Due to a house move I am soon to become a road bike commuter with approx. 18 miles to cover. I could do with loosing a few kgs but am worried about developing the emaciated physique so apparent in many of the road bikers I see. How can one avoid this? Is it simply a product of the high endurance aspects of road cycling over longer distances, or a combination of that and excessive dieting? I have been a mountain biker for over 10 years and have found that it helps one build muscle mass in the upper thigh and arms and shoulders. Any advice on how to avoid the skinny road biker look would be greatly appreciated.
Roughly, your muscles are made up of two types of muscle fibers: fast and slow.
Slow muscle fibers are relatively weak, but they have very high endurance. Long rides at moderate effort level will burn fat and develop your slow muscle fibers.
Fast fibers are... well, much faster, much stronger, but they have much lower endurance. These are the ones you use when sprinting, or doing reps/lifting weights. They also have more bulk, so they're the ones usually associated with the "muscle look".
If you don't work the fast fibers, you'll look like a climber cyclist or a marathon runner (and you'll suck at sprints). If you only work the fast ones, you'll look like a track sprinter with huge thighs. It is good to develop both, aim somewhere in-between. Basically look at pro cyclists, there is not an extra gram of useless body mass on these guys, so pick one you like, check if he's a climber or a sprinter, and train accordingly.
A nice way to do this on a bike is to sprint between traffic lights. This is basically HIIT. It's fun, too. Makes you sweat a lot, so do it on the way home! Pull on the handlebars to work your arms too...
but if you don't want the "cyclist arms", you gonna have to either lift a bit or do some rough MTB, which is a good workout.
Elite cyclists work very, very hard for that emaciated physique! It's not exactly natural -- they're working to drop as much unnecessary weight as possible in order to climb faster. While I think bike commuting should help make you leaner, you won't look anything like Bradley Wiggins unless you're also restricting your diet and training very hard. Don't worry!
+1 on the weight work. I read of a report that demonstrated reduced bone density in professional cyclists, attributed to the very smooth loading/unloading of stress on the leg bones in the pedaling cycle. Google for it. My wife's doctor recommended to her impact exercise (weights, running, etc.) because she's under 115lbs and normal walking doesn't stress her bones enough to delay/offset/combat osteopenia. FWIW I'm 63, 5'-7", 145lbs, upper body like Gimondi. :)