## 4 Discussion
These differences were attributed to differences in muscle path definitions and anatomical assumptions. Taken together, the present results align with and expand on previous literature suggesting that muscle activity predictions in musculoskeletal simulations are highly sensitive to model structure, optimization strategy, and the nature of the task being simulated. These factors may explain why certain muscles demonstrated consistent activation levels while others did not. Moreover, the variation in statistical significance across muscles suggests that some may be more robust to model assumptions than others. Importantly, this study focused on muscle activity as the primary parameter of interest. As defined in AnyBody, muscle activity reflects the ratio of a muscle's force to its maximum force capacity and serves as a normalized indicator of muscle effort. While this metric allows for comparisons across models and individuals, it should be noted that it does not incorporate time delays, co-contraction behavior, or subject-specific neuromuscular strategies. As discussed by [PERSON] et al. and [PERSON] et al., this limitation may contribute to the differences in timing and magnitude observed between simulated and experimental data. Musculoskeletal simulations of an isometric external hip rotation often show different muscle activation patterns between static and dynamic models. These differences arise from several biomechanical factors. First, individual muscles have distinct functions and moment arms that can change with joint position. For example, the _Gluteus maximus_ is a powerful external rotator with a large moment arm [35], but its contraction also produces hip extension that must be counteracted by a hip flexor (such as _Iliopsosa_). A static optimization model (solving one static posture) tends to favor muscles with the most effective moment arms and minimal antagonist involvement [36]. It may therefore under-utilize stabilizing muscles or co-contraction. In contrast, a dynamic simulation (which accounts for time-dependent motion and stability) often recruits a broader set of muscles. The _Iliopsosa_, for instance, may be more active in dynamic models to stabilize the hip joint and center the femoral head during the external rotation effort [37], even though static models might not engage it if no net flexion torque is required. Likewise, the _Gluteus medius_ and minimus--primarily hip abductors--contribute to dynamic hip stability by compressing the joint and preventing unwanted pelvis motion [38]. Dynamic conditions (even during an isometric task) demand such co-activation to maintain joint alignment and resist perturbations, whereas static analyses omit these requirements. Furthermore, the treatment of input data differs between static and dynamic simulations. In static models, calculations are based on instantaneous equilibrium, whereas dynamic models incorporate continuous changes in external forces and joint angles. Although both simulations in this study were aligned at the same time point, the way the two models interpreted input data over time may have contributed to divergence in predicted activation. Although direct validation using EMG was initially considered, it was ultimately excluded from the study for both methodological and practical reasons. A substantial proportion of the analyzed muscles--particularly the deep external rotators of the hip, such as the _Piriformis_, _Obturator internus_, and _Obturator externus_--are inaccessible to surface electromyography (sEMG), which only records signals from superficial muscle layers. Using intramuscular EMG (needle electrodes) in a large sample of healthy participants would raise ethical concerns and introduce considerable logistical complexity. Moreover, surface EMG is associated with well-documented limitations, including cross-talk from adjacent muscles and the integration of signals from multiple motor units within the same recording area. These factors diminish the specificity of EMG and hinder its use for accurately estimating muscle force.