The overall pattern of muscle tension that leads to Achilles tendinitis
While tight calf muscles exert the most direct strain on the Achilles tendon, people with Achilles tendinitis often have a pattern of muscle tension in their lower body that operates like a pulley system, involving the lower back, hip flexors, gluteal muscles, and hamstrings.
Tight lower back muscles—the quadratus lumborum and erector spinae group—pull the top of the pelvis and lumbar vertebrae closer together. This typically brings the pelvis into an anterior (forward) tilt.
Tight hip flexors (including the rectus femoris, iliopsoas, tensor fascia latae, sartorius, and hip adductors) also tilt the pelvis forward, and can play a role in Achilles tendinitis.
When the pelvis is tipped forward, the hamstrings get pulled tight. The hamstrings originate at the bottom of the pelvic bone, run down the back of the thighs, and insert at the tops of the tibia and fibula, the bones of the lower leg.
The resulting tension in the hamstrings pulls on the bones of the lower leg, trying to flex (bend) the knees. This attempt to flex the knees recruits the muscles of the calf that flex the knees as well—the gastrocnemius and the plantaris.
The gastrocnemius, soleus, and plantaris muscles attach to the Achilles tendon, which attaches to the heel bone. So when these muscles are tight, they not only flex the knee, but also plantar flex the ankle and pull the Achilles tendon taut.
If all of this tension occurred while we were lying down, we probably wouldn’t feel any negative effects. But if this pulley system is chronically tight, and you do repetitive movements that put strain on the Achilles tendon—running, jumping, or rising up on your toes—pain and injury to the Achilles tendon is almost inevitable. That’s why it’s extremely important to keep not just your calf muscles but all of the muscles from your lower back down to your feet as loose and flexible as possible.