Substantial deviation from normative data of muscle performance differences between limbs is referred to as bilateral muscular imbalance (21). This bilateral muscular imbalance may be the result of side preference, injury or specific sport demands (14,18), and can consequently increase the risk of injury (6,12,13,16). For example, bilateral muscular imbalances have been associated with higher anterior cruciate ligament injury risk in women (6,13) and elite ski racers (11) as well as increased risk for lower back pain (14). In a prospective study, Croisier et al. (3) showed that professional male football players with untreated bilateral muscular imbalances were 4 times as likely to sustain a hamstring injury.
Furthermore, bilateral muscular imbalances could also have an impact on various mechanical aspects and, consequently, on the relevant strength quality of the lower limbs, subsequently affecting performance (4,9,11,22). For example, it was suggested that athletes turned faster in change-of-direction tests when they were pushing off their dominant leg, with this dominance affecting overall performance (22). Furthermore, the weaker leg applied less force during a countermovement jump (9), altering the pattern of force application and reducing the impulse (11), resulting in lower jump height. Such situations can negatively impact on the athlete's performance due to reduced ability to turn fast or jump high.
Muscular imbalances are typically calculated as ([side 1 − side 2]/reference value) × 100 (equation 1), to provide a percentage value of the difference between limbs. However discrepancy occurs with the values that are inserted into the equation 1. When defining side 1 and side 2, for example, researchers have reported using right and left (15,17), stronger and weaker (10,14), and self-reported preferred and nonpreferred, for sides 1 and 2, respectively (4,18). In addition to the definition of side 1 and side 2, the selection of the reference value (right or left, strong or weak, preferred or nonpreferred limb, or simply an average between the 2 limbs) might also impact on the results (23). It is worth pointing out that “strong” and “weak” have been used to refer to the limb with the better (strong) or worse (weak) performance; the actual performance might be a power-based and not a strength-based per se (10). Concernedly, use of different values in the calculations could mask or inflate the true bilateral muscular imbalance value, potentially making it difficult for practitioners to determine whether an athlete is at a higher injury risk, or whether their rehabilitation or training program is working to reduce the strength deficit (1).
Thus, it is important to determine experimentally whether different calculations can produce significantly different results. Hence, the aim of this study was to compare 5 different muscular imbalance ratio calculations (numerator: absolute difference between limbs, denominator: right, left, strong, weak, average of the 2) using 2 functional tests. Although literature has previously also used preferred side (4,18), no calculation was specifically used for those values in this study, as nonpreferred-preferred will be either on the right-left or strong-weak limb, and the exclusion of nonpreferred-preferred selection prevents repetition. Functional tests were chosen over isokinetic dynamometry assessment, due to their practicality and affordability in testing larger groups as well as kinematic resemblance to sporting movements (10).
Experimental Approach to the Problem
The study was designed to compare the different bilateral muscular imbalance calculations obtained using the absolute difference value between limbs as the numerator and right, left, weak, strong, or average of the 2 limbs as the reference value in the bilateral muscular imbalance calculation ([side 1 − side 2]/reference value) × 100. This was done for 2 functional tests, the triple hop and the 6-m timed hop, as the 2 tests place different performance focus on the lower extremity (maximum distance vs. minimum time, respectively) (19). Bilateral muscular imbalances (as per the equation above) were calculated in all possible 5 combinations, which were then compared for differences between sexes and functional tests.
Twenty-three men (mean ± SD: age 21.6 ± 1.9 years [range 19–24 years], height 1.80 ± 0.06 m, body mass 80.5 ± 13.8 kg) and 11 women (mean ± SD: age 20.8 ± 1.5 years [range 19–23 years], height 1.62 ± 0.03 m, body mass 68.0 ± 6.5 kg) took part in the study. They were all competitive, team game players, and free of any injuries for at least 6 months before testing. The sports the subjects participated in were, for men, football (n = 12), rugby union (n = 9), basketball (n = 2) and for women, hockey (n = 6) and netball (n = 5). The study was approved by the institutional ethics committee of the University of Cumbria, Lancaster, UK, and written informed consent was obtained from all subjects.
