Perceptual & Motor Skills: Exercise & Sport 2013, 117, 3, 665-681. © Perceptual & Motor Skills 2013
INLINE SKATING FOR BALANCE AND STRENGTH PROMOTION IN CHILDREN DURING PHYSICAL EDUCATION1, 2 THOMAS MUEHLBAUER, MATTHIAS KUEHNEN, AND URS GRANACHER Department of Training and Movement Sciences Cluster of Excellency in Cognition Sciences University of Potsdam, Potsdam, Germany Summary.—Deficiencies in balance and strength are common in children and they may lead to injuries. This study investigated the effects of inline skating exercise on balance and strength performance in healthy children. Twenty 11–12-yearold children (8 girls, 12 boys) were assigned to an intervention (n = 10) or a control (n = 10) group. Participants in the intervention group underwent a 4-week inline skating program (2 times/week, 90 min. each) integrated in their physical education lessons. Balance and strength were measured using the Star Excursion Balance test and the countermovement jump test. As compared to the control group, the intervention group significantly improved balance (17–48%, Cohen's d = 0.00–1.49) and jump height (8%, Cohen's d = 0.48). In children, inline skating is a safe, feasible (90% adherence rate), and effective program that can be integrated in physical education lessons to promote balance and strength.
Considering all incidences of pediatric trauma, falls and sports-related accidents appear to be the major causes of injury (Gallagher, Finison, Guyer, & Goodenough, 1984; Shanon, Bashaw, Lewis, & Feldman, 1992). In fact, 28% of all pediatric (6- to 12-year-old) trauma hospital admissions can be ascribed to falls and 17% to sports-related injuries (Gallagher, et al., 1984). Injury rates due to falls and sports-related trauma are higher in children (6- to 12-year-olds) compared to adolescents (13- to 19-year-olds) (Gallagher, et al., 1984). This is most likely due to the fact that the neuromuscular system is not fully developed in children and that many of their basic motor skills are still emerging (Shumway-Cook & Woollacott, 1985; Largo, Fischer, & Rousson, 2003). As a consequence, maturational deficits in static and dynamic postural control have been observed in children compared to adolescents and adults in terms of increased postural sway (i.e., static steady-state-balance) (Hytonen, Pyykko, Aalto, & Starck, 1993), slower gait speed (i.e., dynamic steady-state-balance) (Oberg, Karsznia, & Oberg, 1993), decreased movement quality during walking on toes, heels, and inner/outer soles (i.e., proactive balance) (Largo, Caflisch, Hug, MugAddress correspondence to Dr. Thomas Muehlbauer, Department of Training and Movement Sciences, Cluster of Excellency in Cognition Sciences, University of Potsdam, Am Neuen Palais 10 (Haus 12), D-14469 Potsdam, Germany or e-mail (
[email protected]). 2 No sources of funding were used to assist in the preparation of this manuscript. 1
DOI 10.2466/30.06.PMS.117x29z9
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T. MUEHLBAUER, ET AL.
