So keep going over the sketchy lines with the sharpie or pen. Those lines we made on the ribcage we now draw the fuzz on for detail! We draw box-like lines for the pectoral muscles. Make sure they're a little curved, and don't connect the lines.
The arms should also have a hint of musculature to them. Not much, but they should be divided by rounded sections of muscle. Shoulder, bicep, and fore-arm.