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Relationships Between Asymmetries in Functional Movements and the Star Excursion Balance Test

Overmoyer, Grant V.1; Reiser, Raoul F. II1,2

The Journal of Strength & Conditioning Research: July 2013 - Volume 27 - Issue 7 - p 2013–2024
doi: 10.1519/JSC.0b013e3182779962
Original Research

Overmoyer, GV, and Reiser, RF. Relationships between asymmetries in functional movements and the star excursion balance test. J Strength Cond Res 27(7): 2013–2024, 2013—Lower-extremity functional asymmetries (LEFAs) have been shown to be related to performance and injury risk. However, consistency of expression between tasks is not well understood. The goal of this investigation was to examine relationships in vertical ground reaction force LEFA during standing, bodyweight squats, countermovement jumps (CMJs), and single-leg drop landings along with those produced in the Star Excursion Balance Test (SEBT). Twenty (9 men, 11 women) healthy, recreationally active young adults (mean ± SD age: 21.9 ± 2.6 years; height: 171 ± 8.8 cm; mass: 67.2 ± 1.9 kg) performed all tests in a single visit. Correlations of asymmetries between tasks as a whole group and in subsets with larger levels of asymmetries in each task were examined. Many significant correlations (p < 0.05) were found in the asymmetries between the functional tasks, between the reach directions of the SEBT, and between the functional tasks and the SEBT, though they were of low to moderate strength (|r| < 0.8) in the whole group. Except for standing, correlations typically improved in the subset analyses. Most noteworthy was the CMJ subset, which demonstrated strong relationships (|r| > 0.8) with asymmetries in the squat and with the SEBT. Correlations between reach directions in the SEBT improved in the subset comparisons but would not be considered strong. The results suggest that asymmetries are more likely to be expressed in multiple tasks as the bilateral difference increases, that intensity of effort plays a key role in the expression of asymmetries during bilateral tasks, tasks most relevant to the sport should be used when assessing athletes, and though not replacing functional tasks, the SEBT may serve an important role in the diagnosis of LEFA.

Department of 1Health and Exercise Science

2School of Biomedical Engineering, Clinical Biomechanics Laboratory, Colorado State University, Fort Collins, Colorado

Address correspondence to Raoul F. Reiser, raoul.reiser@colostate.edu.

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Introduction

Functional asymmetries, as discussed here, are side-to-side differences in kinetics and kinematics during performance of otherwise symmetric tasks. Although functional asymmetries might be expected in injured or physically disabled individuals, measurable levels of functional asymmetries have also been found to be commonplace in healthy populations. For example, lower-extremity functional asymmetries (LEFAs) have been documented in landings (36), squatting (16,23), quiet standing (3,31), and various jumping and hopping tasks (1,23,35,41). Functional symmetry is desirable in some sport-related performances (6,20). Also, it has been shown that appropriate screening for asymmetry may serve as a predictor of injury or reinjury (26,37) and in some circumstances help to identify a unilateral deficit (7,25,30,33,35). The LEFA has been shown to be related to a few sources (i.e., factors that may directly cause someone to function asymmetrically) including side-to-side differences in strength (23,35), anthropometry (4,37), neural control (38), and potentially flexibility (18). Although tests that directly relate individual sources of asymmetry to a single movement pattern may be useful for isolating specific bilateral deficits, it would also be useful to understand how one functional assessment or movement pattern might relate to another. By correlating several asymmetry assessment methods, it may be possible to isolate a singular deficit or potentially identify multiple asymmetries at once with more time efficiency.

One common way to assess LEFA is by measuring bilateral differences in vertical ground reaction forces (GRFv) under the feet (3,7,25,26,30,31,33,35,37). Unfortunately, the equipment to accurately and reliably measure GRFv is typically expensive and lacks portability for widespread use. Therefore, additional options to assess LEFA are also desirable. A test that has been used in screening for LEFA, which is relatively inexpensive, portable, and easy to administer, is the Star Excursion Balance Test (SEBT). The SEBT requires maintaining balance with 1 leg while reaching in various directions (aka, excursions) in a star-like pattern with the other. The SEBT has been used to identify injury risk and serve as a measure of unilateral balance and neuromuscular control. See Hertel and coworkers (12) for a review.