gli, Molnar, & Molinari, 2001) as well as impaired ability to compensate for platform perturbations (i.e., reactive balance) (Berger, Quintern, & Dietz, 1985). The reasons for impaired balance performance in children can most likely be explained by not yet fully developed structures within the central nervous system. For example, Riach and Hayes (1987) investigated age-related changes in postural sway in children ages 2 to 14 years and in adults. They observed that children predominately rely on visual information to control balance, whereas grown-ups prioritize the proprioceptive system. Another study determined at what age (6- to 12-year-olds) adultlike integrative sensory information processes occur that regulate postural control (Peterson, Christou, & Rosengren, 2006). The results showed that 12-year-old children were able to achieve scores comparable to those of adults. Further, it was reported that specific postural reaction patterns following perturbation while walking on a treadmill did not appear adultlike before the age of 14 years (Hirschfeld & Forssberg, 1992). Additionally, secular trends in balance and strength performance have been reported for children (Jurimae, Volbekiene, Jurimae, & Tomkinson, 2007; Matton, Duvigneaud, Wijndaele, Philippaerts, Duquet, Beunen, et al., 2007). Thus, the promotion of balance and strength represent two important goals in prevention of falls and injuries. To date, there is only one study available that investigated the effects of 4 weeks of balance training in healthy children ages 6 to 8 years (Granacher, Muehlbauer, Maestrini, Zahner, & Gollhofer, 2011). The authors reported tendencies in terms of small-to-medium Group × Test effects (Δ 7%, Cohen's d = 0.14) but no statistically significant improvements in postural sway. Inline skating, however, may provide a form of exercise that improves balance. In fact, inline skating skills are particularly challenging for dynamic postural control due to the fact that balance has to be maintained over a small base of support while striding (Publow, 1999). In addition, the striding, turning, and stopping techniques of inline skating can be offered in a game-like format that could enhance children's motivation throughout the training period. This appears to be particularly feasible since in recent years, inline skating has become a popular sport among children (Nguyen & Letts, 2001), i.e., associated with several health benefits like aerobic fitness, recreational activity, and social interaction (Hoffman, Jones, Bota, Mandli, & Clifford, 1992; Snyder, O'Hagan, Clifford, Hoffman, & Foster, 1993). To the authors' knowledge, there is no study available that investigated the effects of an inline skating program on measures of balance and strength in children. Given that inline skating challenges postural control, the objective of this study was to investigate the effects of a short-term inline skating program on measures of balance and strength in a cohort of healthy 11- to 12-year-old children. It was expected that a progressive-
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ly designed inline skating program would produce significant improvements in both capacities. Hypothesis 1. Four weeks of inline skating will induce significant improvements in children's balance. Hypothesis 2. Four weeks of inline skating will induce significant improvements in children's strength. METHOD Participants Twenty healthy physically active (approx. 9 hr. per week of everyday and sports-related activity) children participated in this study after the experimental procedures were explained. Participants were recruited from two different physical education classes of the same elementary school. Participants were randomly assigned to an intervention (n = 10) or a control (n = 10) group. Characteristics of the study population are described in Table 1. None of the participants had any history of musculoskeletal, neurological or orthopedic disorders that might have affected their ability to perform balance and strength tests. Parents' and participants' informed consents were obtained before the start of the study. Local ethical permission was given and all experiments were conducted according to the latest version of the Declaration of Helsinki. TABLE 1 PARTICIPANT CHARACTERISTICS Characteristic Age, yr.
Intervention Group (n = 10) M
SD
Control Group (n = 10) M
SD
p
11.2
0.4
11.4
0.5
.36
147.5
10.5
150.4
10.4
.54
39.4
4.8
43.8
5.2
.42
Body Mass Index, kg/m2
17.8
2.5
19.0
3.3
.37
Sex, f/m
5/5
3/7
Stages 1 and 2
Stages 1 and 2
Body height, cm Body mass, kg
Tanner stage* Sports-related activity, hr./wk. Leg length left, cm
9.2
2.7
9.3
2.5
.93
73.1
5.4
74.5
4.6
.54
Leg length right, cm 73.1 5.7 74.2 5.0 .64 Note.—f = female; m = male; *Pubic hair development was self-reported by the participants.
