An often overlooked muscle, the soleus gets a work out constantly as it is engaged to help maintain your balance and postures, but also is highly active with walking and running. This stretch isolates it out from the other plantar flexors such as the gastrocs and allows for a pure stretch of just the soleus.
The soleus muscle is a broad, powerful muscle in the calf beginning just below the knee and attaching to the heel via the Achilles tendon. It works closely with the gastrocnemius muscle, which typically thought of as the calf muscle. The soleus muscle is active every time the toes point down while the knee is bent, such as in every stride. Sunday’s runners will take approximately 26,000 strides during the 13.1 mile race, leaving the soleus prone to strains from 1) over-exertion when the calf muscles are tight or 2) a forceful, sudden contraction when the calf muscles are not properly warmed up.
Runners are very familiar with how to stretch their calf muscles (runner’s stretch), however you rarely see runners warming up or stretching their soleus muscle. For the stretch to be effective, the knee must be bent while the foot is flexed towards the head as seen in this video.