Although the SEBT may be a promising replacement for more complicated screening methods such as GRFv measures, it is not clear if this test measures similar attributes. Task-specific factors have been shown to affect functional asymmetry, including level of intensity (1,5,30,34), speed of movement (8,34,40), and fatigue (16). Furthermore, the form of muscle contraction involved (e.g., concentric vs. eccentric or isometric) may also effect asymmetry expression, as takeoff and accelerational movements have been found to differ in asymmetry level compared with landing and decelerational movements (43). Other factors such as balance requirements, in-phase vs. out-of-phase activities and technical requirements of a task or movement pattern have not been examined, but they may also play a role in the level of asymmetry expressed.

Overall, LEFAs are very common, but there is a general lack of understanding regarding factors that affect the expression of LEFAs during task performance. Furthermore, there is little current information relative to whether LEFA in 1 task will be expressed similarly in other tasks. Therefore, it is questionable as to how much LEFA screening methods can be generalized to other movement patterns. Additionally, although the SEBT may be able to identify lower-extremity asymmetry, it is not even clear if asymmetries expressed within one excursion carry over to another excursion. Therefore, the goal of this investigation was threefold: (a) to explore the relationships between GRFv asymmetries identified in commonly studied functional lower-extremity tasks of various speeds and intensity levels, (b) to explore how asymmetries in SEBT excursions relate to each other, and (c) to explore the relationships between GRFv asymmetries between functional movements and the SEBT. Based on the limited relevant literature, most specifically Newton et al. (23), it was hypothesized that (a) asymmetries will be related between the functional tasks, (b) asymmetries in the excursion directions within the SEBT will be related, and (c) asymmetries will be related between the functional tasks and the SEBT. An understanding of these relationships will improve future screening methods for lower-extremity asymmetry relative to both performance and injury risk.

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Methods

Experimental Approach to the Problem

To accomplish the goals of this investigation, a cross-sectional research design was used. Healthy, active, pain-free subjects with limited previous injury were recruited. During a single visit lasting approximately 2 hours, subjects performed in order the SEBT, a battery of 9 active range of motion (AROM) measures, and 4 functional movements after adequate warm-up and practice. The functional movements included quiet standing, bodyweight squats, countermovement jumps (CMJs), and single-leg drop landings (SLDLs). The LEFAs during the functional movements were assessed via GRFv sampled from dual force platforms. The AROM measures, completed in approximately 45 minutes while in the seated or laying position, were not included in this analysis. Approximately 50% of the subjects returned for a second visit to repeat the measures (at least 2 days after the initial visit, but not later than 31 days). All measurements were made by a single researcher. To test the proposed hypotheses, relationships within and between measures were assessed through correlations. Specifically, GRFv asymmetries between the functional movements were correlated to each other, SEBT excursion direction asymmetries were correlated to each other, and GRFv asymmetries were correlated to the SEBT asymmetries. To understand the role that the magnitude of asymmetry may play in the relationships, correlations were performed with both the whole group and subsets of those that were more asymmetric in each task.

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Subjects

Twenty (9 men, 11 women) nonobese (body mass index <30 kg·m−2), healthy individuals aged 19–30 years (Table 1) participated after initial screening via a brief health and activity questionnaire. All the subjects granted university-approved written informed consent after having the study explained to them. The subjects were identified as individuals who were currently recreationally active in exercise or activities requiring jumping and squatting type movements. To be considered recreationally active and meet inclusion criteria, they must have been involved in activities requiring these movement patterns at least once per week for the past 8 weeks based on self-report and recall. Activities included, but were not limited to, resistance training, step aerobics, basketball, racquetball, baseball, and soccer. Other exclusion criteria included self-reported pain, injury, or soreness to the lower back or lower extremities at the time of the visit. Any injuries must have healed with a return to regular activity at least 4 weeks before participation. Those with a history of back or lower-extremity pain; major previous surgery; bone, joint, or muscular disorder; history of neurological and orthopedic dysfunction; or pain that would limit the ability to perform functional tasks correctly, were excluded. Any subjects with a known reason to perform these activities asymmetrically for anthropometric or orthopedic reasons were also excluded (e.g., known limb-length discrepancy, bilateral corrective devices, braces, anterior cruciate ligament tear, chronic ankle sprains). Pregnant women were also excluded.