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Procedure The intervention was performed during regular physical education classes to ensure compliance and to test the feasibility of integrating such a program in the regular school curriculum. As is typical for German high schools, teaching units in physical education last between 4 and 6 weeks, and physical education is taught twice each week. Given the diversified contents of the mandatory physical education curriculum (e.g., ball games, swimming, gymnastics, track and field, etc.) that have to be realized over the school year, it was not possible to conduct inline skating on a weekly basis over the school year. Given that the German physical education curriculum allows short training periods only, it was decided to conduct a 4-week inline skating program because this period appears to be sufficient to improve balance and strength performance (Granacher, Gollhofer, & Kriemler, 2010). These externally imposed conditions had to be taken into account when planning to integrate inline skating in regular physical education lessons. Before the start of the study, the authors comprehensively instructed the regular physical education teacher about the inline skating methodology. The intervention program was taught by the regular physical education teacher and by an expert on inline skating to keep the student-to-teacher ratio small (two teachers for 10 children). The inline skating program lasted four weeks and comprised two training sessions (90 min. each) per week during regular physical education classes. The program was based on inline skating guidelines provided by Publow (1999). Each session started with a 10-min. warm-up, followed by 70 min. of inline skating. The physical education lesson was completed with a 10-min. cool-down. During the main part of the inline skating program, basic techniques like falling, stopping, striding, and turning were practiced. Progression during the inline skating program was initially achieved by advancing from static (i.e., learning issues of alignment and basic body positions) to dynamic (i.e., active movement of the lower body while maintaining alignment in the upper body) drills, and thereafter by changing from gliding to single-leg push techniques. Table 2 illustrates a detailed description of the training program. Participants in the control group attended their regular physical education lessons (also two times each week for 90 min. each) during the four-week intervention period and were primarily taught in track-and-field (e.g., running, throwing, jumping). Both groups conducted their physical education lessons on different days but during similar hours (i.e., one session in the morning and one session in the afternoon). Testing Prior to testing, all participants underwent a standardized 5-min. warm-up which consisted of bipedal and monopedal balance, submaxi-
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INLINE SKATING TRAINING IN CHILDREN TABLE 2 PROGRESSION DURING THE INLINE SKATING PROGRAM Training Session Session 1
Content
Graphic Explanation
Acquisition of basic positions (i.e., lowering the trunk, deep knee flexion, keeping the body weight back toward the heels) Acquisition of falling and stopping (i.e., heel stop) techniques
Acquisition of Acquisition of basic positions stopping tech(i.e., lowering niques (i.e., heel the trunk) stop)
Gliding on skates in the basic positions Session 2
Repetition of falling and stopping techniques Gliding on skates in the basic positions Acquisition of weight transfer and striding technique (i.e., single-leg push)
Session 3
Gliding on skates
Acquisition of striding technique (i.e., single-leg push)
Repetition of weight transfer and striding technique Consolidation of falling and stopping techniques Acquisition of turning technique
Session 4
Acquisition of turning technique
Repetition of turning technique Consolidation of striding technique Acquisition of skating a slalom parcour Acquisition of skating a slalom parcour
Session 5
Consolidation of turning technique Repetition of skating a slalom parcour Performing games (e.g., duck walk)
Session 6
Performing games (i.e., duck walk)
Consolidation of skating a slalom parcour Acquisition of single-leg gliding Performing games
Session 7
Acquisition of single-leg gliding
Repetition of single-leg gliding Performing a skating parcour using all acquired skating techniques Performing games
Session 8
Performing a skating parcour
Consolidation of single-leg gliding Performing a skating parcour using all acquired skating techniques Performing games
