A garment and system includes a monitoring fabric that exhibits a light reflection property and substantially no light transmission property when the fabric is illuminated with light having wavelength(s) in the range of 400 to 2200 nanometers. The amount of useful light reflected by the fabric into an aperture of acceptance defined with respect to an imaginary axis extending from the fabric relative to the amount of light lost to the aperture of acceptance detectably changes when the fabric stretches in response to motion, as the motion induced by physiological activity (e.g., heart rate). The system includes at least one radiation source and at least one radiation detector, with the detector disposed in the aperture of acceptance. The source and detector may be attached to the fabric in relative positions such that the reception of incident radiation by the detector is directly affected by a change in the amount of useful light reflected by the fabric into the aperture of acceptance as the fabric stretches in response to motion.