It's guns day at the gym. Unfortunately, a brutal weekend has put your left arm out of action for a while. Fear not – new research suggests doing a few bicep curls with just your right arm could benefit both limbs.
A recent study by researchers from institutions in Chile, France, and Australia has found an immobilised arm will hold onto more of its muscle mass, and even gain strength, if its opposite undergoes a month-long regime of resistance training, especially when the exercises are of an 'eccentric' nature.
Any bodybuilder will tell you muscles work as a team to create motion, supporting and anchoring various parts of the skeleton as you crunch and curl that iron. So it shouldn't come as a surprise that body parts not obviously involved in a task might still be getting in on the action.
Very little research has been done devoted to testing this assumption when it comes to opposing limbs like our arms. Of two studies that have measured a significant effect, neither distinguished whether the nature of the exercise makes any real difference.
The specific type of weight training is no trivial matter, however.
Picking up a dumbbell and bringing it to your shoulder with a bend of your elbow is caused by what's referred to as a concentric contraction of the bicep. The fibres contract, and the whole muscle shortens.
Extending your arm, on the other hand, is managed by an eccentric contraction. The bicep's fibres still exert an effort to carry the weight, but collectively the whole muscle lengthens.
Both types of workouts have their place in developing general fitness, but if it's building strength you want, focussing on eccentric contractions seems to be the way to go.
To see whether eccentric contractions are also preferable for an immobilised arm, researchers invited 18 men and 12 women between the ages of 18 and 34 to spend a month with an arm in a sling for eight hours a day (excluding driving, showering, and sleeping).
A third of the group didn't do much else. They went about their lives as if they had a broken forearm.
The remainder were divided in half, with ten individuals performing a mix of concentric and eccentric exercises three times a week, and the other ten engaging solely in eccentric routines.
At the end of the month, the whole group had their immobilised bicep studied against various metrics, including its circumference, strength, and neural input.
"Participants who did eccentric exercise had the biggest increase in strength in both arms, so it has a very powerful cross-transfer effect," says medical researcher and one of the study's authors Ken Kazunori Nosaka from Edith Cowan University in Australia.
"This group also had just 2 percent muscle wastage in their immobilised arm, compared with those who did no exercise who had a 28 percent loss of muscle. This means that for those people who do no exercise, they have to regain all that muscle and strength again."
Just how these exercises helped a still limb stave off atrophy in its muscles isn't entirely clear.
One possibility is that by managing a complex set of contractions of muscles in one limb, the brain is also sending signals to the other side of the body to cause muscles to remain active.
It's also likely that the entire supporting network of muscle contractions extends across the body, having a subtle but non-zero influence on otherwise immobilised biceps.
There's also the question of whether similar effects could occur for other contralateral body parts, such as for each leg.
For those nursing an injury or have experienced a debilitating stroke, balancing the risks and benefits of engaging in specific exercises at any given time could give patients a head start on recovery.
"By starting rehab and exercise in the uninjured limb right away, we can prevent muscle damage induced by exercise in the other limb and also build strength without moving it at all," says Nosaka.
This research was published in the Scandinavian Journal of Medicine & Science in Sports.