Cycling is an aerobic endurance activity. It robs you of you muscle mass if you do not lift weights to counter it. You just have to take a look at the physique of most professional cyclists/runners. While it's not impossible to build muscle mass as an endurance athlete, it's very difficult, and requires that much more discipline in your nutrition. But the key to keeping that precious lean muscle mass is to lift heavy weights. That's essentially what I try to do: 6-8 rep range at 80%max. Front and back squats, dead lifts, dips and pull-ups, overhead barbell press, bench press, and limited core work because doing the other exercises will already engage your core, sometimes quite intensely.