All participants were familiarized with the testing procedures on a session before testing (2). Testing took place on a single occasion at the same time for all participants. Participants were asked to refrain from strenuous exercise 48 hours before testing and to avoid food or caffeine intake for 2 hours before testing. For all tests, 2 trials were performed on each limb and if the coefficient of variation was above 5% (8), a third test was performed; this only happened on 3 occasions. To reduce order bias, the order of which limb was used to perform each test and the test executed was counterbalanced. The average score of the 2 trials (or the closest 2 trials, in case of more than 2 trials) was used for subsequent analysis.
Participants were required to complete both the one-legged 6-m timed test (6-m hop) and the one-legged triple-hop distance test (3 hop) (19). The 6-m hop test requires participants to stand with their toes just behind a starting line and hop as quickly as possible (on the same leg) over a marked distance of 6 m with large, forceful pushes. Participants were allowed to start on their own time and time taken to cover that distance was recorded. Time was measured using infrared timing gates (Brower Timing, Utah) aligned at the starting and finishing lines, set at hip height. The 3 hop test requires the participants to perform 3 consecutive hops on the same leg aiming for maximum distance. Participants' toes were immediately behind the zero mark of a measuring tape, and the distance covered was measured as the distance from the zero mark to the point their heels touched the ground following the third hop.
Bilateral muscular imbalance difference was calculated with 5 different calculations as the absolute difference between the 2 limbs divided by right, left, weak, strong, or average of the limbs and expressed as a percentage.
Normality of data was examined using the Kolmogorov-Smirnov test and confirmed for all variables. A 2 (sex) × 2 (functional test) × 5 (calculation method) analysis of variance was used to examine for differences. Homogeneity of variances was examined using Levene's test and confirmed for all variables. Where differences were found between groups, an independent t test was performed, whereas for differences between tests or ratios, dependent t tests were performed; all pairwise comparisons were adjusted using the Holm-Bonferroni correction (7). Effect sizes (ES) were calculated for all significant differences, with 0.2, 0.5, and 0.8 representing small, moderate, and large ES, respectively (5). All statistical analyses were performed in IBM SPSSv22 (Chicago, IL). Significance level was set at p ≤ 0.05. All data are presented as mean ± SD unless otherwise stated.
The left leg was stronger in 60.9% of the men and 63.6% of the women for the 6-m hop, whereas the left leg was weaker for 47.8% of the men and 45.5% of the women for the 3 hop.
All descriptive statistics for all tests and calculations for both sexes are shown in Table 1.
There was no significant interaction for sex, test and calculation method, test and sex (p > 0.05), but there was a significant interaction of sex and calculation method (p = 0.002, partial η2 = 0.124). Follow-up analysis revealed that when the calculation method using the right leg as the denominator was used, bilateral muscular imbalance was significantly lower (p = 0.039, ES = 0.76) in men (6.1 ± 3.5%, averaged across the 2 functional tests) compared with females (9.1 ± 4.6%, averaged across the 2 functional tests). Finally, significant differences were found between the calculation methods for men (averaged across the 2 functional tests; Figure 1) and women (averaged across the 2 functional tests; Figure 2), with small ES, however (range: 0.07–025).
The aim of this study was to examine the different bilateral muscle imbalance calculations used and, subsequently, the effect they may have on inferences made about an athlete's, patient's or client's bilateral muscular imbalance. The results suggest that, although some differences exist between the bilateral muscular imbalance calculations using a different denominator, the small ES and small mean differences (all <1.5%) suggest that these have little practically significant impact. These findings, along with recommendations on which bilateral muscle imbalance calculation methods to use, are discussed further to enable strength and conditioning coaches looking to use bilateral muscular imbalance assessment for monitoring purposes to be confident in the results obtained.
Although there is an agreement in the literature on the way bilateral muscular imbalances can be calculated, there is a discrepancy on what values are used in that equation 1. For example, studies have previously used left and right (15,17) or strong and weak sides (10,14) to calculate bilateral muscular imbalances. This study suggests that results between studies are comparable, as selection of different reference values did not substantially influence the results as suggested by the low ES.
