Can you improve the health of one part of the body by exercising another part of it? Unlikely as it may seem, the idea is gaining traction among scientists looking for new ways to manage the effects of chronic illness, stroke and even breast cancer surgery.
Researchers at the University of Texas in El Paso, US, are set to test a remarkable new theory that it may be possible for people with osteoarthritis to banish pain in their knees by exercising their arm.
Last week, researchers launched a new clinical trial in which 60 men and women with frequent pain from wear and tear on their knees will test different exercises to see which provides the most relief.
The trial, due to be completed in 2025, will determine whether regular 20-minute sessions of arm exercises (using an “arm cycle” machine like those found in gyms ) are better than cycling for legs to relieve knee pain and increase mobility.
The goal is to try to replicate the results of previous small studies suggesting that arm cycling is better than walking on a treadmill for relieving knee pain.
Regular, moderate exercise is one of the best ways to manage arthritic knee pain because it strengthens the muscles around the joint and provides relief to the damaged area.
But walking or jogging can increase the load on the knee, making it difficult for some to move. “There is no obvious mechanism by which arm exercise is likely to target knee pain,” says Uzo Ehiogu, consultant physiotherapist at the Royal Orthopedic Hospital, Birmingham, commenting on this approach.
“What likely happens is that patients feel fitter, more confident and may be more mobile after a 20-minute arm workout, which may then reduce the feeling of pain in the knees.”
But, in some cases, one member’s training actually has a direct – and fascinating – impact on the other.
Recent evidence for this came in a study published in July in the Scandinavian Journal of Sports Medicine and Science.
Researchers at National Taiwan Normal University in Taipei, Taiwan, recruited 30 volunteers who kept one arm completely still for hours while flexing the muscles of their free arm; others kept both arms still.
After several weeks, those who contracted the muscles of the moving arm lost only 2% of the muscles of the stationary arm.
However, in the no-exercise group, muscle loss in the static arm was 28 percent.
This effect is known as muscle cross-education, where the muscles on one side of the body benefit from the activity of those on the other side. It is commonly used in sports medicine to reduce rapid muscle loss during injury.
Studies show that an injured arm in a sling loses up to 60 percent of its muscle strength in the first five weeks. “But with regular exercise of the other limb – ‘contralateral’ training – muscle wasting will be significantly reduced,” says Mr Ehiogu.
It is not clear how muscle cross-training works. One theory is that it is a “trickle down” effect, whereby giving the healthy limb a thorough workout (by lifting weights with the arm or doing single-leg squats, for example) also generates new connections between the brain and the injured limb.
But the key to its success, says Mr Ehiogu, a spokesman for the Chartered Society of Physiotherapy, is to make the active limb work harder than usual by increasing resistance, that is, lifting a heavier weight.
“You need to work at 80 to 85 percent of your maximum capacity,” he says. “So if you normally achieve ten reps working at 50 percent of your maximum capacity, try doing five reps instead, but at 80 or 85 percent of your capacity.
“If you don’t increase the resistance, you don’t get the crossover effect.”
One of the most intriguing examples of this phenomenon is mirror therapy, used on stroke patients who are paralyzed on one side.
Their damaged arm is placed in a box with a mirror outside; the patient then performs a series of exercises with the good arm to improve strength and grip, while looking at the same arm in the mirror.
This “tricks” the brain into thinking it is working on the damaged arm, triggering connections with the nerves and muscle fibers there.
A 2018 Cochrane review of mirror therapy in the treatment of stroke (which reviews the evidence for medical treatments) concluded that it improved movement in paralyzed limbs enough for patients to carry out daily activities such as cleaning. .
And a study published in June in the Journal of Cancer Survivorship, carried out by Fudan University in China, found that mirror therapy also helps women with limited shoulder function after surgery or radiation therapy for breast cancer.
Both treatments can cause immobility and pain due to tissue removal, nerve damage, and scarring. In the study, half of the 79 participants exercised their unaffected arm daily while looking at it in a mirror; the others did the same without a mirror.
After eight weeks, the mirror group had a better range of motion in the affected arm than their peers. In a report on the findings, the researchers said: “Breast cancer survivors can try mirror therapy for effective shoulder rehabilitation.”