Cycling is a convenient form of exercise that can be performed year-round in most areas. It burns calories, it is aerobic exercise, it improves your cardiovascular system and it strengthens muscles not only in your legs but throughout the entire body. In addition to those benefits, your bicycle can become your transportation.
Legs and Hips
During your pedal stroke, you use the muscles in your legs and hips. More specifically, your gluteus muscles work in the first part of your downstroke as you power down the pedal, and your quadriceps and calves finish the downstroke. Your hamstrings work in the recovery phase, as the pedal returns to the top. At the very top of the stroke -- what you might call 10 o'clock to 12 o'clock on a clock face -- your hip flexors in the front of your hips complete the pedal stroke.
The leg and hip muscles propel us forward in everyday movements. It is especially useful to have strong quadriceps, calves and gluteal muscles to lift and stabilize us in movements involving elevation change, such as hiking and stair climbing.
When you ride outside, your deep core muscles stabilize you as you rock from side to side. Muscles in this category include the transversus abdominis, obliques and multifidi. Keeping these muscles strong helps prevent lower back pain and supports proper posture.
Hand, Wrist, Forearm
Your hands get a fair amount of strength work, as do your wrists and forearms, from holding the handlebars, especially if you ride over bumpy surfaces. Many small muscles get strengthened in this area, but it is also an injury-prone area because of the small size of internal structures in relation to the amount of body weight placed on the hands.
Strengthening the wrists and forearms is very useful in improving grip strength.
Shoulders and Upper Back
Your upper body bears some of your weight when you ride as well, thus muscles throughout your back and shoulders are constantly getting strengthened throughout your ride. More specifically, the deltoids in your shoulders help hold you up, the trapezius in your upper back helps maintain your posture, and the rhomboids and rotator cuffs stabilize your shoulder blades.
Strength in these areas improves and maintains posture in your upper back, helping prevent the typical rounded "computer posture."