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Unilateral Balance Performance in Female Collegiate Soccer Athletes

Thorpe, Jennifer L; Ebersole, Kyle T

Journal of Strength and Conditioning Research: September 2008 - Volume 22 - Issue 5 - p 1429-1433
doi: 10.1519/JSC.0b013e31818202db
Original Research
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Thorpe, JL and Ebersole, KT. Unilateral balance performance in female collegiate soccer athletes. J Strength Cond Res 22(5): 1429-1433, 2008-The Star Excursion Balance Test (SEBT) is a unilateral balance task designed to evaluate dynamic postural control. This investigation explored the relationship between limb preference, strength, and performance on the SEBT in NCAA Division I, female collegiate soccer athletes (nonsoccer, n = 11; soccer, n = 12). Each participant completed maximal, concentric test efforts at a velocity of 90°·s−1 for supine ankle dorsiflexion (ADF) and plantarflexion (APF) and at 60°·s−1 for the seated leg extension (LE) and flexion (LF) and supine hip extension (HE) and flexion (HF). In addition, participants performed maximal SEBT reaches in the anterior, medial, and posterior directions. All testing was completed on both limbs. These results indicate that SEBT performance is similar for both limbs in both groups. The soccer group, however, reached significantly farther than the nonsoccer group, suggesting that the SEBT may be sensitive to training status and/or sport-related adaptations. The concentric strength results indicate that despite group differences in all strength tests, strength in general was not highly correlated to SEBT performance. Thus, neuromuscular factors above and beyond strength may have accounted for the group differences in SEBT performance. The SEBT may be a useful tool for determining the relative effectiveness of an intervention designed to improve postural control.

University of Illinois, Department of Kinesiology and Community Health, Urbana, Illinois

Address correspondence to Kyle T. Ebersole, ebersole@uiuc.edu.

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Introduction

The Star Excursion Balance Test (SEBT) is a unilateral, functional joint-stability task that incorporates a single-leg stance of one leg with a maximum targeted reach of the free leg (3,4-8,12,13). The SEBT has been described as a test of dynamic postural control because it challenges an individual's ability to maintain a stable base of support while performing the reach movement (5). Thus, the SEBT may provide an effective noninstrumented and clinically applicable test for use in assessing lower-extremity balance and neuromuscular control.

Performance on the SEBT is quantified by measuring the distance reached with the free leg, and greater reach distance is associated with greater postural control. Gribble and Hertel (5) have reported that normalizing raw SEBT reach scores to leg length eliminated the gender difference in the raw reach scores. Earl and Hertel (4) have reported direction-specific muscle activation patterns for the muscles of the quadriceps, hamstrings, and lower leg. A few studies (3,6,7,12) have reported that chronic ankle instability is associated with decreased reach distances on the SEBT performance, with conflicting outcomes. Gribble et al. (6) suggest that a reduction in hip and knee flexion angles in the chronic ankle instability group may have been associated with the decrease in SEBT performance. Nakagawa and Hoffman (11), however, have reported an absence of difference in SEBT scores between the injured and uninjured limbs in a population with a history of recurrent ankle sprains. Although the lack of agreement between these may be related to injury severity differences, additional research is warranted to better understand the factors that may influence SEBT performance.

The SEBT was recently demonstrated to have some injury predictive value. Plisky et al. (13) report that healthy boy and girl high school basketball athletes with limb differences in SEBT performance were 2.5 times more likely to sustain lower-extremity injuries during the season. The results by Plisky et al. (13) as well as those involving chronic ankle instability (3,6,7,12) may, in part, be associated with losses of strength. To date, however, no prior study has examined the influence of strength on SEBT performance.

Previous research has reported that, in healthy individuals, SEBT performance is similar across the limbs (5,7). In an injured population, there is limited evidence to support the idea that the SEBT is able to discriminate between the injured and uninjured ankle (6,7). Furthermore, Plisky et al. (13) have provided initial evidence to suggest that limb asymmetry in SEBT performance in apparently healthy basketball players may be associated with future injury. It is unclear, however, whether the SEBT would demonstrate limb differences in a population of well-trained soccer athletes, a sport in which most athletes have a clear limb preference when kicking the ball and performing other sport-related skills.

Although previous studies have found that limb preference has not resulted in limb asymmetries in strength (1,2,9,10,13) for soccer players, it is possible that limb preference may influence a postural control task such as the SEBT performance. To fully characterize the potential applications of the SEBT in an athletic population, the relationship between SEBT performance and strength as well as the influence of limb preference on SEBT performance needs to be explored. Therefore, the purpose of this study was to examine the influence of limb preference and strength on SEBT performance in a group of NCAA Division I collegiate soccer athletes.

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Methods

Experimental Approach to the Problem

The SEBT is a postural control task, and performance on the SEBT may be influenced by strength as well as athletic status. To examine this problem, NCAA Division I collegiate soccer athletes and recreationally active nonsoccer collegiate females were recruited to complete a series of unilateral, lower-extremity isokinetic strength tests as well as the SEBT for both limbs.

