Compensating for loss: running on one tibialis anterior.

Rupture of the tibialis anterior (TA) tendon is rare and may be either traumatic or spontaneous. TA is the main ankle dorsiflexor, providing 80% of the power, as well as contributing to inversion of the foot. We describe a case of an 84-year-old male who was incidentally diagnosed with absence of his left TA at age 46 while preparing for his first-ever marathon.