Unilateral Compound Training Reduces Lower-Limb Strength Asymmetry and Enhances Athletic Performance in Female Handball Players: A Randomized Controlled Trial

Abstract

Lower-limb strength asymmetries are common in handball players and may negatively influence athletic performance while increasing injury risk. This study examined the effects of a six-week unilateral compound training program on lower-limb strength asymmetry, isokinetic strength, the hamstring-to-quadriceps (H/Q) ratio, body composition, and physical performance in competitive female handball players. Thirty highly trained female handball athletes (age: 18.75 ± 1.91 years) were randomly assigned to an experimental group (n = 14) or a control group (n = 16). The experimental group completed a unilateral compound training program consisting of unilateral resistance and plyometric exercises three times per week for six weeks, in addition to regular handball training, whereas the control group continued regular training only. Before and after the intervention, participants underwent assessments of body composition, isokinetic knee strength, bilateral strength asymmetry, H/Q ratios, 20 m sprint performance, change-of-direction (COD) ability, countermovement jump (CMJ), and single-leg CMJ performance. Significant group × time interaction effects were observed for body fat percentage, muscle percentage, muscle mass, knee flexor strength (both limbs), left knee extensor strength, bilateral quadriceps and hamstring asymmetries, H/Q ratios, sprint performance, COD performance, CMJ height, and single-leg jump performance (p < 0.05); the interaction for right knee extensor strength did not reach statistical significance (p = 0.072). The experimental group’s improvements coincided with substantial reductions in quadriceps (13.19% to 7.60%; −42.38%) and hamstring (16.65% to 7.83%; −52.97%) asymmetries, alongside changes in H/Q ratios, isokinetic strength, sprint performance (−4.68%), COD performance (−5.91%), CMJ height (+12.56%), and single-leg jump performance (+11.42–18.36%; partial η 2 = 0.19–0.46 across these primary outcomes). Given the relatively modest sample size, these percentage reductions should be interpreted with appropriate caution, as they may be subject to overestimation. Because the experimental group received three additional weekly training sessions relative to the control group, the observed improvements cannot be unambiguously attributed to the unilateral training modality itself, as opposed to the additional training volume. These findings suggest that adding a supplemental unilateral compound training program to regular handball training may improve lower-limb strength asymmetry and neuromuscular performance. However, because the intervention also increased total training volume, these effects cannot be attributed exclusively to the unilateral training modality itself.