Table 1

Table 1

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Procedures

All the subjects completed the initial visit to the laboratory with 11 of them returning a second time for the repeatability assessment. The subjects were instructed to abstain from heavy exercise of the lower limbs and back for 48 hours before the visit and to limit exercise to normal daily activities for the lower body the day before and day of testing. Caffeine consumption was limited to normal daily intake, and no other ergogenic aids were allowed for consumption during the previous 24 hours leading up to the visit. Requirements and procedures of the second visit were identical to those of the initial visit.

After arriving and verifying eligibility, the subjects were given orientation to the laboratory followed by measurement of bodyweight and height without shoes. The subjects wore clothing appropriate for physical activity (i.e., t-shirt and shorts). A warm-up of not <5 minutes on a stationary cycle ergometer was required for all the subjects. Stretching was not allowed at any point during the study to avoid confounding factors that might be caused by stretching one side more vigorously than the other. After the warm-up, functional leg-length asymmetry and anatomical leg-length measurements were taken with the subject in the supine position on an elevated examination table as described by Hinson and Brown (14) and Evans (10), respectively. Briefly, functional leg-length asymmetry is a measurement of the distance one limb protrudes relative to the other when lying relaxed, whereas anatomical leg-length is a boney measure from the anterior-superior iliac spine to the medial malleolus. Functional and anatomical measurements were taken twice with the left side always measured first during the anatomical measurement and the 2 measurement average used for analysis.

The SEBT was then performed in stocking feet as described by Plisky et al. (28) in 3 specified excursion directions: to the front (Ant), diagonally back and to the medial side of the reaching leg (PostMed), and diagonally back and to the lateral side of the reach leg (PostLat) (Figure 1). The distal aspect of the subject’s great toe was centered at the junction of the star. The subject performed a reach in all 3 directions on the left foot (Ant, PostMed, then PostLat), followed by all 3 on the right foot. This was then repeated for a total of 6 correct practice trials. Criteria for a correct trial were that the heel of the plant foot stayed in contact with the ground, the hands remained on the hips at all times, the subject lightly brushed the measuring tape at the furthest point possible without planting weight onto the reaching foot, and the subject recovered to a single-leg standing position for at least 2 seconds. After the 6 practice trials, 3 more correct trials for which data were recorded were completed in the same manner. The researcher visually identified the furthest point reached on a tape measure. A fourth or fifth trial was granted if excursion distance appeared to still be improving.

Figure 1

Figure 1

Functional movements were then performed wearing shoes consistent with physical activity (i.e., tennis, running, crosstraining, etc.). The GRFv were sampled at 1,000 Hz with two 4060-10 force platforms (Bertec Corp., Columbus, OH, USA) mounted side by side and flush to the surrounding floor. The force platforms were verified to measure within 0.1% of each other within the range of measures of the study (excluding high magnitude instantaneous peak forces during SLDL that could not be replicated adequately for evaluation of the platforms).

Five 20-second trials of quiet standing were first performed with 1–2 minutes of rest between trials. The subjects stood with eyes open, looking straight ahead, hands relaxed at the sides with feet approximately shoulder width apart. The subjects were instructed to stand as naturally as possible, not leaning to one side or the other, with minimal movement.

Second, 6–7 bodyweight squats were performed. Each foot was again positioned approximately shoulder width apart, and squats were performed with approximately 1-second down and 1-second up tempo, with clear pauses in between dictated verbally by the researcher. During squatting, the subjects looked straight ahead, dropping down to visually approximated thighs parallel to the ground position, maintaining hands on hips. The subjects were given brief practice in to obtain correct tempo and form before data were collected.

Third, the subject performed CMJs after familiarization with the movement. Six to 7 maximal effort CMJs were performed with the feet approximately shoulder width apart. Emphasis was placed on jumping as high as possible, with no specific requirements to landing technique or location. The subjects looked forward and maintained their hands on the hips. Tempo and countermovement depth were not dictated, as to allow the subjects to jump as naturally as possible. Again, clear pauses between each repetition were dictated verbally by the researcher.

