Your glutes and legs provide most of the power in cycling.
The thighs of Olympic gold-medalist Chris Hoy have a circumference of 27 inches. Those of track cyclist Robert FГ¶rstemann measure a whopping 34 inches. With numbers like these, you may see cycling as the route to bulking up your lower body. On the other hand, you may fear hopping on the bike because you're worried about developing huge buns and thighs. Although the glutes and quads give power to each pedal stroke, you'll have to put in plenty of miles to get bulky on the bike.
The Muscles Used
The primary muscles used to power each turn of the crank on your bike are the quads at the front of the thigh and the hamstrings at the back of the thigh. The gluteal muscles of your butt also work in conjunction to move your leg in a pedaling motion. When leaning forward to get in the drops of a road bike or when using the aero bars of a track bike, you make use of different areas of the glutes.
Building Through Resistance
For every pedal stroke, your thighs and butt must work against the resistance of the gears. Additional resistance comes from wind, inclines and friction against the road. Resistance requires your leg and butt to work to overcome the force and this work will tone and build your muscles. Pedaling in a harder gear, riding a bike with wider tires, heading up a hill and cycling against a head wind all add resistance and further build up the strength of your thighs and butt. Added weight will also increase resistance since the heavier your are, the harder your muscles have to work to move you through space.
Cycling for Bulk
Although cycling will build strength in your leg muscles and give them a sleek, toned appearance, cycling isn't commonly used to bulk up the thighs and butt. The size of the lower bodies of professional cyclists is the exception. These riders put in hours every day on the bike and often ride at intense speeds. This is coupled with weight workouts to tone their quads to put extra power behind every stroke. The average cyclist does not train with this intensity and will rarely bulk up. If steep hills, long rides and strong head winds are part of your daily routine, you may notice some increase in the size of your butt and thighs, but don't expect the muscles to compare with those of a bodybuilder.
Strengthening Your Stroke
If you want to gain speed and endurance on the bike, add in strength exercises to tone your butt and thighs to your cycling workout. Hip extensions, scissor kicks, squats and lunges work the muscles of your lower body including your thighs and butt. Since these exercises don't rely on anything more than your own body weight, they will tone without adding bulk. If your goal is to add bulk to your lower body, combine cycling with a weight workout that focuses on your thighs and butt. Since core stability will add additional power to your pedal strokes, incorporate exercises that tone your abs and back to your routine as well.