Most major back exercises (rows, pullups, lat pulldowns) incorporate the biceps, as your elbows are flexed to pull the weight toward you or pull yourself up to the pull-up bar. Even the guy with the strongest mind-muscle connection is going to experience, from time to time, his arms giving out before his back muscles have been sufficiently fatigued, resulting in too little lat stimulation to spur lat growth.
Isolate the lats with a single-joint exercise like straight-arm pulldowns in addition to the aforementioned compound moves on back day. Your biceps will get a break, and your lats are guaranteed to bear the brunt of the work.
Arms extended in front of body, hands directly in front of shoulders
1. Hold a lat pulldown bar attached to a high-pulley cable straight out in front of your shoulders with your arms extended.
2. Step back a foot or two— just enough so the weight doesn’t rest on the stack—to ensure constant tension on the muscles.
3. Bend your knees slightly and position your feet shoulder-width apart.
4. Look straight ahead throughout and keep your shoulders down.