I know in Trendelenberg gait the patient’s R hip will drop and this causes the stance leg (the L glut medius) to be weak. I also know that in backward lurch the gluts are weak and in forward lurch the hip flexors are weak. I do not understand why if a patient lurches to the R side as soon as the L foot touches the ground, why would the L glut medius be weak?

The weakened gluteal muscles (notably the hip ABDuctors) is what leads to the pathological gait known as Trendelenburg gait. (From your question), it is not in all cases that the patient’s right hip drops. For clarity, the side of the hip with the weakened muscles is used to qualify the gait e.g. Trendelenburg gait secondary to weak right hip abductors (instead of saying ‘right Trendelenburg’. Bear in mind also that the abnormal gait pattern may be secondary to weak left hip abductors).

A compensated Trendelenburg occurs when the patient’s trunk leans ipsilaterally to the side of the stance leg. An uncompensated Trendelenburg occurs when the examiner observes a contralateral pelvic drop. A normal test result is when the patient is able to maintain a level pelvis without either of these substitutions. It should be noted that a hip hike is not a true Trendelenburg gait.

In a normal subject, the left gluteus medius is responsible for holding the right side of the pelvis level during the left foot stance phase (i.e. the right side of the pelvis does not dip when the right foot is going through its simultaneous swing phase). With a weak left gluteus medius, the patient is unable to maintain the pelvis level the right side of the pelvis dips/drops during left foot stance phase. An attendant lurch to the right side describes an uncompensated Trendelenburg gait.

Reference 1