Many guard passes are difficult because they are designed around controlling hip movement via trapping, pinning, and/or lifting against the powerful piston action of the quadricep and hamstring.
Comparatively, the weaker links are the hip abductor and flexor muscles that move the leg at the hip girdle.
To feel these muscles, balance on one leg and lock the suspended leg at the knee. From here, move it around the ball and socket joint of the hip. As you’ll likely feel, these muscles are extremely functional for their respective purpose, but not nearly as strong at the hamstrings and quadriceps when it comes to moving and supporting mass.
Design passes (and a guard game!) accordingly. ;o)