Finally, 5 SLDLs from a 30.5-cm platform were performed on each foot. The subjects were given an equal number of practice trials on each foot, until they felt comfortable with their ability to perform the movement. The SLDL was performed while maintaining a hands-on-hips position and by stepping forward smoothly off of the front of the platform and sticking the landing on the lead leg with the contralateral leg neither touching the ground nor the platform. The SLDLs were alternated between sides starting with the left leg, landing on the same force platform each time. Other than landing similarly on each foot, the landing technique was not dictated by the researcher to encourage the subjects to land as naturally as possible.

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Data Processing and Analyses

The SEBT was analyzed using the average of the last 3 trials for each reach direction for each lower extremity, and the average of the composite of the 3 reach directions. The composite was created by summing the 3 directions. The maximum value measured for each excursion direction was also analyzed, including a summed composite score of the maximums for each lower extremity. To control for the effect of limb length between subjects, SEBT values were normalized to percent of average anatomical limb length (average of right and left sides). The normalized preferred kicking leg (KL) SEBT score (reaching with KL standing on NKL) was subtracted from that of the nonkicking leg (NKL) score to quantify the presence of asymmetry (% NKL − KL). Lower extremities were separated based on KL and NKL to identify the neurologically dominant side (KL) from the typically more strength dominant side (NKL) (11,21,39).

For the LEFA analysis, GRFv for the standing, squatting, and CMJ trials were low-pass filtered at 15 Hz (fourth-order, recursive Butterworth) to remove any high-frequency noise. To maintain the high-frequency peak of the GRFv during landing, the SLDL forces were not filtered but were visually verified to be free of extraneous high-frequency noise. For the standing trials, the average GRFv of each foot during each of the 20-second trials was normalized to a percentage of the average total GRFv (i.e., the sum of left and right sides). In this way, a score of 50.0% on each side would represent a perfectly symmetric trial. The 5 trials were averaged to obtain a representative score for each leg. Similar to the SEBT, the normalized preferred KL value was subtracted from the NKL value to quantify LEFA (% NKL − KL).

The first 5 acceptable squatting and CMJ trials were used in the analysis. Trials were discarded if the GRFv was not stable at the initiation of the movement or did not stabilize quickly at the end in the squat. The start of a repetition was defined as the time when the total GRFv (sum of right and left sides) dropped below the bodyweight. The end of a squat repetition occurred when the total GRFv returned to the bodyweight after completing the up phase, whereas the end of the CMJ repetition was defined as the point at which the total GRFv reached zero (i.e., the toes left the ground). Average and maximum GRFvs for each foot were measured for each repetition. The average GRFv on each foot was calculated as a percentage of the total GRFv over the entire repetition. The maximum GRFv asymmetry was calculated as the highest force observed for each leg, as a percentage of the maximum instantaneous total. The LEFA measures were created as above from the 5-trial average by subtracting the KL side value from the NKL side value (% NKL − KL).

For the SLDL trials, the start of a repetition was defined as the time at which GRFv exceeded zero when first contact with the ground was made. For each repetition, the following were analyzed: peak instantaneous GRFv, time to GRFv peak (amount of time from repetition start to peak instantaneous GRFv), slope (GRFv peak divided by time to GRFv peak), GRFv average to peak (average GRFv from beginning of the repetition to the peak GRFv), impulse to peak (GRFv average to peak multiplied by time to GRFv peak), and GRFv average over the first 300 milliseconds of the landing. The SLDLs were also then analyzed in a similar manner as listed above, where an average was found on both sides over the course of the 5 landing trials, for each variable measured. Percentage of bilateral total was calculated for each side with LEFA calculated by subtracting KL from NKL (% NKL − KL).

