Angela is putting in some long days at the triathelon so I wrote a little article to fill in for her blog.
The Soleus. A runners turbo booster.
The soleus along with the gastrocnemius make up the calf muscle. The soleus is underneath the gastrocnemius. The primary purpose of this muscle is to articulate the toes and the foot as a whole.
That is not all the soleus does however. Have you ever noticed how you calves ache, burn, stiffen when you stop running? If you start running or walking again it goes away. It is quit a shock to the new runner to discover this unpleasant antithesis of reason. Many no doubt conclude that this whole running business is best suited to fanatics and lunatics and not a good idea after all.
To understand why this happens lets first consider the source of the pain. The common answer is lactic acid. This is not entirely accurate but it is essential accurate for our purposes. Lactic acid refers to the byproduct of anaerobic metabolism. Anaerobic is a kind of secondary method of energy metabolism when our cells cannot get enough oxygen. Like a turbo boost. The accumulation in the body of lactic acid produces intense discomfort.
So the calves hurt because of the accumulation of lactic acid but why don't they hurt while we are running? First lets look at the circulatory elements used to remove the lactic acid. The heart uses positive pressure to push blood through the body it it does not suck. When your heart is trying to remove lactic acid from an area of the body its only option is to pump in more fresh blood to force out the old. When this happens in a non running context like weight lifting we experience a intense engorgement called a pump. This does not happen in the calves during running running though. That is because every contraction of you soleus squeezes the veins in the muscle the veins are full of one way valves that that compel the blood back to the heart. Your soleus is in effect an auxiliary heart helping to move blood out of you calve and keep the concentration of lactic acid low enough to prevent the intense pain.
How amazing is it that every footfall powers a pump to pump lactic acid out of the runners legs? The second you stop running you shut down the pump, the levels of anaerobic by products go up the pain comes.