When extending the hips, which of the following is a synergist (or assistant mover) muscle?

In the context of hip extension, the hamstrings function as synergist muscles. When the hips extend, the primary mover responsible for this action is the gluteus maximus. However, the hamstrings assist in this movement by helping to facilitate the extension of the hip joint. They play a significant role in stabilizing the pelvis and supporting the primary action of the glutes during various movements, such as running, jumping, and squatting.

While the glutes are the primary movers for hip extension, and the quadriceps primarily function to extend the knee, their roles do not align with assisting in hip extension specifically. The soleus, on the other hand, is predominantly involved in plantarflexion of the ankle and does not contribute significantly to hip extension. Therefore, the hamstrings are the correct synergist muscles in the context of hip extension, working alongside the glutes to achieve this motion effectively.