Single-leg training halves strength imbalance in players

Side-to-side strength differences in the legs are common in handball, a sport built on repeated one-legged jumps, landings and hard changes of direction. Large asymmetries are thought to blunt performance and raise injury risk, so coaches routinely look for ways to even the two limbs out. Unilateral training — loading one leg at a time through resistance and plyometric exercise — is the obvious candidate, since it removes the stronger limb's ability to compensate. What has been less clear is whether adding such work on top of an already full training week produces measurable change in a short block.

Thirty highly trained female handball athletes, mean age 18.75 ± 1.91 years, were randomly assigned to an experimental group (n = 14) or a control group (n = 16). The experimental group performed a unilateral compound programme of unilateral resistance and plyometric exercises three times per week for six weeks in addition to their regular handball training, while the control group simply continued regular training. Before and after, participants were assessed for body composition, isokinetic knee strength, bilateral strength asymmetry, hamstring-to-quadriceps ratios, 20 m sprint, change-of-direction ability, countermovement jump and single-leg countermovement jump.

Significant group × time interactions appeared for body fat percentage, muscle percentage, muscle mass, knee flexor strength in both limbs, left knee extensor strength, quadriceps and hamstring asymmetries, H/Q ratios, sprint, change-of-direction, countermovement jump height and single-leg jump performance. Quadriceps asymmetry fell from 13.19% to 7.60% (a relative reduction of 42.38%) and hamstring asymmetry from 16.65% to 7.83% (-52.97%). Performance shifted too: sprint -4.68%, change-of-direction -5.91%, countermovement jump +12.56% and single-leg jump +11.42-18.36%, with partial η2 of 0.19-0.46 across primary outcomes. The interaction for right knee extensor strength did not reach significance (p = 0.072).

The direction of travel matches the general expectation that unilateral loading reduces limb imbalance, and the effect sizes here are large. But the design carries an unusually candid limitation that the authors flag themselves: the experimental group received three additional weekly sessions, so the improvements cannot be attributed to the unilateral modality as distinct from the extra training volume. The small sample also means the headline percentage reductions may be overestimated. A volume-matched comparison — the same extra sessions performed bilaterally — is the obvious missing study.

This is a reason to think that adding structured single-leg strength and jump work to a sport schedule can shift limb asymmetry substantially within a couple of months, at least in young trained athletes. It is not evidence that unilateral exercises are inherently superior to bilateral ones, because the extra work itself may explain much of the result. The population is narrow: elite-level young female handball players already carrying a heavy training load. Anyone tracking their own asymmetry should note how large the baseline imbalances were before the intervention began.