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Subjects

Eleven college-aged, recreationally active nonsoccer females (mean ± SD; 20.5 ± 1.0 years; 69.6 ± 9.8 kg; 168.9 ± 11.8 cm) and 12 NCAA Division I collegiate female soccer student athletes (19.5 ± 1.0 years; 64.9 ± 5.5 kg; 168.6 ± 5.0 cm) volunteered to participate. This investigation was approved by university institutional review board for human subjects. All participants completed a self-report health history questionnaire and signed a written informed consent before testing. All participants were screened for lower-extremity (ankle, knee, hips) bone and joint injuries and abnormalities as well as for conditions (i.e., concussion, inner-ear disorders, upper-respiratory infection, etc.) that may influence balance. Any participant self-reporting the presence of any injury or condition within the last 12 months was excluded from the study. The nonsoccer participants were required to be actively engaged in regular (3-4 days per week) fitness activities (i.e., running, resistance exercises, swimming) but to have no prior formal experience in soccer. The soccer group participants were all from the same, highly competitive NCAA Division I team. The data collection for all participants was completed relative to the off-season time period for the soccer group (January to February) and before the start of any formal spring season and conditioning program. All subjects attended two testing sessions separated by 48-72 hours. The first test session evaluated muscle strength and allowed for practice of the SEBT. The second test session involved SEBT testing. All strength and SEBT testing was done on both limbs, and each participant did not have prior experience with the SEBT.

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Anthropometric Measurements

To normalize the SEBT reach distances, leg length was quantified as the distance (cm) from the anterior superior iliac spine to the center of the ipsilateral medial malleolus with the participant in a supine position. Limb preference was determined by (a) asking each participant which limb she would use to kick a ball, (b) noting which limb each participant led with when asked to walk up a flight of stairs, and (c) placing a ball in front of each participant and noting which limb the participant used to kick the ball.

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Isokinetic Strength Testing

Before the start of the strength tests, each participant completed a 5-minute warm-up on a cycle ergometer at a load of approximately 60 W. All strength measures were completed on the Humac Norm Isokinetic dynamometer (Computer Sports Medicine, Inc., Stoughton, Mass) with the participant positioned in the Humac Norm chair according to the guidelines provided by the manufacturer using testing velocities consistent with previous literature (1,9,10,14). Each participant completed four practice trials followed by four maximal, concentric test efforts at a velocity of 90°·s−1 for supine ankle dorsiflexion (ADF) and plantarflexion (APF) and at 60°·s−1 for the seated leg extension (LE) and flexion (LF) and supine hip extension (HE) and flexion (HF). A 2-minute rest period was provided between each velocity. The order of limb (right and left) and movement (ankle, leg, hip) was counterbalanced across all participants.

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Star Excursion Balance Test Protocol

To minimize the potential for a learning effect (8), participants were provided initial SEBT instruction and practice (six reaches in each direction) in test session 1 on completion of the strength measurements. On returning to the laboratory for test session 2, participants were reminded of the instructions and were provided another set of six practice reaches in each direction before the test reaches. The SEBT procedures were similar to those used in previous investigations (4,8). However, only the anterior (A), medial (M), and posterior (P) reach directions were used in this study (Figure 1). While maintaining a single-leg stance, each participant was asked to maximally reach along the identified line with the contralateral limb and lightly touch the line with the distal part of the foot. All reach trials began with both feet in contact with the ground and with the stance leg appropriately positioned in relation to the center of the SEBT grid (4,8). The participants were instructed to keep their hands on their hips while performing this task. Failed test criteria included (a) if the reach foot was placed in contact with the ground for support, (b) if the stance foot was moved or lifted, (c) if equilibrium was lost during any part of the reach or return phase, and (d) if the point of contact was to either side of the taped line. A total of five test reaches were performed in each direction. Reach distance was quantified by measuring the distance (cm) from the center of the crosshairs to the point of distal foot-ground contact marked in ink by the investigator. Star Excursion Balance Test performance for each direction was represented by an average of the three best reach distances and normalized to leg length. A 1-minute rest was allowed between directions. The order of the reach direction and starting limb were counterbalanced for all participants.

Figure 1

Figure 1

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Statistical Analyses

For each concentric muscle action, peak torque was normalized to body weight (kg) and used as the representative strength value. To examine the limb (right, left) and group (soccer, nonsoccer) differences in normalized strength, separate two-way mixed factorial analyses of variance (group × limb) were performed for each concentric muscle action (ADF, APF, LE, LF, HE, HF). To examine the limb (right, left) and group (soccer, nonsoccer) differences in normalized SEBT reach distance, separate two-way mixed factorial analyses of variance (group × limb) were performed for SEBT direction (A, M, P). Pearson product-moment correlation analysis was performed to explore the relationship between SEBT performance and the lower-extremity strength measures. An alpha level of 0.05 was used for all analyses.

