I have read that in a lat pull down, we should not pull the bar behind the neck. If this is true, could you please explain the scientific rationale for this?
This is a great question of biomechanics. First, I will mention that in this industry, I've found it is a good idea to resist using blanket words like "bad," "always" and "never" when referring to exercises. Usually, the most appropriate approach is to simply state that there may be a healthier, more efficient option that is more conducive to the individual's goals strength goals. That being said, yes, the front of the neck pull down will probably be a safer option in most, if not all, instances. Reason being as follows:
- To perform a pull down behind the neck without any shoulder girdle and/or spinal compensation, an individual must posses the ability to achieve 100-120 degrees of ACTIVE external shoulder rotation while abducted. When this ROM is lacking (as will be in the vast majority of people), the force from the machine is pulling the shoulder joint toward external rotation which places large amounts of stress on the anterior shoulder capsule! You'll see this lack of ROM displayed in the individuals performing the exercise behind the neck as compensatory cervical and/or lumbar flexion!
- Assuming training the lat is your goal, one must take into account the functional anatomy of the situation. The lat attaches anteriorly on the superior portion of the humorous and then winds back, fans out and attaches into the thoraco lumbar fascia (low back). Hence, the lat does not run directly from top to bottom... it runs from top/front to bottom/back, at an angle. An attempt to pull straight down - as in a behind the neck pull down - to train the lats simply does not follow the lats fiber alignment, there by not following the lats angle of pull! This would make the front of the neck pull down the more appropriate choice based on its anterior to posterior angle of motion. REMEMBER, A MUSCLE'S ABILITY TO PULL IS DICTATED BY ITS FIBER ALIGNMENT (I.E. ITS ORIGIN AND INSERTION), NOT BY THE MACHINE CHOSEN!
To learn more about biomechanics and their practical application to exercise, take time to explore the "Advanced Workout" and "Programs & Assessments" article categories on the PTN Content Library. I also recommend the Resistance Training Specialist Manual by Tom Purvis.