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mal plyometric, and skipping exercises. Thereafter, balance and strength performance were assessed using the Star Excursion Balance Test and the countermovement jump height test. This sequence of measurements was applied to keep the effects of neuromuscular fatigue minimal during preand post-testing. Pre- and post-tests were conducted for both groups at the same time of day (i.e., in the morning). Pre-tests were completed two days before the first training session and post-tests started two days after the last training session. The examiner was not blinded to the purpose of the study but he was blinded to group allocation. Balance performance.—The Star Excursion Balance Test is a test of dynamic balance that incorporates a single-leg stance on one leg while at the same time performing a maximum reach with the other leg (Kinzey & Armstrong, 1998). Excellent values for test-retest (ICC = .84–.92), intratester (ICC = .82–.96), and inter-tester (ICC = .81–.93) reliability were reported for the Star Excursion Balance Test (Hertel, Miller, & Denegar, 2000; Munro & Herrington, 2010). ICC values calculated from the current data revealed good test-retest reliability for the left (ICC = .74–.97) and the right (ICC = .70–.94) leg. The Star Excursion Balance Test was conducted with the participant standing at the centre of a grid placed on the floor, with eight lines extending at 45° increments from the centre of the grid. The eight lines were labeled according to the direction of excursion relative to the stance leg: anterior, anterior-lateral, anterior-medial, lateral, medial, posterior, posterior-lateral, and posterior-medial. The grid was constructed in a gym using a protractor and adhesive tape. Prior to testing, a verbal and visual demonstration of the testing procedure was given to each participant. To perform the Star Excursion Balance Test and ensure that stability was achieved through adequate neuromuscular control of the stance leg, the participant maintained a single-leg stance, at the centre of the grid, while reaching with the contralateral leg (reach leg) as far as possible along the appropriate vector, lightly touching the farthest point possible on the line with the most distal part of the reach foot. The participant then returned to a bilateral stance while maintaining equilibrium. Participants undertook the test barefoot. The examiner manually measured the distance from the centre of the grid to the touch point with a tape measure in centimetres. Test results were documented after each reach by the same examiner. As recommended by Robinson and Gribble (2008), participants performed one practice trial in each of the eight directions for each leg to become familiar with the task, followed by three test trials. Participants were allowed 15 sec. rest between reaches. The mean of three reaches for each leg in each of the eight directions was used for further analyses. Reach leg (i.e., right leg, left leg), order of excursions performed (i.e., clockwise, counter-clockwise), and direction of the first excursion (i.e., an-
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terior, anterior-lateral etc.) were counterbalanced to control for any learning or order effect. All trials were then performed in sequential order in either the counter-clockwise or clockwise directions. Trials were discarded and repeated if the participant (1) did not touch the line with the reach foot while maintaining weight bearing on the stance leg, (2) lifted the stance foot from the centre grid, (3) lost balance at any point in the trial, (4) did not maintain start and return positions for one full second, or (5) if a participant was judged by the examiner to have touched down with the reach foot in a manner that caused the reach leg to considerably support the body. Given that leg length correlates with reach distance and that betweensubjects comparisons (i.e., intervention group vs control group) were computed, participants' left and right leg length (i.e., from the most distal end of the anterior superior iliac spine to the most distal end of the lateral malleolus) were measured (Gribble & Hertel, 2003). Thereafter, excursion distances were normalized to participants' leg length. Normalization was performed by dividing each excursion distance by the participant's leg length, then multiplying by 100 (Gribble & Hertel, 2003). Thus, normalized values can be viewed as a percentage of excursions distance in relation to the participant's leg length. Strength performance.—To measure strength, participants performed maximal vertical countermovement jumps. For this purpose, participants stood in an upright position 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. Participants performed three countermovement jumps with a resting period of 1 min. between jumps. For each of these trials, participants were instructed to jump as high as possible. The mean jump height (in cm) was taken for further data analysis. To record the jump height, the Optojump photoelectric cell system was used (Microgate, Bolzano, Italy), which consists of two parallel bars (a transmitter unit and a receiver). Bars were placed approximately 1 meter apart and parallel to each other. The transmitter contained 32 light emitting diodes, which were positioned 0.3 cm from ground level at 3.125-cm intervals. The Optojump system measured the flight time of countermovement jumps with an accuracy of 1/1000 sec. (1 kHz). Optojump software (Version 1.5.1.0) was used for quantification of jump height. Compared with a force plate, the Optojump system demonstrated strong concurrent validity (ICC = .99) and excellent test-retest reliability (ICC = .98) for the estimation of vertical jump height (Glatthorn, Gouge, Nussbaumer, Stauffacher, Impellizzeri, & Maffiuletti, 2011).
