The low bar back squat can be very demanding on the shoulder and arms. With the bar positioned on top of the Rear Deltoid, the Humerus is externally rotated under load.
Inadequate mobility of the Thoracic Spine can cause Scapula Dysfunction (inability to retract Scapula to stabilise the shoulder). Shortened anterior musculature of the Pectoralis Major and Minor can pull the Clavicle forward and internally rotate the Humerus, which is counter productive to the position required to hold an externally rotated position for the low bar back squat.
This may lead to a poor low bar position, an inability to hold upper back tension, stressed shoulders and musculature, compensation leading to Bicep pain, Ulnar nerve impingement or tendonitis around the elbow. These symptoms will severely impede your training.
Here are my recommendations for your pre-squat mobilisation warmup.
Soften and lengthen Pectoralis Major and Minor with a hockey ball. Target the insertions at the Humerus and Clavicle.
Soften and lengthen Latissimus Dorsi (lats) with a foam roller and static stretch. Soften the Rhomboids (muscle between the medial border of the Scapula and Spinous Process) with a hockey ball. Mobilise your Thoracic Spine by extending over a foam roller (hands supporting the Cervical Spine).
Activate the musculature responsible for retraction of the Scapula by using a band pull apart.
These are preventative measures. But, if you already suffer from the above mentioned symptoms then you can.
Use the above mobilisation warmup before squatting.
Ice the area in the case of tendonitis. Reduce training loads temporarily to allow recovery.
Use a safety squat bar or even the duffalo bar to allow you to get in training load.
Experiment with different hand placement. (Ensure you are not attempting to hold a low bar position that’s TOO low, I see this a lot!). Last case, talk to your coach and obtain a referral to Allied Health.