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Statistical Analyses

Means and SDs were compiled for all variables. To show the overall magnitude of asymmetries, mean and SDs of absolute levels of asymmetries were also compiled independent of NKL and KL. Paired t-tests were also performed on all NKL vs. KL values to determine the existence of significant bilateral differences. Pearson’s correlations were performed comparing asymmetries (% NKL − KL) in each individual and combination of excursions in the SEBT, each GRFv LEFA parameter of the functional movements, and anatomical leg-length differences both within and between different test types. Anatomical leg-length difference was included to verify that this was not a factor in our population related to asymmetries in any of the tasks. The same Pearson’s correlations were also performed on subsets of the total population to concentrate on the most asymmetric subjects in each functional task and the SEBT. Asymmetry level cutoffs were set so that the more symmetric subjects were eliminated. This subsetting left approximately half of the total subjects in each group. These cutoffs were asymmetries of 3.0, 4.0, 3.0, 0.6, and 6.0% NKL − KL in standing, squatting, and CMJ GRFv averages, and average GRFv for the first 300 milliseconds in SLDL and composite SEBT scores, respectively. Finally, intraclass correlations (Chronbach’s Alpha, α) were used to establish intersession repeatability of all measures reported. Statistical analysis was conducted in PASW version 19.0 (SPSS, Inc., Chicago, IL, USA) with significance set at p ≤ 0.05. Correlations with |r| ≥ 0.800 were considered to be strong.

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Results

Eighteen of the 20 subjects indicated that the right leg was their KL, and 2 indicated the left leg. General subject characteristics tended to be highly repeatable from day to day for the 11 subjects (6 women and 5 men) who returned for a second visit, except for absolute functional leg-length difference (Table 1). Repeat visits averaged 19.0 ± 9.5 days after the initial visit. Although small in magnitude, the anatomical leg length of the NKL was on average significantly longer than the KL (p < 0.001). This anatomical leg-length difference did not significantly correlate with any of the GRFv LEFA values from the functional movements or the SEBT bilateral differences (p ≥ 0.054).

Ignoring side, small absolute levels of GRFv LEFA existed in the functional movements (Figure 2); however, no significant between-leg differences existed when compared as NKL to KL (p ≥ 0.141). On average the GRFv LEFAs were between ±3% NKL − KL except slope to peak in the SLDL, which was 4.4% NKL − KL. The GRFv LEFA exhibited high repeatability in standing (α ≥0.926) and CMJ (α ≥0.959), with more moderate repeatability in squatting (α = 0.595–0.827) and SLDL data (α = 0.570–0.910, except absolute bilateral differences in impulse to peak where α = 0.212).

Figure 2

Figure 2

All the tests were performed by all the subjects; however, because of technical error, the data collected for the SLDL on 2 male subjects were deemed unusable, reducing the number of subjects to a total of 18 for the SLDL only. Because of this, subject numbers ranged within the subsets created with those displaying higher levels of LEFA depending on whether one or both of those subjects were excluded based on symmetry levels. There were 11–13 subjects for the standing, 12–13 for squatting, 9–10 for the CMJ, 10 for the SLDL, and 8–10 for the SEBT subset.

Many significant correlations between tasks in GRFv LEFA measurements existed (Table 2). In the whole group analysis, these correlations were of only low to moderate strength (|r| < 0.8). The subset of those with higher levels of GRFv LEFA during standing showed correlations that were generally very similar to the whole group, whereas the subset of those with higher levels of GRFv LEFA during the CMJ displayed noticeably increased levels of correlation compared with the whole group, especially when comparing with their squat asymmetries where correlations were now quite strong (|r| > 0.8). The subsets with increased levels of GRFv LEFA in the squat and SLDL both showed smaller improvements in correlations relative to the CMJ subset.

Table 2

Table 2

Small absolute levels of bilateral differences in SEBT scores existed (Figure 3), but again, none of the NKL vs. KL comparisons were statistically significant (p ≥ 0.230) when examined as a 3-trial average or a single trial maximum (Table 3). Individual leg scores and bilateral differences in SEBT scores showed good repeatability (α ≥ 0.751 and α ≥ 0.752, respectively). Again, absolute differences had a broader range in repeatability, mostly being moderate to good (α = 0.621–0.727, with PostLat 3 trial average and single trial maximum composite being much lower [α = 0.036 and 0.380, respectively]).

Figure 3

Figure 3

Table 3

Table 3

When analyzed as a whole group, significant correlations between excursion directions existed among most bilateral differences in SEBT scores, with correlations of mild to moderate strength (Table 4). The exception was between PostMed and PostLat excursions, which did not correlate significantly with each other (p ≥ 0.291). When the subset of more asymmetric subjects during the SEBT was analyzed, the correlations again typically exhibited noticeable increases in strength, many of which became reasonably strong, even when comparing PostMed to PostLat.

