Final answer:
A weak gluteus medius can cause an athlete to display a Trendelenburg gait, where the hip drops on the side opposite to the weakness during the stance phase, leading to a side-to-side motion.
Step-by-step explanation:
An athletic trainer observing an athlete with a weak gluteus medius may notice a specific type of gait called the Trendelenburg gait. This altered walking pattern is characterized by a dropping of the hip on the side opposite to the weak gluteus medius during the stance phase, when the foot is on the ground. It is caused by the inability of the gluteus medius muscle to adequately stabilize the pelvis during single-leg support.
During a normal gait cycle, the gluteus medius contracts to prevent the pelvis dropping on the opposite side. However, with weakness in this muscle, an individual compensates by leaning toward the weak side to reduce the demand on the gluteus medius. This compensatory action leads to the characteristic side-to-side motion of the Trendelenburg gait.