Why exercising your 'good arm' can also help the one in a sling

Injured limbs need rest. They are often kept in a sling or cast to immobilise them as a way to promote healing. But that can mean smaller and weaker muscles several weeks later. It takes a long time to rehabilitate these muscles and muscle strength and function may not be fully restored for some people.

Experts are learning more and more about the “cross-education effect” where training one side of the body results in an increased strength of the opposite side of the body. Our recent study shows it can also stop muscle wasting in the “unused” arm.

So, how can we harness that effect?

How it works

First discovered 100 years ago, the mechanisms underpinning the cross-education effect have not been fully clarified yet. But it is likely associated with neural adaptations in the motor cortex of the brain that controls movement in the body.

Researchers have reviewed almost 100 studies and showed the average cross-body transfer ratio between the strength gain in the trained muscle to non-trained muscle ranged from 48% to 77%. So, if your trained arm strength increased by 20% after training the same muscle of your non-trained arm strength might increase by 10% even though you did nothing with that arm.

Such changes could be due to increased cortical excitability (the brain activity to control movement), reduced cortical inhibition (the signal to stop movements), reduced inter-hemispheric inhibition (the signals that direct movement instructions to one side of the body but not the other), changes in voluntary activation or new brain regions getting switched on.

It appears the type of muscle contraction in the training affects the extent of the cross-education effect.

There are three types of muscle contractions:

• isometric (static) where the force produced by a muscle is equal to the load to the muscle, such as holding a dumbbell
• concentric (shortening) in which force is greater than load, such as lifting a dumbbell
• eccentric (lengthening) in which force is less than load, such as lowering a dumbbell.

Muscles can produce greater force during eccentric than isometric or concentric contractions. And less fatigue is induced during eccentric than other contractions. Resistance exercises – when muscles work against a weight or force – increase muscular strength and endurance using these types of muscle contractions.

Several studies report exercise consisting of eccentric-only muscle contractions (say, lowering a dumbbell but not lifting it) produces greater cross-education effect than exercise consisting of concentric-only (lifting only) or concentric-eccentric contractions (lifting and lowering).

One study showed eccentric exercise training affected brain-spine responses and stopping (inhibition) signals of the untrained limb to a greater extent than concentric training.