Table 4

Table 4

Many significant correlations were found between SEBT bilateral difference and LEFA in the whole group, though of only mild to moderate relationships (Table 5). Strength of correlations again typically increased when analyzing the subsets of the more asymmetric subjects in individual tasks, with the most notable increases in the CMJ subset with strong relationships existing with their CMJ asymmetries and both maximum PostLat and composite scores. The standing subset again exhibited little change relative to the whole group results.

Table 5

Table 5

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Discussion

The threefold goal of this investigation was to (a) explore the relationships between GRFv asymmetries identified in commonly studied functional lower-extremity tasks of various speeds and intensity levels, (b) explore how asymmetries in SEBT excursions relate to each other, and (c) explore the relationships between asymmetries between functional movements and the SEBT. A healthy, active population was examined to gain a fundamental understanding of the relationships that exist between functional tasks and the SEBT. The GRFv asymmetry levels as a whole group were on the order of ±3% NKL − KL, with the individual excursion asymmetries being on the order of ±2% NKL − KL. As expected, small levels of asymmetries existed within this population during functional tasks (16,41) and when performing the SEBT (28), and the expression of GRFv LEFA was typically repeatable from day to day (16,36) as was SEBT performance (22,28). Of the functional measures, the SLDL had the greatest range of repeatability, suggesting that not all GRFv LEFA from this task could be used reliably as an assessment tool. Lower repeatability obtained could be because of the high reliance on dynamic balance during an eccentrically controlled impact and the possibility that there was a higher degree of freedom related to successful task completion in comparison to the others. In general, the repeatability of absolute level of asymmetry was lower than relative levels that maintained a specific relationship to the dominant side. This was most likely the result of being consistently dominant on the same side from one day to the next but demonstrating slightly different magnitudes of asymmetry each day. Because knowing the specific side that expresses dominance is critically important, reduced reliability of absolute levels is of limited practical importance except when at or near established cutoffs for injury risk. It is anticipated that those with higher levels of asymmetry (i.e., greater bilateral deficits than those observed in this population) would be even more repeatable than healthy subjects with more moderate levels of asymmetry.

Although the SEBT raw scores of our subjects were comparable with those of other studies in the PostLat and PostMed directions, they tended to be approximately 20% lower in the Ant direction (28). It is not entirely clear as to the source of this discrepancy. However, we have anecdotally found that small differences in testing criteria can make a noticeable difference in score, including stance foot starting position, hand position (i.e., on hips or freely moveable), if the heel can lift from the ground, or if a subject must touch the tape measure or just hover above it with the foot. These observations underscore the importance for strict compliance to all aspects of the SEBT, because small deviations might alter performance and inadvertently create an asymmetry.

Overall, the results regarding how LEFA in one task relates to another partially support our hypothesis that relationships would exist. Statistically significant correlations existed in many of the comparisons, though they were typically low to moderate level (|r| < 0.8). When only the more asymmetric subsets were assessed, correlations typically improved. Most notable were the strong relationships that emerged between the GRFv LEFA in the squat and CMJ in the subset of more asymmetric CMJ performers. When subsets of the more asymmetric GRFv LEFA performers were examined, correlations typically increased between the functional tasks. This may be because of a decreased ability to compensate by other means when a deficit is larger, regardless of the source(s) of the asymmetry and requirement of the task.

The strongest relationships emerged between the squat and CMJ in those that were more asymmetric during the CMJ; this is most likely because of the similar kinematic and balance requirements of the 2 movements even though one was performed slowly with relatively low load while the other is ballistic with maximal effort. However, this may be the reason why the correlations between these 2 movements were not as high in the subset that was more asymmetric during the squat. The lower intensity of the squat may have allowed compensations to be made that could not be made during the maximal effort task, supporting the evidence that intensity level has an effect on asymmetry (5,34,40). Although the GRFv LEFA correlations were strong in the more asymmetric subsets between the squat and CMJ, they were inversely related. This contrasts with the findings of Newton et al. (23) where the correlation between these tasks was positive (r = 0.734). Newton et al. (23) used squats of 80% of 1 repetition maximum, instead of the unloaded (bodyweight only) squats used in this study. In addition to intensity level potentially explaining the differences, speed of the motion (8,34,40) and the slightly different range of motion, and thus potential flexibility requirements (17), could also be contributing factors. The existing inverse correlation could also be based on the previously noted differentiation between lower extremities with 1 leg the motor dominant (KL) and the other strength dominant (NKL) (11,21,39). As the relative contribution of motor skill and strength changes, it may result in a switch in dominant force production. This hypothesis further supports why Newton et al. (23) found a positive correlation in the asymmetries, because both were of high strength demand.

