Possible Clues:

• Rowers work them, briefly
• Chin-up targets, briefly
• Pull-up targets
• Chin-ups and pull-ups develop them
• Rowing beneficiaries, for short
• Muscles worked by rowing
• Muscles worked by pull-ups, briefly
• Pulldown muscles, for short
• Muscles worked by pull-downs, for short
• Muscles worked in pull-ups and pull-downs