The aim of this study was to examine the validity of automatically stored exercise-data from the elastic band sensor© compared to a gold-standard stretchsensor during exercises commonly used for rehabilitation of the hip and knee. The design was a concurrent validity study. Participants performed three sets of 10 repetitions of six exercises with both sensors attached to the same elastic exercise band. These were knee extension, knee flexion, hip abduction and adduction, hip flexion and hip external rotation. Agreement between methods was calculated for: date, time-of-day, repetitions, total and single repetition, and contraction-phase specific time-under-tension (TUT). Files from the elastic band sensor© contained identical dates, time-of-day and number of repetitions for each exercise set compared to the gold-standard. Total TUT and total single repetition TUT were highly correlated with the stretch-sensor (r=0.83-0.96) but lower for contraction-phase specific TUTs (r=0.45-0.94). There were systematic variations between the methods ranging from 0.0-2.2 seconds (0.0-6.3%) for total TUT and total single repetition TUT, and between 0.0-3.3 seconds (0.0-33.3%) for contraction-phase specific TUTs. The elastic band sensor© is a valid measure of: date, time-of-day, number of repetitions and sets, total TUT, and total single repetition TUT during commonly used home-based strength training exercises. However, the elastic band sensor© does not seem able to accurately measure TUT for specific contraction-phases.