Intensity of effort could also explain why the asymmetries expressed when standing correlated the lowest with the asymmetries of the other functional tasks and why the relationships did not improve within the subsets. Of the tasks examined, standing required the least muscular effort, dynamic balance, and range of motion. When standing, the majority of a person’s bodyweight is supported through the skeleton rather than by muscular moments. Instead, muscles primarily work isometrically, to maintain joint position and balance. Furthermore, with joints not positioned near extremes of their ranges of motion, it is not anticipated that passive joint stabilizing structures such as ligaments and fascia would produce an asymmetry during standing, while potentially being the source of asymmetry in other movements. Therefore, at least in healthy subjects, sources producing asymmetries when standing are the least likely to also play a role in the asymmetries expressed during more dynamic movements.

The SLDL is a reasonably intense exercise, but it does not show correlations to other tasks that are as strong as the CMJ to squatting. This could be because of several differences between the tasks. Most notable is the unilateral vs. bilateral limb involvement. With only a single leg performing the task, performance cannot be deferred or compensated by the opposite side. As a result, asymmetries are very likely to be expressed differently between the 2 tasks. Additionally, there is a much greater reliance on eccentric muscle effort in the SLDL with shared reliance on eccentric and concentric action in the CMJ and squat. Muscle contraction type has been suggested to play a role in the expression of asymmetries (43). The increased reliance on dynamic balance could also play a role as could the lower repeatability of asymmetries within the SLDL compared with the other movements, as previously discussed. Examination of the relationships within the 7 parameters extracted for the SLDL suggests that all of these may not be necessary. It may also not be necessary to extract both average and instantaneous maximum levels of asymmetries from squatting and the CMJ. However, until a more thorough assessment of relationships is performed, it would be premature to limit any analysis.

Overall, the results regarding the asymmetries in SEBT excursion scores partially support our hypothesis that relationships would exist between each direction. Correlations were often significant although not strong (|r| < 0.800). The correlations between asymmetries did rise in the subset comparisons, although still not to the level where the relationship would be considered strong. The limited strength in the directional comparisons of asymmetries within the SEBT both within the whole group and in the SEBT subgroup supports the notion that these 3 directions assess different physical attributes (13). Thus, the source(s) of asymmetry within a given excursion may be reasonably independent of the source(s) of another excursion. However, given an even more asymmetric population, correlations might markedly increase. Because the composite score is formed by summing the individual directions, it was not surprising to find relatively high correlations with it and each direction. Although it is not clear what source(s) are responsible for the asymmetries in each direction, as knowledge relative to requirements for success in each direction is limited (9,15,29), these results suggest that each direction is important and necessary when interpreting the composite score. Because of a lack of consensus within the literature (27), both the 3-trial average and maximum SEBT scores were assessed. In most cases, correlations between directions were very similar with each method, and correlations between average and maximum measures were extremely strong. As a result, no clear recommendation in this matter is available from the results of this investigation, though it may not be necessary to compute both a 3-trial average and a maximum SEBT score.

Overall, the results regarding the asymmetries in the GRFv LEFA relative to those of the SEBT excursion scores partially support our hypothesis that relationships would exist between them. Correlations again were often statistically significant but never strong in the whole group comparisons (|r| < 0.800). Again, correlations typically increased in strength when assessing the more asymmetric subsets. Limited relationships of the SEBT asymmetries with those of the bilateral tasks could be expected for similar reasons as to why the SLDL GRFv LEFA were not more strongly related to standing, squatting, and CMJ. However, strong relationships emerged within the CMJ subset where GRFv LEFA in the CMJ correlated highly with maximal PostLat and composite SEBT asymmetries. At present, it is unclear why the relationship emerged between the CMJ and just the PostLat direction of the SEBT, though it is anticipated that the relationship to the composite is because of its inclusion of the PostLat direction. It is also unclear why the GRFv LEFA of the SLDL are not more highly correlated with those of the SEBT, because they are both unilateral tasks. Two potential reasons are the differences in intensity between them and the fact that the SEBT actually incorporates both legs even though stance is maintained on a single limb.