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Statistical Analyses Data are presented as group means and standard deviations. A multivariate analysis of variance (ANOVA) was used to detect baseline differences between study groups in all test variables. Balance and strength parameters were analyzed in a separate 2 (group: intervention, control) × 2 (test: pre, post) repeated measures ANOVA. Post hoc tests with Bonferroni-adjusted α were conducted to identify the comparisons that were statistically significant. Additionally, Cohen's d was calculated (Cohen, 1992). According to Cohen (1988), d = 0.2 indicates a small, d = 0.5 a medium, and d = 0.8 a large effect. An a priori power analysis (Faul, Erdfelder, Lang, & Buchner, 2007) with an assumed Type I error of 0.05 and a Type II error rate of 0.20 (80% statistical power) was conducted for measures of balance (Taube, Bracht, Besemer, & Gollhofer, 2010) and revealed that 10 persons per group would be sufficient to observe medium Group × Test effects. The significance level was set at α = 5%. All analyses were performed using Statistical Package for Social Sciences (SPSS) Version 19. RESULTS All participants received treatment or control conditions as allocated. Participants in both groups completed their lessons (i.e., intervention group = inline skating program, control group = regular physical education lessons) and none reported any training-related injury. The attendance rate during sessions amounted to 90% and 92% for the intervention and control groups, respectively. Overall, no statistically significant baseline differences were observed between the two groups for any of the test variables. After four weeks of inline skating lessons, all participants of the intervention group were able to successfully execute the specified skating techniques in a parcour without external assistance. Balance Performance Irrespective of the tested leg or the test direction, all but one Group × Test interaction was statistically significant, except for the posterior-medial direction of the right leg (see Table 3). In addition, all main effects of Test but not of Group were significant. Post hoc analyses indicated significant improvements in balance from pre- to post-test in the intervention compared to the control group (change = 17–48%; for all directions but one, p < .05, d = 0.00–1.49). Hypothesis 1 was supported. Within-group analyses showed a larger effect for the intervention group (left leg: d = 1.59–3.90, right leg: d = 1.43–2.32) than for the control group (left leg: d = 0.65–1.54, right leg: d = 1.16–2.05), irrespective of the reach direction. Strength Performance In terms of jump height, the statistical analysis revealed a significant Group × Test interaction (F1, 38 = 4.10, p < .05, d = 0.48). In addition, the main
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53.6
56.7
Posterior-lateral, %
Posterior-medial, %
63.3
Posterior, %
Posterior-lateral, %
Pre
9.0
6.5
6.9
7.0
7.8
7.1
6.5
8.5
13.0
9.9
8.9
8.9
7.5
7.9
5.0
SD
82.9
75.2
68.0
90.9
82.9
96.4
99.2
84.1
75.2
80.2
91.9
72.0
96.9
82.7
100.3
M
Post
10.3
8.7
6.7
7.9
7.4
7.1
5.8
10.1
11.2
11.0
11.3
10.3
8.7
7.4
7.9
SD
67.9
65.9
56.8
78.8
69.9
87.2
88.6
58.8
52.6
60.8
74.4
52.9
82.4
69.5
85.1
M
Pre
9.8
7.9
10.0
5.3
3.8
3.9
6.7
12.6
14.5
12.0
9.8
15.3
6.6
8.1
10.2
SD
81.2
75.6
68.4
89.0
77.1
95.2
97.3
78.2
67.5
71.4
87.9
62.8
91.9
76.9
95.7
M
Post
Control Group (n = 10)
9.3
9.1
11.4
8.4
2.9
6.0
7.7
11.6
14.1
13.1
8.6
14.0
6.0
6.8
10.6
SD
.005 (0.75)
.048 (0.50)
.052 (0.48)
.041 (0.52)
.001 (1.28)
.006 (0.73)
.010 (0.67)
.004 (0.77)
.005 (0.75)
.015 (0.64)
.001 (1.11)
.001 (1.49)
.001 (1.15)
.001 (1.06)
.001 (1.11)
G×T
.001 (3.92)
.001 (3.64)
.001 (3.73)
.001 (3.22)
.001 (3.44)
.001 (2.56)
.001 (2.51)
.001 (4.55)
.001 (4.11)
.001 (3.20)
.001 (3.92)
.001 (4.31)
.001 (3.35)
.001 (2.73)
.001 (3.76)
Test (T)
p (Cohen's d)
.73 (0.08)
.61 (0.12)
.60 (0.13)
.98 (0.00)
.54 (0.15)
.47 (0.17)
.64 (0.11)
.69 (0.10)
.46 (0.18)
.23 (0.29)
.75 (0.08)
.47 (0.18)
1.0 (0.00)
.70 (0.10)
.97 (0.00)
Group (G)
Posterior-medial, % 54.8 10.8 70.2 9.1 55.7 11.6 70.8 12.0 .94 (0.00) .001 (3.12) .89 (0.03) Note.— = Cohen's d for the interaction effect Group × Test. Normalised Star Excursion Balance Test means dividing each excursion distance by the participant's leg length, then multiplying by 100 (Gribble & Hertel, 2003).
