While running and cycling mostly use the same muscles, the mechanics of running and cycling are different and use the muscles in different ways. During running, the muscles of the lower body help to move the body through four stages -- heel strike, single limb support, push-off and leg swing. Cycling has only two stages, the power phase and the recovery phase. It is during the power phase of cycling that the leg muscles are used the most.
Running on Level Ground
When running on level ground, the muscles that are most activated are the adductors, the quadriceps and the hamstrings. These muscles assist with knee flexion, hip extension and stabilizing the pelvis, which protects your spine. The abdominals are active when running at all times to help stabilize the pelvis and maintain balance during the different stages.
Cycling on Level Ground
Cycling on level ground uses all the muscles in the legs, but at different times during the power, or crank, phase. During the top of the pedal stroke, the outer quadriceps and hamstrings are working the hardest. At around 90 degrees of the pedal stroke, the gluteus maximus, hamstrings, calves and inner quadriceps increase activity. Toward the bottom of the pedal stroke the semimebranosus -- a hamstring muscle -- is activated the most to begin knee flexion of the recovery phase. The abdominals, triceps and shoulders help to support the upper body when the hands rest on the handle bars.
During uphill running, your inner thigh, hamstrings, gluteals, calf and quadriceps all get involved. This is due to the greater forces required to extend the knee and hip while running uphill. The additional force production accounts for the greater effort you feel in your heart and lungs. You are also burning more calories running uphill because of increased energy needs in your muscles. Your core muscles are engaged to stabilize your pelvis and hold your spine in alignment.
Since you are likely to stand on the pedals when cycling uphill, the greater muscle activation will result in greater energy demands. In a study published in the February 2007 "Journal of Applied Sciences," researchers showed that the gluteus maximus and quadriceps demonstrated a prolonged period of activation during the power, or crank, phase while standing as compared to sitting. The upper body muscles are used more when cycling uphill due to the rocking back and forth of the bike. This rocking motion requires the use of the shoulders, biceps, triceps and abdominals.
Advantages of Cycling and Running
Because cycling does not involve impact, it is easier on your joints and muscles. However, running demands more energy, and is more effective for burning calories. Cross training by running on some days and cycling on others can help balance out your muscles, and give your joints a break.