Professional cyclists' thighs resemble tree trunks. Sprinters are usually hulking riders whose huge muscles work together to pull on the handlebars and explode into the pedals for a powerful sprint. Climbers stay rail-thin to limit gravity's effect on hill, but have well developed leg muscles to propel them up mountains. While you may not dream of winning the Tour de France, you can still develop a cyclist's bulging quadriceps and gluteal muscles on a stationary bike.
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The quadriceps and gluteal muscles are the primary cycling muscles, but the hamstrings, hip flexors, calves and shin muscles also help pedal. The quadriceps are responsible for pushing down on the pedals, which generates the most power in the pedal stroke. Because you can push down on the pedals with all your might, cycling with heavy gears causes physiological adaptations and muscle growth similar to weightlifting.
To build leg muscle on a stationary bike, you have to pedal with a lot of resistance. Some indoor group cycling programs recommend a resistance where you can't move the pedals faster than 60 pedal revolutions per minute, or at an intensity you can't maintain for more than a minute or two to stimulate the greatest quadriceps-building effects. Sprints--pedaling as hard as you can for less than a minute--also force your legs to put out a tremendous amount of power and can build strength and muscle. Recover between sprints or heavy resistance sets by pedaling easily for 30 seconds to a few minutes. "Hovers," or pedaling in a seated position while holding your bottom about an inch above the seat are another way to build quadriceps strength on an upright exercise bike.
The American College of Sports Medicine recommends doing muscle-building workouts two to three times per week on non-consecutive days to give your muscles a chance to recover and build new muscle tissue. Between tough muscle-building workouts, do lower intensity "endurance" rides to recover and develop your cardiovascular system.
Because cycling is cardiovascular exercise, a muscular endurance-building activity and a strength-building activity, it can be a means to reach many fitness goals. If you simply want to change the way your legs look, then cycling helps burn fat in addition to developing your quadriceps muscles. Cycling is also a non-impact activity that is easy on your joints. You can take your stationary bike fitness outside and ride up hills or sprint to catch the next green light. Other exercise modalities should be included in your fitness regimen in order to more fully address the muscle groups of the upper body, such as the back.
Because big-gear riding can put a lot of strain on your knees, it is imperative to avoid injury by adjusting your bike properly. Your seat should be high enough that your knee is fully extended without locking out--you should still have a slight bend in your knee--at the bottom of the pedal stroke. Keep your toes and knees facing straight ahead as you're pedaling. Make sure that your upper body is still and relaxed and not swaying from side to side or plunging forward in a "pecking" motion while you pedal.