Postural control is defined as an ability to maintain upright body position and control body movements and can be classified as either static or dynamic (19,28). There is evidence to suggest that successful development of task-specific skills requires several foundational skills, including postural control. Postural control is an important factor in regular training of young athletes leading them to the stages of maturity. However, athletes, coaches, and trainers often overlook the benefits of postural control as a training technique (17).
Balance can be disturbed by several factors including vibration, neuromuscular impairments, and fatigue (6). There is epidemiological evidence that more than half of the injuries occur late in athletic practices and that 58% of those are the result of noncontact mechanisms, suggesting that fatigue is considered to be a crucial element in injury-related sensory motor changes (13,14,27). Fatigue increases the threshold of muscle spindle discharge, disrupts afferent feedback, alters conscious joint awareness, and consequently impairs the proprioceptive and kinesthetic properties of joints (5,16). These data suggest that somatosensory changes that occur secondary to fatigue may alter neuromuscular control. This altered control may in turn cause deficits in postural control such as decreases in stabilization time, increased postural sway, and difficulties in maintaining the base of support during the dynamic task (10,11,30). Referring to the definitions of static and dynamic postural control, static postural control measurement tools may be of limited value to distinguish underlying injury mechanisms. Conversely, dynamic postural control is important to athletes and sport teams because disturbances in dynamic balance are associated with reactive and compensatory movements that are linked to lower extremity injuries (2). Thus, measuring dynamic postural control may be very informative in terms of reducing injuries that are linked to sensorimotor control.
Studies have typically focused on comparing the effects of fatigue on dynamic balance performance of injured and uninjured athletes. Steib et al. (24) examined changes in dynamic balance of athletes with a history of ankle sprain compared to uninjured controls after a fatiguing treadmill running protocol. Fatigue-induced changes of dynamic postural control were greater in injured than those in uninjured athletes (24). Few studies have investigated dynamic balance changes and their underlying mechanisms during fatigue in healthy athletes (2,11,24,30). In a study by Zech et al. (30), male team handball athletes ran on a treadmill to induce physical fatigue followed by a star excursion balance test (SEBT). Their results demonstrate that sensorimotor mechanisms responsible for recovering dynamic postural control in healthy male athletes remain predominantly intact when they are fatigued (30). Although their results may not be directly applicable to the female athletic population, their results support that fatigue does not change dynamic postural control in healthy male athletes. Proper balance control in athletes may link mainly to better muscular synergy that minimizes displacement of the center of gravity. However, it remains unknown whether novice or healthy individuals demonstrate the same postural control strategies as experienced players do when fatigued. The assessment and the periodic monitoring of dynamic postural control during training sessions can be important in young novice individuals to help them prevent injury occurrences that are related to being fatigued. To the authors' knowledge, no study has investigated postural control changes after being fatigued in healthy nonathletes compared with healthy athletes. This type of study is needed to help convince trainers that a novice individual's response to dynamic balance when in a fatigued state is different than that of an experienced athlete.
This study is aimed at comparing changes in dynamic postural control after fatigue between young female athletes and nonathletes. We hypothesized that fatigue would not change the dynamic balance of female athletes and that the female athletes would have better dynamic postural control in SEBT than that of the female nonathlete group.
Experimental Approach to the Problem
The primary objective of this observational study was to compare the dynamic balance of female athletes and nonathletes in response to a fatigue protocol. Fifteen female athletes and fifteen female nonathletes were recruited for the study. Pre-SEBT was measured; all subjects then performed a whole-body fatigue protocol, after which post-SEBT was measured. Rating of perceived exertion (RPE) was measured using the Borg scale immediately before, mid-way through (i.e., after the third station), and again after performing the fatigue protocol (i.e., immediately before the post-SEBT).
