A cycling class is good cross-training for regular treadmill users.
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You cross-train to prevent repetitive muscle patterns, which can lead to overuse injuries. One way to cross-train is to alternate stationary cycling with treadmill walking or running. Running and cycling both use your legs to raise your heart rate and thus activate many of the same muscles. But these muscles are activated in different ways.
Cycling: Thighs Front and Center
Stationary cycling primarily uses muscles at the front of the thigh, including the tensor fascae latae, the sartorius and quadriceps. The tibialis anterior in the front of the shin is also highly active during each pedal stroke. (ref3)
Treadmill Walking and Running
Walking on the treadmill activates the quadriceps more than when you walk on regular terrain, but not as much as when you use the stationary cycle. (ref1) Running and walking on the treadmill use virtually the same muscles, but you engage the soleus muscle -- the smaller muscle of the calf -- more during a walk. (ref 4) The hamstrings experience greater activation during a treadmill workout when compared to a stationary cycling workout, especially when sprinting. (ref5)
Running on a flat treadmill mostly activates the adductors and gracilis of the inner thigh, the biceps femoris of the quadriceps and the semimembranosus of the hamstrings. Running uphill on a treadmill increases activation in these muscles and stimulates the glutes and the gastrocnemius of the calf. When changing a stationary cycle from a level grade to an 8 percent grade, there's little change in muscle activation; the gastrocnemius and biceps femoris work at both incline levels. When riders change from seated to standing pedaling at an 8 percent grade, the hip and knee extensors are activated to a greater degree.
Although stationary cycling and treadmill workouts use many of the same muscles, they engage them in a different way. When you repetitively put muscles through the same patterns, certain parts of your body become strong and others become weak. Including both stationary cycling and treadmill work in your routine discourages this type of imbalance. (ref2)