Context: Neck pain (NP), neck injuries, and concussions are more prevalent in female athletes than in their male counterparts. Females exhibit less neck girth, strength, and stiffness against a perturbation. As part of the clinical examination for individuals with NP, ultrasound (US)-based imaging of the cervical muscles has become common. Muscle size or thickness and stiffness can be measured with US-based Bmode and shear-wave elastography (SWE), respectively. Information on reliability, normative values, and sex differences based on US-based muscle size or thickness and stiffness in young and athletic individuals is limited. Objective: To evaluate sex differences in US-based muscle size or thickness and biomechanical properties of the cervicalflexor and -extensor muscles. Design: Cross-sectional study. Setting: Laboratory. Patients or Other Participants: A total of 13 women (age= 23.7 6 1.9 years, height = 167.1 6 6.1 cm, mass = 63.8 6 5.6 kg) and 11 men (age = 25.6 6 4.9 years, height = 178.7 6 8.3 cm, mass = 78.9 6 12.0 kg). Main Outcome Measure(s): The same examiner collected all measures, using US B-mode to scan the cross-sectional area and thickness of the longus colli (LC), sternocleidomastoid (SCM), cervical-extensor muscles, and upper trapezius (UT) muscle. The US SWE-mode was used to measure the stiffness of the SCM and UT. Independent t tests or Mann-Whitney U tests were calculated to determine sex differences. The intraclass correlation coefficient (ICC) measured intrarater test-retest reliability. Results: Men had thicker SCMs than women (P = .01). No sex differences were present for longus colli cross-sectional area, cervical-extensor muscle thickness, or UT thickness (P . .05). In addition, no sex differences were evident for SCM (P = .302) or UT (P = .703) SWE stiffness. Reliability was good to excellent (ICC = 0.715-0.890) except for SCM SWE stiffness (ICC ? 0.554). Conclusions: The only sex difference was in SCM thickness. However, smaller SCMs in women did not result in less SCM SWE stiffness. We provided normative values for USbased imaging of the cervical-flexor and -extensor muscles in young and athletic men and women. [ABSTRACT FROM AUTHOR]