The WHOOP monitoring platform continuously measures physiological strain and the body’s response to that strain to provide personalized training recommendations based on an individual’s response to various loads. The ability to action continuous data by applying it to dynamic training recommendations is a unique feature of the WHOOP system, and is primarily made possible by the proprietary “Day Strain” algorithm. Day Strain is a measure of user-normalized total cardiovascular load that jointly accounts for both obvious (example: from workouts) and non-obvious (example: from emotional stressors and activities of daily life) inputs. Day Strain therefore allows WHOOP to quantify the effects of down time, something not possible in cases of fitness tracking wearables designed to be removed after exercise.
The first section of this report provides a detailed explanation of the WHOOP Day Strain feature. The remainder of the report then demonstrates the significance of non-workout strain both by illustrating the magnitude of the cardiovascular load accrued outside the workout and by demonstrating the practical significance of this additional information by analysis of the predictive power of Day Strain as compared to the predictive power of workout Strain alone for key recovery metrics such as next-day resting heart rate and heart rate variability.