How does one keep upper body exercises from putting excessive strain on the neck or tzius muscles?

I am a personal trainer and have a client who has a hard time stabilizing weight for upper body exercises and sometimes abdominal exercises without it putting strain into her neck (specifically tzius area) Could this simply be due to an underdeveloped tzius muscle so it is having a hard time providing stabilizer strength? Or could it be something to do with her everyday activity..sleep habits, posture etc.. She is in excellent health, does yoga at least twice a week, and has a very lean build. (about 5'11" with very low BMI and lean build...bordering on gangly)