Final answer:
Secondary movers in the wrist are considered secondary because they provide assistance to the primary movers, aiding in the execution of movements such as thumb lateral motion and wrist extension, but do not carry out the majority of the movement.
Step-by-step explanation:
Secondary movers at the wrist are considered secondary because they assist the primary movers in performing actions but do not play as significant a role. The primary movers are the major muscles that undertake the initial movement or maintain the position of the wrist, hands, and fingers. The secondary movers, often located in the deep posterior compartment of the forearm or the superficial posterior compartment, support these actions, such as when the thumb moves sideways or the wrist straightens away from the body. The effectiveness of the movement relies on the coordinated efforts of both primary and secondary movers.
The mobility of the first metacarpal bone, independent from the other metacarpal bones, plays a critical role in thumb mobility. The fourth and fifth metacarpal bones also exhibit limited anterior-posterior mobility, which is crucial during power gripping actions.