Perhaps the delayed action of the gluteus medius allows an adductory moment of the pelvis, moving the center of gravity medially. This could conceivably place additional stress on the achilles tendon (via the lateral gastroc) to create more eversion of the foot from midstance on
“The results of the study demonstrate altered neuromuscular control of the GMED and GMED in runners with Achilles Tendonitis. During running, GMED typically activates before heel strike so as to stabilize the hip and the pelvis. In runners with Achilles Tendonitis, GMED is activated with a delay, which consequently might affect the kinematics of knee and ankle resulting in rear foot inversion. Similarly, GMAX is activated with a delay and for a shorter duration in runners with Achilles Tendonitis. GMAX is the primary hip extensor and via a kinetic chain, a decreased hip extension moment might be compensated by an increased ankle plantarflexion moment which could potentially increase the load on the Achilles tendon.”
Franettovich Smith MM1, Honeywill C, Wyndow N, Crossley KM, Creaby MW. : Neuromotor control of gluteal muscles in runners with achilles tendinopathy. Med Sci Sports Exerc. 2014 Mar;46(3):594-9.
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