I had a little soreness at the very top of my left arm where it attaches to the shoulder. I'm not sure why, but I backed off on my extensor action a tad and it seems to have helped. Dont know if I was doing something wrong, letting a little tension creep in to the shoulder/arm connection or something.
I like to Hit from End too but am wondering if it is a good fit with an Angle of Approach or whether it is best stay on the Arc given that the #3 pp is on the knuckle in Startdown. Not sure, its a question I have for Luke or Yoda. From Top with Drive Loading and the #3pp on the first joint of the right index finger the Angle of Approach works really nicely. But I love that feeling of Float Loading and have trouble stopping at Top. I dunno? Still searching for my pattern.
Is there any way your soreness could be from a quick rotational swingers turn in Startdown, your low under the TSP angle left arm getting stretched to the max? The solution to this would be a Slow Startdown from Top with an active Drive or thrust of #1 against the #3 pp in the first joint at Release. Classic drive loading, Hitting, Angle of Approach procedure. Just an idea. Maybe send some tape to Luke for a proper diagnosis.
If you're like me the soreness in that particular place is a bit of a worry. What ever causes it must be removed from your swing lest you get a chronic golf related injury. Dont want to suffer like Ken Venturi did or anything.