"To extend the hip" means straightening or moving the leg backward at the hip joint.
Sentences with «to extend the hip»
From this position, stand back up to the start position by extending your hips towards the bar and again keep it close or in contact with your legs. (traineatgain.com)
Return back to the start position by bending the knee and then extending the hip until the foot is on the floor again. (sportsinjuryclinic.net)
Keep your chest up and back straight as you pull the bar off the floor by fully extending your hips. (americanfootballinternational.com)