the observed effect sizes for COP displacements (f = 0.14), for jumping height (f = 0.25) and for plantar flexor strength (f = 0.18) indicate small to medium interaction effects and thus a tendency toward an improvement in the investigated parameters.
However, the results of the statistical analysis of the present study are in contrast to findings reported in the literature regarding the effects of BT on postural control and strength performance in different cohorts. In fact, Granacher et al. (14) found improvements in postural control, increases in RFD of the leg extensors as well as an enhanced jumping height in squat jump (SJ) and CMJ after 4 weeks of BT implemented in PE lessons in healthy active adolescent high-school students (age 19 ± 2 years). In addition, Taube et al. (43) observed a modified reflex activation during stance perturbation and a significant increase in jumping height in SJ and CMJ after 6 weeks of BT in young elite athletes (age 15 ± 1 years). Furthermore, Bruhn et al. (4) found similar results regarding jumping height in SJ after 4 weeks of BT in a cohort of sport science students (age 23 ± 2 years). Finally, Heitkamp et al. (22) investigated the impact of a 6-week BT program in healthy active adults (age 32 ± 6 years) on variables of static postural control and observed that BT significantly improved 1-legged stance balance. This was also shown for subjects with chronic ankle instability (34) and for older adults (35). Thus, it can be stated that findings in the literature regarding the impact of BT on measures of postural control and strength performance are consistent for study populations in adolescent and above adolescent age. What makes prepubertal children different from adolescents, adults and seniors in terms of their adaptive potential after BT? The answer can most likely be found in maturational processes of the postural control system.
Postural control has been defined as the control of the body's position in space for the purpose of balance and orientation (40). The ability to control posture can be described as a dynamic process across the life span. There is evidence that young children and elderly adults show the largest magnitudes of postural sway, when measured on a force platform. Therefore, a U-shaped dependency between balance and age can be postulated (24). Upon closer examination of the literature on maturational processes of the postural control system during childhood, it can be stated that the age of 7 years represents an important milestone in maturation of the postural control system because adult balance strategies begin to appear (38). The participants of the present study are slightly below or right at the age of 7 years. The large between-subject variability in COP displacements at baseline might indicate that some students already adopted adult-like balance strategies, whereas others were still carrying out immature, child-like strategies.
For many years, the control of posture was solely attributed to automatic or reflex controlled muscle activations (23). However, today it is well known that attentional resources are necessary to effectively stabilize the body's center of gravity over the base of support (45). Therefore, it can be postulated that the control of posture demands the complex processing and integration of sensory information provided by vision, proprioception, and the vestibular system on a spinal and supraspinal level (31). A study of healthy children between the ages of 3 and 6 years using computerized dynamic posturography indicated that balance control changes from being primarily visual-vestibular to being somatosensory-vestibular between these ages, but that the transition to adult responses for all sensory conditions is not complete by the age of 6 years (13). Furthermore, it was reported that children younger than seven and a half years, when tested on a moving platform, are less able to suppress inappropriate visual and somatosensory inputs (12). A neurophysiological study in children investigating electromyography (EMG) responses to muscle stretch in the upper limb indicated that long latency EMG responses, probably involving supraspinal pathways, are present at 2-3 years of age, but that the duration of these long-latency EMG responses did only show adult-like values after the age of 6-8 years (1). Berger et al. (2) found similar results when children aged between 1 and 8 years were subjected to unexpected displacement of the treadmill in posterior direction while the child was standing still. The authors reported that from the age of 8 years on, EMG responses no longer differed from those obtained in adults. Based on these results, it can be concluded that hierarchically lower level and primarily spinal motor centers mature before higher level supraspinal motor centers. Both mechanisms however are necessary for the targeted coordination of balance control. Given these findings in the literature in combination with our results regarding large between-subject variability in COP displacements, one may speculate that important mechanisms for balance control involving supraspinal motor centers were not fully developed in the participants of this study. Therefore, our participants may have primarily relied on mechanisms involving hierarchically lower spinal motor centers for balance control. These compensatory reactions however are not particularly suitable for coordinated postural control because they are less modifiable (2). In addition, Taube et al. (42) reported in a recent study that supraspinal rather than spinal mechanisms seem to be responsible for the training induced postural improvement in adults (age 25 ± 3 years) after 4 weeks of BT. The authors suggested an improved regulation of human erect posture in terms of a shift from cortical to subcortical areas. Given the already described maturational deficits in the postural control system of prepubertal children, it can be speculated that our subjects lacked an important neurophysiologic prerequisite for adaptive processes after BT. This again might explain why we found tendencies but not statistically significant results as opposed to BT studies with older study cohorts.
Another reason could be that the attentional focus necessary for the performance of balance exercises and thus for inducing training-related adaptive processes was not given at all times during training in our study population. Throughout the training period, the 2 teachers were under the impression that children performed balance exercises with high levels of attentional focus and concentration only when a teacher was standing right next to them. As soon as the teacher spotted a different student, the performance quality of the balance exercise decreased. Therefore, it could be argued that the postural control systems of our participants were not mature enough to participate in BT or that the student-to-teacher ratio was too high.
We acknowledge that this study has some limitations that warrant discussion. First, the sample size applied in this study is relatively small. However, the a priori power analysis revealed that 15 participants per group are sufficient to obtain a statistically significant interaction effect for the investigated balance parameter. In addition, other researchers investigating similar study objectives with a similar study design used even smaller sample sizes and found significant interaction effects in terms of the impact of BT on measures of balance and strength in high school adolescents (14) and young elite athletes (43). Second, we observed tendencies but not statistically significant results. Therefore, one may argue that the training period was too short to induce adaptive processes in the postural control system of prepubertal children. However, other studies showed that adaptive processes are possible after 4 weeks of BT implemented in PE (14). In addition, PE teaching units last between 4 and 6 weeks, which is why we chose this rather short period. Third, because of the methodological approach applied in this study, we cannot directly infer on the underlying neuromuscular mechanisms responsible for the observed results. Therefore, future studies should involve the application of H-reflexes and transcranial magnetic stimulation to find out if adaptive processes take place on a spinal level as suggested in this study and or rather on a supraspinal level.
The results of this study illustrate tendencies but not statistically significant improvements in postural sway, jumping height and RFD of the plantar flexors after 4 weeks of BT implemented in regular PE classes in healthy prepubertal children aged 6-7 years. Immaturity of the postural control system could account for the observed findings. Indeed, it was reported that adult-like balance responses are not present until the age of 8 years (1,2). To the best of our knowledge, this is the first study that investigated the effects of BT on variables of postural control and strength in healthy prepubertal children. Based on our results, BT alone cannot be recommended for the promotion of balance and strength in children. Recently, it was found that resistance training resulted in significant increases in measures of strength and improvements in selected motor skills in children and adolescents (10,25). Therefore, resistance training could be applied alone or in combination with BT to induce effects on both, balance and strength variables and to prevent children from falling.
The authors would like to thank the participating students and teachers for their enthusiasm.
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