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Results

On the basis of the limb preference tests, the right limb for all participants was considered the preferred limb. Tables 1 and 2 describe the normalized group mean strength and SEBT reach data, respectively. There was no significant (p > 0.05) group-by-limb interaction or a main effect for limb for each muscle action (ADF, APF, LE, LF, HE, HF) and SEBT reach direction (A, M, P). There was a significant main effect for the between-subjects factor of group (collapsed across limb) for each strength measurement and SEBT performance in the A and P directions. Follow-up analyses indicated that the soccer participants were significantly stronger than the nonsoccer participants, and they reached significantly farther in the anterior and posterior directions. Bivariate correlations ranged from r = 0.00 to 0.66 and from r = −0.58 to 0.20 for the nonsoccer (Table 3) and soccer participants (Table 4), respectively.

Table 1

Table 1

Table 2

Table 2

Table 3

Table 3

Table 4

Table 4

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Discussion

The results of this investigation indicate that there was a balance in strength across the limbs and that limb preference did not result in limb differences in SEBT performance. The lack of limb difference in the strength of the quadriceps and hamstring muscle groups was consistent with previous investigations involving soccer athletes (1,2,9,10,13). The current findings also suggest that there was no limb difference in ankle (APF, ADF) and hip (HE, HF) strength. Previous investigations have not reported ankle and hip strength values for both limbs in soccer athletes, thus it is difficult to draw comparisons with the current results. In the present investigation, there was no limb difference in SEBT reach performance for either group. The similarity in reach performance for both limbs in both groups (collegiate soccer and nonsoccer) was consistent with the findings of previous investigations (5,7) and provides further support that healthy limbs will demonstrate similarities in SEBT reach performance.

Recently, a prospective study by Plisky et al. (13) has reported that reach deficits between limbs of boy and girl high school basketball players were associated with increased risk for lower-extremity injury during the season. Chaiwanichsiri et al. (3) have reported that when the SEBT was used as a training task as part of the rehabilitation for a grade 2 ankle sprain (3 days per week for 4 weeks), single-leg stance time increased in the involved limb more than twice than in the control group (grade 2 ankle sprain, but no inclusion of SEBT training). Gribble et al. (6) have demonstrated that SEBT performance deteriorated after completion of a fatigue task. These findings (3,6,11) suggest that changes in strength or general muscle function will influence SEBT performance. Similarly, the greater SEBT performance in the present study by the soccer group may be associated with a training effect. That is, the regular training performed by the soccer group resulted in greater lower-extremity strength and, possibly, neuromuscular control across both limbs. As a result, the soccer group had greater SEBT reach distances. Additional research is warranted to determine the sensitivity of the SEBT to training-both strength and training programs designed to improve neuromuscular control and/or postural stability.

Correlation analyses in the present investigation were performed within each group to explore the relationship between reach directions as well as strength and SEBT performance. Hertel et al. (7) used correlation analyses to determine redundancy across the eight reach directions. In the present investigation, the correlations between reach directions varied by group (Tables 3 and 4). These findings suggest that any redundancy in information captured at each reach direction was, in part, associated with the training status of the participants, where greater redundancy was observed in the soccer participants. In the nonsoccer participants, each the A and P directions seemed to provide unique information relative to postural control during the SEBT.

In general, correlation analyses indicated low to, at best, moderate correlations between SEBT performance and lower-extremity strength. For the nonsoccer group, the posterior reach direction was the only direction with significant strength relationships (ADF, APF, HE). In the soccer group, the only significant correlation was between HE and posterior reach. Furthermore, the correlations between strength and reach performance for the soccer group were not only weaker than the nonsoccer group, but also negative. The negative relationship is difficult to explain, but it may suggest that the relationship between SEBT performance and strength is limited, and additional factors such as muscle activation may have a stronger relative relationship to SEBT performance. It is also possible that the absence of a relationship between strength and SEBT performance for many of the strength variables may be attributable to the fact that there was not enough variance within the data to detect any relationship. Alternatively, the SEBT is a two-phase task consisting of a down (eccentric) and up (concentric) phase. The present study quantified maximal strength through concentric, isokinetic muscle actions, and quantification of eccentric strength may result in a stronger relationship between SEBT performance and strength (14).

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Practical Applications

The results of this investigation provide further support that SEBT performance will be similar for both limbs in a healthy population. The results also suggest that the SEBT may be sensitive to level of training, because the soccer participants' reach was significantly greater than that of the nonsoccer participants. The fact that the soccer participants were significantly stronger in all lower-extremity strength tests did not seem to be related to SEBT performance, and it may simply reflect the differences in training status. Lastly, the group differences in reach performance suggest that the SEBT may be sensitive to training and could, perhaps, be a useful tool for determining the relative effectiveness of an intervention or training program designed to improve postural control.

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Acknowledgements

The authors would like to thank the women's soccer team at the University of Illinois for their participation and support of this study.

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References

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Keywords:

strength; limb preference; postural control

© 2008 National Strength and Conditioning Association