"To target these muscles" means to focus on or direct exercises or activities specifically towards working and strengthening those particular muscles. Full definition
It was useful in targeting muscle groups that I have had trouble working out before. (perfecthealthdiet.com)
Build muscle strength and relaxation: If you have fallen arches, those muscles might be weak, work on them with yoga poses specifically targeting those muscles in the lower legs. (mindbodygreen.com)
This will increase the recovery of targeted muscle during the workout. (fitnessandpower.com)