Great news! Ive been lifting for decades and the LBM drop I see lines up with what calorie restriction alone would cause, so overall its pretty small. Strength has gone down though even if the muscle itself is still there.
The big drop in body weight from GLP-1 drugs has people worried about losing lean body mass and muscle. This paper covers four animal studies plus a small human trial. In obese mice the drugs cut fat mostly and cause only a modest drop in lean mass, with liver shrinking more than muscle. Absolute muscle and strength fall a bit, yet relative to body weight both improve and running gets better. The muscle proteome changes differently than with simple calorie cuts during immobilization. People with obesity using these drugs show better body composition and no strength decline. In middle-aged mice and men the medicines lower absolute muscle numbers slightly while improving overall composition and movement.