Knees are joints most prone to injury and also the most complex to heal. It can take a few days to recover from a sprain in the knee to months for more severe injuries. The joint, which joins the thigh bone to the two bones in the shin and also bears the weight of most of the body when it is upright, depends on the proper functioning of muscles in the legs, feet and even in the back and the abdomen for its well being.

An injured knee injured once and healed is also susceptible to relapse. So how do you know when your knee is strong enough to put it through physical activity? One researcher is listening to knees to find out.

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Omer Inan, an electrical engineer at Georgia Tech University in the United States, once suffered knee pain himself. Inan used to be a discus thrower with a rigourous schedule of physical activity that particularly strained his knees. He would often feel that his knees were creaking and popping as he put more stress on them.

When the Defense Advanced Research Projects Agency, a US government body, called for research proposals for wearable technologies for helping rehabilitation, Inan decided to work on knees, especially the sounds that stressed knees make.

Inan and his colleagues at Georgia Tech developed a knee band to which they strapped microphones and vibration sensors to record the sound emanating from a knee as it moves. They heard a lot of cracking, grinding and squishing, which they attribute to the bones in the joint rubbing against each other or against cartilage. In a study published in the journal IEEE Transactions in Biomedical Engineering, they describe how a healthy knee produces more consistent sounds while and injured knee makes erratic noises.

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Inan’s device can help orthopaedic surgeons and physiotherapists better assess the recovery in an injured knee and suggest more rest or more activity accordingly.

Inan describes his novel experiment in this Scientific American video.