I've been boxing recreationally 4-5 years and I have weird issue with shoulder and I hope someone can help or relate. While i keep my lead arm infront of chin in normal stance, after 10 minutes of punching bag or sparring or just holding the arm up, shoulder of that arm gets so tired and weak (doesnt really hurt) that i cannot keep it up anymore or throw any punches. I dont remember when it exactly started but I feel its like that since 2 years, i thought firstly its just getting used to boxing and keeping arm up so long but it never got any better. I went to couple of physical therapist who fixed and strengthented my rotator cuffs, shoulder blades, biceps and worked my shoulder in million different ways ( electricity, lot of exercise to strengthen it) but nothing helped. I took 2 months off without doing anything about boxing but after 2 months I still had the same issue. I do proper warm ups and stretches, and I can work normally my shoulder, biceps, rotator cuffs ( ive strengthened it alot in past year) and all around shoulder in normal exercises in gym without any issues, its only when holding the arm up in boxing stance and throwing punches that after 3 rounds shoulder gets completely dead and tired and useless.