Fifteen healthy female nonathletes (mean age ± SD: 16.1 ± 1.8 years; height: 161.23 ± 8.3 cm; and weight: 52.3 ± 1.1 kg) from the local high school volunteered to participated in this study. Fifteen female athletes (handball and basketball players; mean age ± SD: 16.1 ± 1.1 years; height: 164.03 ± 4.6 cm; and mass: 53.1 ± 6.7 kg) were recruited through advertisement in local clubs and word of mouth to participate in this study. Because gender effect is one of the factors that influences balance performance and the risks of lower extremity injuries, in this study, only females were selected to avoid a possible gender effect on the postural measures (7,18,22). Individuals were included as nonathletes if they were not actively participating in any sport or participated only occasionally (defined as no more than once a week). The athletes were all competing at the provincial level and had been involved in sport-specific training for at least 7 hours per week for the previous 2 years. All subjects and their guardians were briefed about the study and objectives of the research. Written informed consent was obtained from subjects who were 18 years and from parents or guardians if they were minor-age participants. They were asked to fill out a questionnaire about their physical activity level, health status, and neurological and musculoskeletal disorders or injuries. Participants were excluded if they had a history of cerebral concussions, vestibular disorders, injury to either ankle, lower extremity injuries for 3 months before testing, ear infection, upper respiratory infection, or head cold at the time of the study and previous balance training. All testing procedures were approved by the Shahid Rajaee Teacher Training University's human ethics board and were conducted according to the Declaration of Helsinki.
The subjects were informed about the objectives of the study, fatigue protocol, RPE scale, SEBT, and order of the tests on the test day. They were allowed to practice the SEBT 6 times to minimize the learning effect. The pre-SEBT was completed first, followed by the fatigue protocol, and then the post-SEBT. To determine the effectiveness of the fatigue protocol, the RPE score was measured immediately before the pre-SEBT, after the third station, and right after the seventh station of the fatigue protocol. All assessments were administered by the study personnel who were blinded to the group and information of the subjects.
Assessment of Dynamic Balance Control
Dynamic postural stability is usually evaluated by single leg-hop test (20), stability biodex (3), and posturography system (31). These procedures assess dynamic postural control in a functional situation, but they do not measure the subject's stability skill (1,3,8,23,25,26). The SEBT is the simplest and cheapest procedure with several advantages: it does not require any professional equipment; it has high validity and good reliability compared with other protocols; it evaluates functional abilities and lower-body performances in 8 directions; and consequently, subject stability skills and dynamic balance control are assessed (15,23).
The SEBT was used to assess the dynamic balance control of subjects. It consists of taping a star pattern on an even surface with 8 projections, each standing at a 45° from each other. The 8 directions are anterior, anteromedial, medial, posteromedial, posterior, posterolateral, lateral, and anterolateral (15). The foot that is normally used to kick a soccer ball was considered to be the dominant foot for the subjects. The length of dominant leg (anterior superior iliac spine to medial malleolus) was measured in centimeters to normalize the data to leg length. Normalized data allowed us to compare the results between subjects without the effect of the participant's height. The subjects stood on their dominant foot in the center of grid, whereas the nondominant foot was used to reach as far as possible in each of the 8 directions (15). The subjects were then directed to lightly touch on the ground along the line using the most distal portion of their foot and then to return to the double-leg stance without allowing contact to disturb their base of support. To avoid the effect of recovery after fatigue protocol, the entire post-SEBT measurement was completed in less than 6 minutes. The percentage of a subject's reached distance for each direction was calculated as an average of her 3 performances in each direction (centimeter) divided by her dominant leg length (centimeter) × 100. This was calculated for each subject's dynamic balance performed for each direction (11,15). The trial was discarded, and the test was repeated if the subject removed her dominant foot from the center of the grid or used the reaching leg for support (15). Sensitivity of the SEBT is well established, and many studies have confirmed that the SEBT is a reliable and a valid test to assess dynamic balance (15).
Protocol to Produce Fatigue
The fatigue protocol (7-station exertion protocol) designed by Wilkins et al. (26) was used to fatigue the subjects (25,26). It is a test that mimics the type of fatigue that takes place during actual athletic practice and game situations. The protocol is designed on and around a basketball court and consists of 7 stations. The subjects jogged moderately at a self-selected pace for 5 minutes in station 1. In station 2, they sprinted up and down the length of a basketball court for 3 minutes. Station 3 was 2 minutes of push-ups. Sit-ups were performed in station 4 for 2 minutes. Station 5 was 3 minutes of 30-cm step-ups. Station 6 was another 3 minutes of sprinting up and down the length of basketball court. The final station was 2 minutes at the fastest speed that each subject could run.
