Objectives There are several reports suggesting that forward head posture contributes to alterations in scapular kinematics and muscle activity, leading to the development of shoulder problems. Currently, it is unknown whether forward head posture alters the thickness of the scapular muscles. The aim of this study was to compare the thickness of the serratus anterior and upper and lower trapezius muscles at rest and during loaded isometric contractions in individuals with and without forward head posture. Methods Twenty individuals with forward head posture and 20 individuals with normal head posture participated in this case-control study. Three separate ultrasound images of the serratus anterior and upper and lower trapezius muscles were captured under 2 randomized conditions: at rest and during a loaded isometric contraction. Results The thickness of each muscle significantly increased from rest to the loaded isometric contraction (P?<?.001). The only difference between the groups was that the thickness of the serratus anterior muscle at rest in the normal-posture group was larger than that in the forward-posture group (P?=?.01). Conclusions Forward head posture appears to be related to atrophy of the serratus anterior muscle, which may contribute to the development of shoulder problems. Further research is required to identify more about the association of forward head posture with the imbalance of shoulder girdle muscles and the impact of head posture on upper quadrant pain.