Hiking and running are very similar. The body is upright, moving in a straight line, one foot after the other. But the difference in speed between running and hiking affects how the muscles are used. Hiking usually incorporates more ascents and descents than running, which also changes your muscle usage pattern.
Hiking uphill involves some of the body's biggest muscles, the gluteus maximus and the quadriceps, also known as the butt and thighs. You'll also feel your hamstrings, and the gastrocnemius, plantaris and soleus, the muscles in your calves. Depending on how steep your ascent, your heart might also be working hard. You'll be using your abdominal and the lumbar and dorsal muscles in your back to keep you upright. If you're carrying a heavy pack, you'll feel your back muscles even more.
When you hike downhill, your muscles don't work as hard as they did on the way up. Now gravity is pulling you down, especially if you're carrying a backpack. The quadriceps will work like brakes here, trying to keep the shock out of the joints. The faster you go, the more impact you'll feel. Hiking downhill is a frequent cause of knee pain. Taking smaller steps, walking sideways or using a walking stick may help.
Many leg muscles work together when running. Your quadriceps, hamstrings and gluteus maximus are called into action. The hip flexors -- both your iliacus and your psoas major -- lift your leg for every step. Your soleus, gastrocnemius and plantaris are responsible for flexing your knee and plantar flexing your ankle -- that is, coming into a position where your toes are below the ankle.
A 2012 study at North Carolina State University discovered one interesting difference between moving at a walking and running pace. Using high-speed motion-capture tools, ultrasound imaging and a treadmill, researchers focused on the behavior of the gastrocnemius while walking and running. This calf muscle engages early in your stride cycle, stretching your Achilles tendon, which rapidly recoils and moves you along. Researchers discovered that the faster you go, the more this muscle speeds up. But with speed, the gastrocnemius loses power and efficiency. However, when you change your gait from walking to running, the gastrocnemius slows down and grows more efficient. This may explain why at certain speeds it's much easier to jog than to walk.