Electromyographic (EMG) activity of the vastus medialis (VM), vastus lateralis (VL), semitendinosus (ST) and biceps femoris (BF) muscles will be recorded during knee extensions performed in a knee extension machine until contraction failure. The relative loading will be 10 repetition maximum (RM). The absolute load (kilograms) corresponding to 10 RM is defined a minimum of 3 days before the day where the EMG-data are recorded. The primary outcomes will be normalized EMG amplitude and median power frequency for each 10th (10%, 20%, 30% failure, etc.) of the set to failure.