17.10.2026 · 6 min read
The left and right legs rarely show absolutely identical results. The difference can be assessed by single-leg jump height, power, squat strength, change-of-direction time or other measures. But the "weaker leg" in a jump will not necessarily be the weaker leg in a turn or a strength test.
Both the size and the direction of asymmetry depend on the task and the metric, and can change over time and after fatigue. A number such as 12% is therefore not a diagnosis in itself. First you need to make sure that the test is reliable and that the observed difference exceeds its usual variability or measurement error.
Thresholds of 10–15% are often used as a guideline, but the systematic review stresses that a universal boundary is hard to justify for all movements, ages and sport disciplines.
The first question is whether the numerical asymmetry can be reduced. The second is whether athletic performance will improve. The third is whether the risk of injury will fall. These outcomes are not identical.
A programme can bring the legs' scores closer together without improving sprinting or jumping. Performance may improve in both legs while the percentage asymmetry stays the same. Finally, an association between asymmetry and injuries in observational data does not mean that correcting it will necessarily prevent damage.
Martos-Arregui and colleagues searched for studies of healthy athletes aged 12–30 with lower-limb training lasting at least four weeks. To be included, a study had to measure both the change in asymmetry and at least one measure of physical performance.
One type of study compared an individualised stimulus — for example, starting each set with the weaker leg or giving it additional volume — with identical loading of both sides. The other compared general unilateral and bilateral programmes without individual adjustment.
The review included 11 studies and 297 athletes from football, basketball, karate, handball and other disciplines. The size of individual samples was 18–66 people. Participants were predominantly men, and the protocols, tests and methods of identifying the weaker leg differed markedly.
A reduction in asymmetry was observed more often in programmes where the stimulus was assigned according to the initial weaker side, and in unilateral training compared with general and bilateral programmes. Simple strategies included starting a set with the weaker leg and a greater training volume for it.
Most programmes also improved some performance measures. But the transfer was specific: unilateral and asymmetry-focused programmes more often gave more in unilateral tasks, whereas bilateral training could be more advantageous in bilateral tests.
The review was a qualitative synthesis, not a single meta-analytic figure of superiority. The variety of programmes and outcomes does not allow one dose, exercise or percentage of extra volume to be prescribed for all athletes.
Bettariga et al. in 2022 pooled eight studies and found small or moderate reductions in asymmetry after various training interventions. Both unilateral and bilateral strength work could reduce the difference, so the mere fact of using one leg did not yet prove the superiority of individual correction.
The 2025 meta-analysis by Sun et al. included eight studies and 157 athletes. In a subgroup, unilateral plyometrics reduced asymmetry with a standardised effect of 0.71, but the number of studies was small, and conclusions about injury prevention went beyond the measured outcomes.
The new review adds a useful distinction: unilateral training that is the same for everyone and a programme specifically assigned on the basis of an initial test are not the same thing.
Only four studies had a low risk of error in generating the random sequence, and only one clearly described allocation concealment. Blinding of assessors was explicitly stated in three studies. In four studies roughly 20% to more than 40% of participants dropped out.
The methods of identifying the dominant or weaker leg differed: jumps, change of direction, squats, the preferred take-off or kicking leg. Some studies from the same group may have used overlapping samples. All of this limits confidence and transferability.
It is especially important that the review did not test a reduction in injury rates as an end point. The phrase "may potentially reduce risk" remains a hypothesis.
If a coach sees a persistent difference, a reasonable order of action looks like this:
Pain, instability or a sudden change after an injury require clinical assessment, not just a corrective exercise. In a healthy athlete, striving for zero difference at any cost can distract from strength, technique and management of overall load.
Training the weaker leg separately is sometimes justified if the weakness is confirmed by a reliable and relevant test. The data support cautious individualisation, but not a universal threshold and not an automatic "one extra set with the weaker leg" programme for everyone.
Tags: injury prevention, performance, strength training, asymmetry, unilateral training
Research on training, nutrition and recovery: the key points worth knowing. One email a week with the new Sport in Luxembourg articles: title, short description and a link to read.
We use cookies for visitor analytics. You can accept or decline their use. Learn more in our Privacy Policy
