It has been proposed that neck muscle activation may play a role in head response resulting from impacts in American Football. The importance of neck stiffness and active musculature in the standard Linear Impactor Helmet Test was assessed using a detailed head and neck finite element (FE) model from a current Human Body Model (HBM) compared to a validated Hybrid III head and neck FE model. The models were assessed for bare-head and helmeted impacts at three speeds (5.5, 7.4, 9.3 m/s) and three impact orientations. The HBM head and neck was assessed without muscle activation, and with a high level of muscle activation representing a braced condition. The HBM and Hybrid III had an average cross-correlation rating of 0.89 for acceleration in the primary impact direction, indicating excellent correspondence regardless of muscle activation. Differences were identified in the axial head acceleration, attributed to axial neck stiffness (correlation rating of 0.45), but did not have a large effect on the overall head response using existing head response metrics (HIC, BrIC, HIP). Although responses that develop over longer durations following the impact differed slightly, such as the moment at the base of the neck, this occurred later in time and therefore did not considerably affect the short-term head kinematics in the primary impact direction. Though muscle activation did not play a strong role in the head response for the test configurations considered, muscle activation may play a role in longer duration events.