Can we get the strength benefits with half the work? New evidence suggests so. When pushing yourself in your training on a regular basis, injuries can happen. But giving up training altogether when faced with setbacks could mean you’re missing out on potential strength gains.

The new study, published in The Scandinavian Journal of Medicine & Science in Sports, examined what happens to the strength and size of one arm when we don’t train it, but train the opposite side.

Researchers of the study aimed to investigate whether heavy unilateral (single limb) training influenced strength changes in the non-working arm when both arms are training using the same heavy weight principle. The researchers primarily focussed on the neural adaptations and understanding whether training one arm can boost strength in the other arm, even if it’s not being directly trained. This effect, known as ‘cross-education’ (or ‘cross-transfer’) is thought to happen because of changes in the nervous system rather than the muscles themselves. The researchers wanted to see if this effect could make the untrained arm even stronger when it was also doing the same type of heavy training.

The researchers looked at 160 healthy participants aged 18-35 years who had not engaged in resistance training in the six months prior. Muscle strength and thickness were measured before and after the intervention. Participants were randomly assigned to one of four groups:

• D + ND: Training both dominant and non-dominant arms
• D only: Training the dominant arm only
• ND only: Training the non-dominant arm only
• Control: No training

Participants completed three training sessions per week over six weeks for a total of 18 sessions, performing unilateral bicep curls. Each session involved:

• A maximum of five attempts to determine their 1 rep max
• Four sets of heavy resistance training (8-12 rep max range) at approximately 70-80% of their 1 rep max

The findings of the study included:

• Strength: In the non-dominant arm, the D + ND group and the ND only group both gained about the same strength (2.7 kg and 2.6 kg). These strength gains were much higher than those in the D only group (1.5 kg) and the control group, which actually lost strength. In the dominant arm, the D + ND group gained 2.5 kg, and the D only group gained 2.3 kg. Both did better than the ND only group (1.3 kg) and the control group (-0.1 kg).
• Muscle growth: Muscle growth occurred only in the trained arm. The untrained arm did not show any increase in muscle size due to cross-education.
• Neural adaptations: The cross-education of strength was driven by neural changes, not by increases in muscle size.

The researchers concluded that unilateral high-load resistance training increased strength in the opposite untrained arm (cross-education of strength), without changes in muscle thickness.

If the very worst happens and you find yourself injured, it pays to keep lifting heavy with the non-injured limb. This may not increase muscular size of the injured limb, but it will cause neural changes to ensure you don’t lose strength in either. This is supported by further evidence in the field of cross-education, published in Medicine & Science in Sports & Exercise.

Similarly, the researchers found strength improvements in non-working limbs when the opposite limb is training. However, interestingly they also found that performing eccentric exercises maximised the ‘cross-education’ effect, as compared to concentric phase of the lift. This new evidence encourages us to keep up our strength training when injured. You can include the best unilateral exercises for your programme, here.