Abstract: A wearable optical device is described for optically detecting parameters of interest within muscle, such as during physical activity or when at rest. The parameters of interest include oxygenation level and/or hemoglobin concentrations in some situations. The detected parameters, such as oxygenation level, may be used to assess physical performance, such as the extent to which the muscle is utilizing aerobic or anaerobic processes. Methods for determining the parameters of interest, such as oxygenation level, from the detected optical signals are also described, and feedback may be provided to a user.