High test-retest reliability scores were observed between baseline and pretraining evaluations for all tests: at the preferred limb, isometric peak torque (r = 0.981), concentric peak torque (r = 0.958), eccentric peak torque (r = 0.949), and sum of muscle thickness (r = 0.893); and at the nonpreferred limb, isometric peak torque (r = 0.966), concentric peak torque (r = 0.932), eccentric peak torque (r = 0.934), and sum of muscle thickness (r = 0.940).
Before starting the resistance training program, volunteers presented similar values between preferred and nonpreferred limbs for concentric peak torque (Figure 1; p = 0.301, ES = 0.23). However, the preferred limb showed superiority in terms of isometric peak torque (Figure 2; p = 0.041, ES = 0.36) and eccentric peak torque (Figure 3; p = 0.021, ES = 0.45). At pretraining evaluation, mean isometric and eccentric peak torque values of the nonpreferred limb were 4.9 and 5.8% smaller compared with the preferred limb, respectively.
Bilateral symmetry remained constant throughout the training program for concentric peak torque (Figure 1). Between-limb symmetry in eccentric peak torque was reached at the fourth training week (p = 0.534, ES = 0.08) and remained unchanged until the end of the training program (Figure 3). Similar isometric peak torque values were found after 8 training weeks (p = 0.253, ES = 0.12) and also did not change until the final evaluation (Figure 2). After 12 training weeks, the percent differences between the preferred and the nonpreferred limbs were 0.73% (ES = −0.03) for concentric, −0.62% (ES = 0.09) for isometric, and −1.93% (ES = 0.21) for eccentric peak torques, respectively.
Although there is no consensus in the literature, strength imbalances between the preferred and nonpreferred limbs smaller than 10% have been considered “normal” (18). Despite some contrary results (19), strength asymmetries seem to impair functional (23) and sports performance (17). Lower-limb asymmetries have also been related to increased injury risks in sports practice (12), and systematic isokinetic evaluations have a remarkable place for injury prevention in elite athletes (25). In addition, knee extensor strength deficits in previously injured limbs predict functional performance at the time of sport return (31) and may help clinical decision making to optimize sports participation after surgery/immobilization/rehabilitation.
Our initial expectation about the nonpreferred limb having smaller peak torque values compared with the preferred limb was partially confirmed because participants of this study presented significant deficits at the nonpreferred limb in isometric and eccentric tests, but not in the concentric test. Assuming that the subjects were not engaged in any kind of systematic exercise with asymmetric kinetic patterns, the torque asymmetries may result from the preferential recruitment of the preferred lower limb for a range of mobilization task during daily life that is able to affect maximal strength capacity. In addition, these findings further support the notion that lower-limb strength asymmetry is dependent on the contraction type (11,22). Therefore, because daily and sports activities are not restricted to one type of muscle action, it is advisable that isokinetic evaluations contemplate concentric and eccentric tests to get a reliable comparison between preferred and nonpreferred (or injured and uninjured) limbs.
Finally, reduction of strength asymmetry between lower limbs after an isokinetic training program is a novelty of our study. Future studies should address the mechanism behind the specific adaptation of preferred and nonpreferred limbs. In addition, the impact of other resistance training regimes (e.g., free weights, gym machines, elastic bands) with unilateral and bilateral exercises and responses from other muscle groups with clinical significance (such as the hamstrings) should be investigated, especially considering that our training was performed in an isokinetic dynamometer, which does not resemble real-life training situations. Finding effective programs to counteract bilateral asymmetry may be an important step to decrease injury risk and to improve athletes' performance.
Our findings support that the nonpreferred lower limb presented strength deficits compared with the preferred limb, and this asymmetry is not attributed to the amount of muscle mass. Therefore, health professionals should have greater concerns on the muscle functional status than the limb circumference or other measures of muscle mass. Unilateral tests in different types of exercises (or muscle actions) are useful to identify bilateral asymmetries. Once the functional deficit is diagnosed, symmetry patterns may be restored with relatively short periods (4–8 weeks) of unilateral training programs using the specific exercise where the strength deficit was identified. In addition, subjects with bilateral strength asymmetries should focus on unilateral exercises to avoid potential compensations of the preferred leg in bilateral exercises.
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