Your soleus muscle makes up a large portion of the back of your lower leg, calves. It lies underneath the gastrocnemius, which is the muscle most people associate with the calves because its muscular shape is easier to see. Your soleus is what allows your foot to move up and down, and that motion is used for many daily activities.
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The main movement of your soleus is the flexing of your foot. You use this muscle whenever you stand on your toes as if reaching high. Ballet dancers use the soleus when they rise up onto their toes into a point position, and swimmers contract the soleus to keep their toes pointed as they kick through the water.
The soleus attaches beneath your knee joint and into your heel, so when your knee is bent during driving, your soleus is what allows you to press the pedals. If you sit up close to the steering wheel, you will use more of your soleus. The more you straighten your leg and sit away from the wheel, the less the soleus is involved with pressing the gas and brake pedals.
You use your soleus for walking and running. These movements require repeated pointing and flexing of your foot, so the soleus contracts and releases every time you place your foot onto the ground and pick it up again. If you run on your toes, you will activate the soleus to a greater degree.
Jumping or hopping on your toes uses your soleus muscle. The soleus contracts to aid in balance and power during jumping movements. It is also used when you skip, which includes quick bursts of jumping from one foot to the other.