Trendelemburg gait:

Glut. medius and minimus - Contract when other foot is off the floor to prevent pelvic tilt towards the unsupported side. e.g. right sided muscles contract when left foot off floor.

Paralysed muscles (e..g due to Sup.gluteal nerve lesion) = Trendelemburg sign

Patients will compensate for this by using their upper body mass to lean towards the affected side to stop the leg from hitting the floor. E.g. in a right sided lesion patients will walk and lean to the right side when left foot is off the floor to prevent the left foot from drooping.