Running and cycling use similar lower body muscles, but in very 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 -- during which you push down on the pedal -- and the recovery phase -- when your leg returns back to the top of the pedal stroke.
Running on Level Ground
Running on level ground activates the adductors at the inner thigh, the quadriceps at the front of your thighs and the hamstrings at the back of your thighs. These muscles assist with bending your knee, extending your hip and stabilizing your pelvis.
The abdominals activate through all phases when you're running to help stabilize the pelvis and maintain balance.
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 -- or biggest buttock muscle -- hamstrings, calves and inner quadriceps increase activity.
Toward the bottom of the pedal stroke the semimebranosus -- a hamstring muscle -- is activated the most to bend the knee for 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. Your abs work on overdrive too. 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're also burning more calories running uphill because of these increased energy needs in your muscles.
Since you're 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 issue of the 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 involves no impact, it's easier on your joints and muscles. However, running demands more energy, so it's slightly 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.