Elastic bands embedded with colour-changing sensors can reveal how hard muscles are working by measuring the acidity of sweat on skin.

The device could be used to improve worker safety in physically demanding jobs, such as the construction industry, as well as to help sportspeople optimise their training, says John Rogers at Northwestern University in Illinois.

“If you see your pH dropping down to very low levels, it’d be a good idea to stop working out,” says Rogers. “Otherwise you end up with sore muscles. But it’ll also tell you if you haven’t worked out enough.”

During high-intensity exercise, muscles produce a chemical called lactate. With continued exercise, levels of this in the blood start to shoot up as people near their limits of physical endurance. This can lead to a burning sensation in muscles, a sudden loss of energy and extreme fatigue. The fitter people are, the longer and harder they can exercise before blood lactate rises to high levels.

For this reason, the blood lactate levels of elite athletes are often monitored during training. This requires getting samples from a finger prick, so many groups are trying to develop a non-invasive alternative.

The solution Rogers and his team have come up with is an elastic band with a series of tiny reservoirs that fill with sweat at different intervals. Inside each reservoir is a sensor that changes colour depending on things such the concentration of lactate or the acidity of sweat. The results are read by taking a photo with a smartphone or attaching an electronic monitor.

When the sweat monitors were tested on 12 volunteers on exercise bikes, the team found that sweat lactate levels didn’t correspond well with blood lactate levels, but that the acidity of sweat did.

“What we found is that the pH of sweat is a better indicator of lactate concentration in blood than the lactate concentration in sweat,” says Rogers.

However, this was only the case for sweat from skin near the working muscles, which in this case was monitored by the bands being around the ankles. Measurements by bands on the wrists of the cyclists didn’t reflect blood lactate levels.

“It’s not a solution to everything that one might want to know, but it is providing a new type of data that wasn’t available previously,” says Rogers.

A company he co-founded, called Epicore Biosystems, is already producing disposable patches that measure sweat loss and electrolyte levels, which it says can help prevent dehydration. These are being used by some companies to monitor people doing physical work in very hot and humid conditions, as well as by athletes, says Rogers.