What difference would it make if you put your hands in a supinated rather that pronated position during straight arm pulldowns performed on the Free Motion Cable Cross machine? I am currently researching this concept and would like to have your expert opinion.
Good question. My question back to you is, what's the goal? Let's take a look at the basic posture and mechanics of the movement. A PRONATED GRIP, depending on how the trainer is cueing the client, may encourage internal rotation at the GH (glenohumeral) joint, thereby encouraging maximum shortening of the latissimus dorsi. The problem with that rationale is the posture that is most likely being encouraged is one of rounded shoulders and protracted scapulae. This is otherwise known as Upper Crossed Posture and is all too common in today's seated and non-mobile day and age.
Encouraging a SUPINATED GRIP, or even simply a "neutral" grip, may reinforce external rotation at the GH joint, thereby encouraging retraction and depression of the scapulae, or rather, supination of the shoulder girdle. Either way, it is going to primarily depend on how/what the trainer is cueing and ultimately the abilities and goals of the client (i.e., posture, CORE STRENGTH, etc.).
With regards to being under load, the "CARRYING ANGLE" (CA) is also something to consider. The CA can be observed in an individual by having one drop the arms straight to the sides and turn the palms forward (supination). There will be a natural deviation laterally (outward) of the lower arm/wrist. This is due to the angle at which the ulna and radius meet with the humerus at the elbow joint. It is wise to load the elbow joint with great regard for the CA (such as in straight arm pulldowns, curls with a bar, pull downs, shrugs, etc.) to avoid any poor mechanics under load that may cause injury.
Good luck with your research!