Statistical difference was revealed between sexes for the calculation using the right leg as the denominator. This is somewhat surprising, as no other calculation revealed any sex differences. Furthermore, the patterns of stronger and weaker legs in our sample between the sexes were very similar for both functional tests, thus excluding the possibility of a substantially higher percentage of stronger right leg in one group compared with the other as a potential reason. As no explanation for this finding can be currently offered, it may be a recommendation that the right leg is used as a denominator in studies that want to compare between sex bilateral muscular imbalances, as it was the only one that was able to distinguish between each group's bilateral muscular imbalance.
Furthermore, some statistical differences were found between comparisons, both for males and females. However, these comparisons had low ES, suggesting a potentially low practical significance. Indeed, when one examined the values in Table 1, the differences in bilateral muscular imbalances range from 0.4 to 1.2%. Although what constitutes “substantial deviation” from normative data is difficult to determine (21), studies have reported a difference of 15% in countermovement jumping (9) performances, as a threshold for substantial deviation between limbs. With this threshold in mind, consider a female athlete performing the 3 hop test and having the bilateral imbalance calculated as 9.2% using the strong leg as denominator. Using the weak leg as a denominator, this bilateral muscular imbalance would only increase to 10.4%; given the inherent measurement error, it is unlikely that the difference in these values would lead to different interpretations of the athlete being “at risk.” This contradicts our hypothesis that the reference value used in equation 1 could impact on the results. Although for standardization purposes the same reference value should be used, comparisons between results that have used a different numerator (i.e., right, left, weak, strong, or average of the 2) should be possible, as little difference would be present from the use of a different reference value.
Using 2 different tests, 6-m hop and 3 hop, that had the same overall aim (power, speed, balance, and lower limb control) but different emphasis (time v distance) produced comparable results, suggesting that the ultimate aim of each test had no effect on the measured outcome and they assess the same muscle qualities (10). As both are suggested as tests of bilateral muscular imbalance, the results of this study suggest that using one of them is sufficient to provide bilateral muscular imbalance ratios, thus increasing testing efficiency of large groups. As the 6-m hop test is more prone to measurement errors with a stopwatch (2) but more difficult to conduct with timing gates, the use of the triple hop test is recommended.
Functional tests are a practical and easy way to assess bilateral muscular imbalances, with the advantage that they mimic sporting movements, thus providing assessment in a more sport-specific manner, compared with dynamometry (10). However, this type of assessment prevents the identification of specific individual muscle or muscle groups imbalances (10,15). In addition, an element of postural balance is inevitably included in the assessment, as the participant has to balance themselves on their foot before they are able to hurl themselves toward the next hop. As such, and although a large muscular component is included, the results represent more of a “movement imbalance.” A potential solution can perhaps be the use of functional tests for large group assessment, with the participants recording higher percentage differences undergoing a more thorough dynamometry assessment.
It has been previously reported that different sports yield different bilateral muscle imbalances (e.g., American football (24) and soccer (20)). The convenience sample used in this study did not allow to separate for different sports or positions. However, as the same functional test performance was used for all the difference calculations, this effect should have been minimal and not impacted on the results.
Finally, suggestions have been made (1) to use the symmetry angle, proposed by Zifchock et al. (23), as a means of achieving a bilateral muscular imbalance score without the need for a reference value (23). This paper adds to the choices available in bilateral muscular imbalance calculations by offering some practical recommendations for those strength and conditioning coaches, sport therapists, or athletic trainers who prefer to continue using more conventional bilateral muscular imbalance calculation methods for e.g., simplicity.
This study examined the different bilateral calculation methods by using 2 different functional tests. The results suggest that (a) for comparisons between sex, the right leg should be used as the reference value (denominator) in calculations, (b) the calculation method (i.e., the different reference value used for the denominator) makes little practical difference when calculating bilateral muscle imbalances, and (c) the 2 different functional tests used in the study (i.e., the triple single leg hop and the 6 m timed single leg hop) provide the same information when bilateral muscular imbalances are concerned. Strength and conditioning coaches can use these findings when they are assessing their own athletes as well as when comparisons between studies are made.
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