Abstract: A wearable lactic threshold and entrainment exercise device (LTEExD) is described herein. LTEExD may assist subjects in improving athletic performance. Users adjusting their exercise regimen in real-time with the assistance of LTEExD often exercise more efficiently and perceive exertional status as feeling like a ‘second wind’. LTEExD collects physiologic data, and uses a signal analyzer to convert the data from the time domain to the frequency domain primarily utilizing fast fourier transform based spectral analysis to accurately determine certain physical variables. Variables are determined, integrated, and compared by the signal analyzer to determine whether lactic threshold has been reached and/or entrainment has occurred.