pick up two 3 lb weights and hold tem out in front of you with your arms straight and perpendicular to the ground. You'll reach muscle failure pretty quick
And be much less likely to injure yourself than trying to do three sets of military presses with heavy weights. It just requires a different type of exercise, I wouldn't want to do 300 curls with light weights either. I can reach muscle fatigue with some seemingly simple yoga exercises very quickly.
