There are many factors that can contribute to Achilles tendonitis and plantar fasciitis. The number one factor is usually an overuse of the gastroc-soleus complex. The two muscles in your calf join together at the Achilles tendon in the back of the heel. When this structure gets overused, it can pull the heel bone (calcaneus) in a way that irritates the plantar fasciitis in addition to it just getting irritated right at the Achilles tendon.
The real question we ask as physical therapists is “Why Are You Using Your Calf So Much!?” Ironically, your calf being tight may be the immediate source of your pain but more often than not your pain is probably coming from a lack of strength or range of motion somewhere else entirely. I have found throughout my experience in physical therapy and taking two intensive courses on running injuries that there are two main places that can cause an overuse of your calf muscle: 1. Hip Extension Range Of Motion and 2. Great Toe Extension Range Of Motion
In the gait and running cycle, we have phases called “terminal stance” and “pre-swing,” which is when our toe just barely comes off the ground to take that next step forward. This particular position requires a certain range of motion at the hip and big toe. The hip should be able to achieve at least 10 degrees of extension and the big toe should be able to achieve at least 60 degrees (if walking) and up to 90 degrees (if running) of great toe extension.