An elastic band developed by Northwestern University, in Illinois, can show how strenuously muscles are working by measuring the acidity of perspiration on the skin. The band can be placed around the user’s working muscle, such as the ankle of a cyclist or the wrist of a tennis player.

A series of tiny reservoirs collect sweat at different intervals, while a sensor measures the acidity, and a smart phone or electronic monitor reports the results. A low pH may indicate the need to stop exercising to avoid sore muscles, whereas a high pH suggests the user has not worked out enough.