The shoulder’s large range of motion is due to the low congruency of the glenohumeral joint, whose stability relies mainly on rotator cuff muscle activity. The effect of joint congruence on shoulder biomechanics remains unclear. We used a sphere-on-sphere glenohumeral model combined with a Force-Dependent Kinematics algorithm to simulate muscle and joint forces while considering humeral head displacements. Our innovative simulations showed an increase in humeral head displacements and rotator cuff muscle forces when joint conformity decreased. Our model aligns with in vivo observations and highlights the importance of joint congruence on stability. It provides insights to improve our understanding of shoulder biomechanics.