Joint motion and respiration monitoring. The strain sensor is attached on a knee (a), an elbow (b), and a wrist (c) for monitoring the strains induced by joint flexion. (d) The strain sensor is attached on an index finger for sensing the strains induced by holding objects of different sizes. (e) The strain sensor attached on a knee can detect different states of physical activities, including squatting, flexing, jumping, and walking. (f) The strain sensor is attached on the abdomen for monitoring respiration. (g) The integrated strain sensor is attached on an elbow. When the elbow is not flexing (left), or the flexion angle is small (middle), the LED stays off. When the elbow flexion angle is too large (right), the LED turns on.