The antonym of the term "upper arms" could be the lower limbs or the lower extremities. While the upper arms refer to the upper part of the arms, from the shoulders to the elbows, the lower limbs refer to the lower part of the body, from the hips to the feet. They are different in terms of location and function, and they have opposite characteristics. The lower limbs are designed for mobility, stability, and weight-bearing, whereas the upper arms are designed for manipulation, flexibility, and dexterity. Thus, if you want to improve your upper arms, you may focus on exercises that target the biceps, triceps, and shoulders. However, if you want to strengthen your lower limbs, you may opt for exercises that target the quads, glutes, and calves.