53.0
62.5
Medial, %
67.3
76.8
Lateral, %
Anterior-lateral, %
Anterior-medial, %
84.1
82.2
Anterior, %
Right leg
67.7
64.5
Posterior, %
Lateral, %
Medial, %
77.4
51.9
Anterior-medial, %
80.8
66.2
Anterior-lateral, %
M
Anterior, %
Left leg
Balance Leg/Reach
Intervention Group (n = 10)
TABLE 3 EFFECTS OF INLINE SKATING PROGRAM ON NORMALISED STAR EXCURSION BALANCE TEST PERFORMANCE FOR INTERVENTION GROUP COMPARED TO CONTROL GROUP
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effect of Group (F1, 18 = 4.10, p < .05, d = 0.48) but not Test (F1, 38 = 2.87, p = .11, d = 0.40) was statistically significant. Post hoc analyses indicated a significant increase in jump height from pre- to post-test in the intervention (change = 8%, p < .05) compared to the control group (Fig. 1). Thus, Hypothesis 2 was supported as well. Within-group analyses showed a larger effect for the intervention (d = 0.29) compared to the control group (d = 0.19). DISCUSSION To the authors' knowledge, this is the first study that investigated the effects of a short-term inline skating program on balance and strength performance in healthy children. Four weeks of progressive inline skating resulted in statistically significant improvements in balance and jump height tests. Balance Performance Given that there is no study available that scrutinized the effects of inline skating in a school setting on measures of balance and strength in children, the findings have to be compared with results originating from a similar training regimen (i.e., conventional balance training). In general, the findings are in agreement with results from studies that investigat-
FIG. 1. Effects of a four-week inline skating program on countermovement jump height for the intervention compared to the control group.
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ed the effects of balance training in different age groups, e.g., Granacher, et al. (2010) reported significant improvements in balance performance (i.e., reduced postural sway during one-legged stance on stable ground, with changes = 15–18%, d = 0.50–0.56) after four weeks of balance training implemented in physical education lessons in healthy adolescent highschool students (M age = 19 yr., SD = 2). In contrast to these and our results, Granacher, et al. (2011) did not observe any significant improvements in variables of balance (change = 7%, d = 0.14) and strength (change = 6%, d = 0.25) following four weeks of balance training in children ages 6 to 7 years. This discrepancy is most likely due to the different methods applied in these studies. Whereas Granacher, et al. (2011) measured static balance (i.e., displacements of the center of pressure) before and after balance training, we examined reaching distances in several directions, which represents a measure of dynamic balance (Kinzey & Armstrong, 1998). It is possible that static and dynamic balance skills are regulated by different neuromuscular mechanisms and therefore an improvement in one may not be reflected in the other. In fact, Drowatzky and Zuccato (1967) observed small correlations (rs = .03 to .26) between measures of static (e.g., stork stand) and dynamic (e.g., sideward leap) balance in children ages 12 to 14 years. Further, Granacher, et al. (2011) argued that immaturity of the postural control system in children could be responsible for their non-significant findings. What are the underlying neuromuscular mechanisms responsible for the observed improvements in postural control following inline skating training? To the authors' knowledge, there are no studies available that have applied electrophysiological testing (e.g., transcranial magnetic stimulation, electroencephalography) in children to identify the underlying neurophysiological mechanisms responsible for training-induced balance improvements. Due to this void in the literature, the following discussion will review data collected in young healthy adults. According to Taube, Gruber, and Gollhofer (2008), it can be argued that spinal and supraspinal adaptations most likely account for the investigated findings. In an early report on training-induced improvements following balance training, Gollhofer (2003) assumed that adaptive processes following balance training mainly take place on a spinal level due to the high intermuscular activation frequencies observed during stabilization tasks on unstable platforms. In an original work, Taube, Gruber, Beck, Faist, Gollhofer, and Schubert (2007) investigated cortical and spinal adaptive processes in young participants (M age = 25 yr., SD = 3) following four weeks of balance training. From pre- to post-training, cortical excitability (i.e., motorevoked potentials) decreased when perturbation impulses were applied while standing on a treadmill. The authors suggested an improved regula-
