During initial screening and assessment, a client has shown excessive tightness in the hip flexors.

During initial screening and assessment, a client has shown excessive tightness in the hip flexors. The ACE certified Personal Trainer has been working on improving the client's flexibility of the psoas, rectus femoris, and adductors. After three weeks, little progress has been made. Which exercise could be implemented to help improve the client's mobility of the hip flexors?