Work in the area of postural alignment suggests that asymmetries may result from multiple different and potentially interrelated sources, and that an asymmetry in one location may cause an asymmetry to develop in another as part of a compensation mechanism (17). Considering all the degrees of freedom within the body, this may further explain why correlations between functional tasks of different demands and with the SEBT were of low to moderate strength. As a result, deficits may be compensated for in a multitude of different ways and, therefore, expressed with different GRFv LEFA levels. This also may explain why the low, but expected (4,19), levels of anatomical leg-length difference in the whole group did not correlate with the LEFA and SEBT asymmetries. Finally, this may also explain why no significant differences were found in the t-tests comparing KL with NKL, even though different roles for each limb have been noted (11,21,39).

It should also be noted that a variety of potential algorithms exist when computing asymmetry levels for a given variable, with each having their strengths and weaknesses (32,42). We chose algorithms with the goal in mind to compare measures from different tasks to each other, putting all in units of % NKL − KL. However, even with the same units, there are slight differences in how the asymmetries have been and could be computed. Therefore, there may be methods that are more suitable for an individual task or that may show more suitable relationships to performance and injury risk within specific populations. There may also be a need to use other variables besides, or in addition to, GRFv during functional tasks or the SEBT when assessing asymmetries and their relationships to performance and injury risk. Furthermore, the possibility also exists that analysis of movements by breaking them into specific phases may be beneficial. This might be useful as asymmetries may affect one part of a movement pattern more so than another part, especially because joints near their end range of motion or as type of muscle contraction varies (i.e., concentric vs. eccentric or isometric). Overall, however, we expect our results to be highly generalizable to the healthy, young adult population, with the results furthering our understanding of functional asymmetries. Future research will be needed to establish clear diagnostic relationships and whether purported asymmetry levels in the range of 10–15% (2,24) are accurate.

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Practical Applications

In a young, healthy population with no reason to be asymmetric, it appears that the functional tasks studied and the SEBT are not correlated highly enough to make accurate predictions from one assessment to another. Notably, GRFv LEFAs expressed when standing do not correlate even moderately with those observed during more dynamic tasks, suggesting that it should not be prioritized as a measure of asymmetry in many cases. However, as the level of asymmetry increases in the dynamic tasks studied, asymmetries seem to be more highly related to each other. In those with greater levels of asymmetries during the CMJ, CMJ and squatting GRFv LEFA may be predictive of each other. This is also the case in CMJ GRFv LEFA and SEBT PostLat asymmetries, and CMJ GRFv LEFA and SEBT composite score asymmetries. Task demands, source(s) of asymmetry, and ability to compensate for a deficit all appear to play a role in how asymmetries are expressed during functional movements and within the SEBT. Based on the principle of specificity, it is prudent to use tasks to assess asymmetries that replicate movements of the relevant sport or activity that an individual participates in as closely as possible. If time and resources permit, a battery of tasks, as opposed to a single task, is likely the best way to establish the presence of functional asymmetries within an individual. The low to moderate GRFv LEFA correlations between tasks help explain why a measured asymmetry might pose a strong potential injury risk for one sport or population, but it might not in a sport or population not using the same movement patterns. Finally, although based on this information, the SEBT would not be recommended as a replacement for functional LEFA assessments, it appears that it does have a place, especially with populations performing jumping activities on a regular basis. Also, the 3 common excursions of the SEBT appear to be relatively independent in regards to asymmetry screening and therefore may be individually useful in the diagnostic process.

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Acknowledgments

The authors thank all the volunteer subjects who freely gave their time and effort to this unfunded study. No professional relationships exist with any of the companies whose products or testing procedures were used. The results do not constitute endorsement of the used products or procedures by the authors or of the National Strength and Conditioning Association.

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Keywords:

asymmetry; ground reaction force; countermovement jump; standing; bodyweight squat; single-leg landing

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