Cycling is an endurance activity, not a hypertrophy one.
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Cycling can do a body good. It is a low-impact form of cardiovascular exercise that can be done indoors or out, depending on your preference. Whatever type of bike you choose the motion is the same: circular using your legs to move the flywheel and raise your heart rate. You may see some changes to your calf muscles from cycling, but high tensions will not build a lot of muscles in your calves.
Round and Round
Your calves are active during cycling, but are not the primary movers. Your quadriceps and hamstrings do most of the work when pedaling, and just work harder when you increase the tension. Your calf muscles, or gastrocnemius, does assist the hamstrings with knee flexion, but its primary job is to plantar flex the foot, or point your toes. Your ankles stay bent during cycling, so the calves will not get significant tension to build muscle.
The Visible Curve
Even though you may not build your calves, you will see changes to your legs from cycling. Increasing the tension on your bike increases the intensity at which you work. This translates to more calories burned and potentially a decrease in body fat. You can see improved definition in your whole body, including your calves. The endurance of your legs also improves, and with that, there is some improved muscle tone. So your calves can become more visible, and look tighter.
Building, or increasing, muscles is also known as muscle hypertrophy. Hypertrophy is an increase in muscle fiber size, according to Dr. Len Kravitz writing for IDEA Health & Fitness Association. This increase in size is due to targeted resistance training that overloads and stresses the muscle. To see changes in your calves you need to push them beyond what they are used to, and do it the right way. Lifting weights will build muscles in your calves.
Get It Done
Add a few calf exercises outside of cycling to really build them up. Train two or three times each week on nonconsecutive days, and perform three to six sets of six to 12 reps with a challenging weight. Start with a standing barbell calf raise. Place a barbell across your upper back and shoulders, and stand on the edge of a step or platform with your heels hanging off. Keep your knees straight and raise up on the balls of your feet. Lower down until you feel a stretch in your calves for one complete rep. Try a single leg calf raise as well. Start this exercise with just your body weight and add dumbbells as needed. Balance on your right foot on the edge of a step or platform. Rise up on the ball of your foot and slowly lower down for one complete rep. Perform up to 12 on the right and then switch to the left foot.