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tion of human erect posture in terms of a shift from cortical to subcortical areas. Thus, it seems that supraspinal rather than spinal mechanisms are responsible for training-induced balance improvements. Even though inline skating is a popular sport, it is often associated with a high potential for falls and fall-related injuries, particularly in children (Powell & Tanz, 1996; Nguyen & Letts, 2001). Injuries during inline skating primarily occur due to a loss of balance (Schieber, Branche-Dorsey, Ryan, Rutherford, Stevens, & O'Neil, 1996; Nguyen & Letts, 2001). The results of the present study show that significant improvements in measures of balance can be achieved without sustaining any injuries during the training period. It can be speculated that the small teacher to student ratio (i.e., 2 to 10) contributed to these positive results. This should be taken into account when integrating such a program in the regular school curriculum (i.e., physical education classes). Plisky, Rauh, Kaminski, and Underwood (2006) found that training induced decreases in normalized excursion reach distances were significantly associated with lower extremity injuries in students ages 14 to 18 years. This may indicate that inline skating could have an injury-preventive effect. However, further epidemiological studies are needed that elucidate this issue. Inline skating offers a training regimen somewhat different from conventional balance training. More specifically, traditional forms of balance training use unstable or soft surfaces to challenge postural control. However, this is hardly related to everyday balance-threatening situations like standing in a driving bus or slipping when wet. Additionally, there is evidence in the literature (Yaggie & Campbell, 2006) that the capacity of balance training to transfer training-induced effects to sport-related activities is limited. In contrast, skill-specific training (i.e., inline skating) has the potential to improve the ability to skate inline as well as balance and strength performances. In the current study, only the inline skating program produced statistically significant performance improvements. The control group (which was engaged in track-and-field exercises but not in balance-challenging exercises) showed only tendencies but not statistically significant improvement in performance on the Star Excursion Balance Test. To the authors' knowledge, there is no study available that has investigated the effects of a track-and-field program consisting of running, throwing, and jumping on balance and strength performance in children. This is why studies using a single-item training approach (i.e., effect of jumping or sprinting training) have to be consulted. For example, Chaouachi, Ben, Othman, Hammami, Drinkwater, and Behm (2013) investigated the effects of an eight-week plyometric training on Star Excursion Balance Test performance in children (M age = 14 yr., SD = 1). Results showed significant improvements
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in balance performance (change = 6%, d = 0.64) following training. The reported Cohen's d was higher (0.64 indicates a medium effect) compared to that of the control group in the present study (0.19 indicates a small effect), indicating that the integration of the three components running, throwing, and jumping in the track-and-field program limited balance development compared to a single-item plyometric training. Strength Performance To the authors' knowledge, this is the first study that observed significant improvements in strength (i.e., enhanced jump height) in 11- to 12-year-old children following four weeks of inline skating. Our results were in accordance with other studies that tested the effects of a similar training regimen (i.e., balance training) on strength among other age groups. In fact, Granacher, et al. (2010) reported significant increases in countermovement jump height (change = 5%, d = 0.73), squat jump height (change = 5%, d = 0.73), and rate of force development (change = 34%, d = 0.78) of the leg extensors after four weeks of balance training in adolescent boys and girls (M age = 19 yr., SD = 2). Further, Taube, Kullmann, Leukel, Kurz, Amtage, and Gollhofer (2007) observed significant improvements in countermovement jump height (no report of Cohen's d) and squat jump height (no report of Cohen's d) after six weeks of balance training in young elite athletes (M age = 15 yr., SD = 1). Finally, Bruhn, Kullmann, and Gollhofer (2004) found significantly improved squat jump height (no report of Cohen's d) after four weeks of balance training in a cohort of physical education students (M age = 23 yr., SD = 2). From a practitioner's perspective, these studies indicate that inline skating not only enhances dynamic balance but that these adaptive processes also translate to measures of dynamic strength performance. A thorough literature search revealed that there are no studies available investigating the effects of inline skating on measures of strength. This is why results from balance training studies have to be consulted to interpret our findings. The increase in countermovement jump height can most likely be explained by improvements in intra- and intermuscular coordination of the lower leg extensor muscles (Taube, Gruber, et al., 2007). Due to the fact that it is not possible to increase muscle mass during four weeks of balance training, Gruber and Gollhofer (2004) suggested that the increase in muscle strength of the leg extensors following balance training may arise from enhanced reflex contributions acting on a spinal level. More specifically, the authors speculated that withdrawal of presynaptic inhibition of sensory (Ia) afferents, belonging to the motor neurons of the acting muscles, could account for the enhanced strength performance. Given the similarities in exercise demands (e.g., weight-shifting over a
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small base of support, stop and go movements) between inline skating training and balance training, it is assumed that the aforementioned reasons could be responsible for the observed vertical jump improvements. During the same experimental period, the control group conducted a track-and-field program implemented in physical education lessons. No significant improvements in balance and strength performance were observed in the control group following training. This somewhat unexpected result can most likely be explained by differences in the training regimen between the two groups. Compared to the intervention group, those in the control group participated in a track-and-field program in which they learned the proper techniques for essential or basic skills like running, throwing, and jumping. Performance in these techniques involves large parts of the lower- and upper-extremity muscles. In contrast, inline skating training primarily focusses on lower extremity muscles. Further, a training period of four weeks implies that adaptive processes most likely occur on a neural rather than a muscular level (Taube, Gruber, et al., 2007). This suggests that despite the running and jumping exercises performed during the track-and-field program, the training duration was too short to induce improvements in jump performance. Limitations and Conclusions Two potential limitations of this study warrant discussion. Firstly, the sample size of the study was relatively small. However, other researchers who investigated comparable objectives with a similar study design used comparable sample sizes and found significant Group × Test effects (Bruhn, et al., 2004; Taube, Kullman, et al., 2007; Granacher, et al., 2010; Taube, et al., 2010). Nevertheless, future studies should incorporate larger sample sizes to confirm the present results. Secondly, the training period of this study was relatively short. However, other studies showed that adaptive processes are possible following four weeks of different types of balance training (Gruber, Taube, Gollhofer, Beck, Amtage, & Schubert, 2007; Taube, Gruber, et al., 2007; Granacher, et al., 2010). For example, Gruber, Gruber, Taube, Schubert, Beck, and Gollhofer (2007) investigated neural adaptations following four weeks of balance training using H-reflex recordings. They observed significantly reduced reflex excitability after training, with reductions in the ratio of the maximum H-reflex to the maximum efferent motor response. The results of this study illustrate that a short-term inline skating program, implemented in regular physical education classes, is a safe (i.e., no training-related injury) and feasible (i.e., attendance rate of 90%) training modality that produces statistically significant improvements in balance and strength performance in healthy children. Based on the findings, implementation of inline skating in the regular school curriculum is suggest-
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ed to provide an appealing, motivating and effective training regimen for children. Future research may link the current results with their underlying neuromuscular mechanisms. REFERENCES
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