This study aimed to investigate how muscle activation and intervertebral compressive forces during walking and running are altered with different head supported mass (HSM) types. A detailed neck musculoskeletal model was adapted and simulations were performed using existing motion data. It was found HSM wear required increased muscle activations, with the highest increase in running. Extensor activation increased particularly for the HSM with its center of mass (COM) in front of the head’s COM and flexor activation was significantly higher in running than in walking. Intervertebral compressive forces increased with HSM wear and the heaviest HSM caused the highest increase. During walking, the computed maximum compressive force at C7 was 129.7N without an HSM, and 163.6N and 208.5N with HSMs with a mass of 1.43 kg and 3.12 kg respectively. For running, it was 275.7N without an HSM, and 349.1N and 451.2N with the two HSM types. Overall muscle contributions to the compressive force varied over the gait cycle and were higher in running (26–110%) than in walking (18–58%). It was concluded that neck musculoskeletal loading increases with HSM wear, which is affected by HSM mass and mass distribution.