The device is a combination of two separate sensors that, when applied to the intact skin over exercising skeletal muscle, detects, transmits and analyzes an index of activity. Using established near-infrared spectroscopy technology (in the public domain) and Patent pending non-invasive cardiac output assessment technology, this invention derives a personalized index of exercise effort and predicts exhaustion prior to depletion of muscle and cardiac ability. The invention demonstrates these findings to the user in real time.