What factor is primarily increased when moving forward in the water with arms behind the back and then out to the side?

When moving forward in the water with arms initially behind the back and then extending them out to the side, the primary factor that increases is frontal resistance. This change in arm position alters the swimmer’s profile in the water.

When the arms are behind the back, the swimmer has a streamlined position, presenting less surface area to the water, which reduces drag. However, once the arms are extended out to the side, this increases the frontal area that comes into contact with the water, thereby increasing the resistance faced by the swimmer. This increased resistance can affect the efficiency of the swimmer's motion through the water, making it harder to maintain speed.

Other factors such as depth, speed, and buoyancy may not be as directly influenced by this specific change in arm position. Depth pertains more to vertical positioning in the water, speed is a result of various factors including technique and propulsion, and buoyancy relates to the swimmer's ability to float, which is less affected by the position of the arms in this context. The primary focus with extending the arms out to the side is indeed the increase in frontal resistance encountered.