The transfer of strength from a trained limb to an untrained one is a well-known phenomenon, thought to be due to neural adaptations, and offers potential benefits for athletes seeking to maintain strength in an injured limb while it is out of action. But does this apparently miraculous transfer of strength affect both sides equally? Probably not, according to a team of Canadian researchers, who studied transfer of arm strength in right-handed people and found it operated only from right to left – ie from the dominant to the non-dominant side.

Thirty-five strongly right-handed women were randomly assigned to left-hand training, right-hand training or non-training (the control condition). The strength training consisted of a high intensity handgrip exercise, four times a week for six weeks. Peak torque, muscle thickness and muscle electrical activity were assessed before and after training in both limbs.

Initially, the researchers had expected that training the weaker (left) side of the body would result in greater strength gains and a proportionately greater cross transfer to the right side. However, while strength training increased trained limb strength more in the left limb than the right (41.9% compared with 25.9%) significant transfer of strength to the untrained limb occurred only in the group doing right hand training – a substantial improvement of 32.9%. In other words, the subjects’ left arms benefited almost as much from cross-education as they did from being trained themselves! The left-hand training group and the control group, on the other hand, showed non-significant changes in the untrained limb ranging from 9.3% to 12.2%.

The researchers concluded that ‘cross-education with isometric hand strength training occurs only in the right to left direction of transfer in right- handed individuals’. Moreover, because this process is not associated with an increase in maximal muscle fibre activity, they also concluded that the mechanism is probably associated with ‘neural adaptations which may occur in higher brain centres such as the motor cortex.’

More research on cross-education is needed with left-handed people and also on the effects of ‘handedness’ on transfer of strength in the lower limbs. Meanwhile, this study could have practical implications for people undergoing rehabilitation for injury or impairment on one side of the body, where the training of an uninjured limb may contribute to an increase in strength of an injured/impaired one.

Med Sci Sports Exerc 2005; vol 37, no 9, 1594-1600