Are single-leg and single-arm exercises worth the time?
For muscle growth, single-limb and two-limb exercises are effectively interchangeable. For strength, the gains track how you trained, so unilateral work is worth the extra sets only when single-limb performance, a real injury, or an equipment constraint makes it the better tool.
The cleanest direct evidence pools nine trials that matched unilateral and bilateral programs for volume and effort. Muscle growth came out a wash — no meaningful difference in either direction. Strength followed a strict specificity rule instead: bilateral training won bilateral strength tests, unilateral training won unilateral tests, and neither transferred especially well across. A larger review of 14 studies found the same shape — a large advantage for single-leg training in single-leg jumping, a smaller one for bilateral training in bilateral strength — while sprint and change-of-direction times were indifferent to the choice.
So the muscle doesn’t know the difference. The test does.
The bilateral deficit is not a problem to fix
Add up your two single-leg maxes and the total usually beats your two-leg max. This gets sold as proof that bilateral lifting leaves force on the table and that unilateral work reclaims it. In jumping, roughly three-quarters of that gap is explained by the legs shortening faster in a two-leg jump — a force-velocity effect, not suppressed neural drive. Nothing is going to waste, and no one has shown that training the deficit away makes you better at anything.
Where it does earn the time
Cross-education is the real phenomenon: train one limb and the untrained one gets stronger too, around 12% on average across 31 randomized trials, more in the legs than the arms and most with eccentric work. The adaptation is neural — the idle limb doesn’t grow — which makes it useless as a growth shortcut and genuinely valuable when a limb is casted or post-op, where working the healthy side blunts the losses on the injured one.
Asymmetry, the other common justification, is thinner than advertised. Left-right imbalances show a weak negative association with sprinting and change of direction, essentially none with vertical jumping, and there is no threshold above which you are broken.
Keep bilateral lifts as the backbone if that is what you want to be strong at. Add unilateral work for a specific reason: a sport that lives on one leg, a side that is genuinely behind after injury, or because a split squat loads your quads without taxing your lower back. Doubling every set in the name of symmetry mostly just costs you an hour.