Abstract: Wearable sensors and uses thereof are disclosed. Preferred sensors include surface EMG electrodes for measuring electrical muscle activity, by which muscle contraction is measured. Such muscle contraction can be used to determine a person's posture, especially when the contraction being measured is that of core muscles. An alert preferably is provided to a person in real time when an incorrect posture is detected. By providing such biofeedback, a person is trained in maintaining a good posture. Such posture training is believed to heal alleviate or even present lower back pain. Wearable sensors of the invention also can be used to measure contraction in muscles during exercise, including running and strength training, to gain information for use in improving such activities.