Quantifying the Amount of Fatigue
Rating of perceived exertion was measured using the Borg scale to quantify the amount of fatigue felt by subjects (25). The Borg scale was developed to allow individual to rate their level of exertion. The test has 15 scores (6–20 scale) and is associated with heart rate and V̇o2max, which are reflective of the exercise intensity and workload. The higher values mean more workload, more V̇o2max, and more feeling of fatigue (23). The test was taken just before the subjects' pre-SEBT to ensure that they were not already fatigued. It was also taken at 2 additional points: right after the third station and again after the seventh station of the fatigue protocol to measure the effectiveness of the fatigue protocol (23).
Statistical analyses were performed using SAS software version 9.2 (Version 9.2; SAS Institute Inc., Cary, NC, USA). To represent statistical significance, the cutoff value was set to be p ≤ 0.05 for all measures. Mean and SEM were calculated for the pre-SEBT and post-SEBT performances in each of the 8 directions, and also pre-RPE, middle-RPE, and post-RPE scores. A 3-way factorial analysis of variance (ANOVA) design with 2 (group) × 2 (time test) × 8 (direction) was used to compare the athletes' and nonathletes' pre-SEBT and post-SEBT performances in 8 directions. One-way repeated-measures ANOVA was performed for pre-RPE, mid-RPE, and post-RPE scores. Tukey's post hoc multiple comparison testing was used to test pairwise differences between the mean values.
The sample size calculation was based on the result of a study by Filipa et al. (9) who examined the effect of neuromuscular training on dynamic balance of female athletes. Using the results, the number of 15 participants for a power of 85% was estimated to be sufficient to detect differences in outcome.
Measures of the Perceived Fatigue
The RPE score was recorded just before the first station (pre), immediately after the third station (mid), and after the seventh station (post) of the fatigue protocol. Data analysis showed that the effect of time was significant (p < 0.001), and post hoc pairwise comparisons showed that differences in RPE scores between pretest and midtest (p < 0.001), pretest and posttest (p < 0.001), and midtest and posttest (p < 0.001) were significant. Rating of perceived exertion scores, as shown in Table 1, increased with an increase in workload. The RPE score after the seventh station (post) for all subjects was greater than 15.
Dynamic Balance Performance
Three-way ANOVA (2 [group] × 2 [time] × 8 [direction]) analysis on SEBT data showed that the main effect of group (p = 0.001), direction (p = 0.0001), time (p = 0.01), and the 2-way interaction of group × time (p = 0.005) and group × direction (p = 0.05) was significant. However, the 2-way interaction of direction × time (p = 0.26) and 3-way interaction of group × time × direction (p = 0.35) were not found to be significant.
The balance performance of the nonathlete group decreased after fatigue, and the amount of reduction was significant (p = 0.0003). In contrast, the athlete group had no significant changes in balance performance before and after the fatigue protocol (p = 0.78). Overall, the athlete group had better balance performance in pretests and posttests; differences between the 2 groups in pre-SEBT (p = 0.005) and post-SEBT (p = 0.0001) were significant (Table 2). The interactions of groups by directions were significant (p = 0.05) (Table 3). The Tukey's post hoc test showed that the differences of balance posttest performance between athlete and nonathlete participants in medial (p = 0.0072), posteromedial (p = 0.0121), and the posterior (p = 0.004) directions were significant. However, the differences in the other 5 directions (anteromedial, anterior, anterolateral, lateral, and posterolateral) were not found to be significant between the 2 groups. Moreover, both groups had maximum and minimum balance performances in posterior and lateral directions, respectively (Table 3).
The results of the Borg 6–20 RPE scale data analysis revealed that all subjects had RPE scores of more than 15 at the end of the seventh (final) station of fatigue protocol, and the mean of RPE scores in both groups increased over time. Results of RPE suggest that the protocol used was sufficient to induce fatigue as all RPE values exceeded a score of 15 (23).
