I'd like to hear more about how exactly a lat pull down tracking behind the head strains the shoulder.
Cable Lat Pulldowns can be a great alternative to pullups. For individuals who have access to that piece of equipment can greatly benefit from a lat pulldown tower, especially if they cannot do full pullups yet. Having said that, I think you should stay away from pulling the bar down behind your head - keep the bar in front of your face, coming down to your chest.
Most people do not have the required shoulder flexibility to do a posterior lat pulldown correctly. This, in turn, can lead to bad form and increased potential for injury (shoulder impingement) because the upper arms (humerus bones) are not really designed to track that far behind your body.
If anyone is promoting the use of a posterior cable pulldown, then they must be doing it because they believe that there is either more muscle being activated or you can 'attack' a muscle in your back in a better way...both cases are untrue. In fact, studies have shown that an anterior cable pulldown or pullup activates far more muscle out of the lats, and if you wish to train some of the other muscles in the back that assist in shoulder blade adduction (rhomboids) or humerus adduction (teres major), then you can try changing hand grips, as long as you still maintain the proper transit of the bar in front of your face.
Can a posterior pulldown be done correctly? Sure. But in my opinion, the cons outweigh the risks, and you can achieve far better muscle activation and strength from other forms and keeping things much safer.
I have found a study that was done on varying hand grips on the lat pulldown machine. Here is their "Practical Applications" portion of the paper.
LD = Latissimus Dorsi
WGA = Wide Grip Anterior
WGP = Wide Grip Posterior
PM = Pectoralis Major
CG = Close Grip
Because the primary purpose of the lat pull-down exercise is the development of increased strength during shoulder adduction, it is of great importance to prescribe the handgrip position that elicits the most activity from the muscle primarily involved with this downward movement, namely the LD. The results of this study indicate that the wide grip hand position with the bar pulled anteriorly to the chest (WGA) recruits more motor units, and therefore requires more work from the LD than any of the other conditions tested. Therefore, this handgrip position should be used to provide a greatest amount stimulus and a greater development of the LD than other handgrip positions. This finding may be especially important because it brings into question the necessity to use the WGP position, which has been cited as a condition that increases the potential for injury to both the gleno-humeral joint and cervical spine.
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