There was a significant difference among the groups in pre-SEBT. Therefore, the analysis of covariance was tested considering pre-SEBT as a covariate to adjust its effect on post-SEBT. The results of covariance analysis and 3-way ANOVA were statistically the same. However, covariance analysis controls the effect that is not of primary interest and does not allow for comparison within group changes in SEBT (i.e., pre-SEBT to post-SEBT changes within each group). Thus, in this study, the results of 3-way ANOVA are presented.
The results of this cross-sectional dynamic postural control study support the hypothesis that nonathletes display greater fatigue-induced impairments in postural control compared with athletes. The SEBT reach distance diminished because of fatigue in the nonathlete group, and the amount of decrease was significant (p = 0.0003). However, there were no significant changes in the SEBT performance of the athlete group because of fatigue, suggesting that athletes have better dynamic postural control stability management when fatigued.
Fatigue-related impairment in dynamic postural control of athletes has been previously demonstrated by a number of researchers (4,10,24,30). Steib et al. (24) investigated changes in dynamic postural control in healthy and injured athletes, and their results support our findings that there are no differences in mean SEBT before and after fatigue performance for healthy athletes. Gribble and Hertel (10) used an isokinetic dynamometer to induce the fatigue in the sagittal-plane movers of the hip, knee, and ankle followed by a 3-dimensional SEBT test of an athletic population. In contrast to our findings, they reported that SEBT reach distance in athletes diminished significantly in medial and posterior directions (10). It is most likely that the different fatigue protocols, different types of subjects, or gender-confounding effects may be responsible for the different results. For example, Gribble and Hertel (10) used a group of females and male physically active subjects to be the athlete group who remained active at least 3 times per week for 30 minutes. Conversely, in our study, athletes were all women who were competing at the provincial level and had been involved in sport-specific training for more than 7 hours per week for the previous 2 years.
Although our results showed that fatigue did not alter dynamic postural control scores of athletes in any direction, nonathletes postural control was reduced significantly in the medial, posteromedial, and posterior directions. This study attempted to report on dynamic postural control of healthy female nonathletes after fatigue and compare it with female athletes, an area on which published data are currently unavailable. Thus, we cannot offer an extensive direct comparison of the results of this study with other work. However, it has been demonstrated that lower SEBT reach distance is associated with an increased risk of lower limb injury (30), and SEBT has the potential to predict lower extremity injury (12). Different muscles are involved in the balance performance of different directions (8). The SEBT involves contractions of the hamstrings and quadriceps muscles in all directions. The quadriceps muscles consist of vastus medialis, rectus femoris, vastus lateralis, and vastus intermedius muscles. Vastus lateralis is mostly involved in the dynamic balance performance of medial and posteromedial directions (8). However, the biceps femoris muscle, which is a member of the hamstring musculature, is involved in the posterior, posterolateral, and lateral directions. Star excursion balance test reach distances have been also linked to knee flexibility (21), neuromuscular coordination, and strength (18). Therefore, persisting sensorimotor control deficits rising from physical fatigue may link to poor flexibility, strength, and neuromuscular coordination in this group of muscles. This would eventually result in poor dynamic balance performance of female nonathletes and increases the risks of injuries. Coaches and health care professionals should evaluate and monitor dynamic postural control of young inexperienced individuals during training session and with progressing the exercise time to prevent injury occurrences. Closer examination of the results could also help develop muscle-specific training for novice and inexperienced individuals. Athletic trainers may also consider neuromuscular training (29) followed by exercises aimed at increasing fatigue resistance for these groups of muscles.
Nonathletes presented greater fatigue-induced impairments in dynamic postural control performance compared with athletes. The impairments were in the medial, posteromedial, and posterior directions.
This finding underlines the importance of dynamic postural control stability in trainings and emphasizes on assessment and periodic monitoring of dynamic postural control of the novice with progressing the exercise time to prevent fatigue-related sport injuries. The finding of this study will also help coaches develop muscle-specific training for novice individuals to prevent injury at an earlier age by focusing on the 3 aforementioned directions and aimed at training to increase fatigue resistance. Given that gender effect is one of the factors affecting dynamic postural control, this study attempted to investigate impacts of physical fatigue on dynamic postural control of female athletes and nonathletes; thus, results of this study cannot be generalized to the male population. It would be helpful, therefore, for future studies to compare the fatigue-related impairments in dynamic postural control between men and women.
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