Running is essentially eccentric. And I don’t mean that runners are odd (they have nothing on triathletes). What I mean is that the impact of running is handled by the body eccentrically. The muscles contract and lengthen at the same time as they control the weight of the body in its descent back to earth. See, what comes up eventually comes down—at two to seven times body weight depending on the speed and form of the runner. And this load must be eccentrically controlled ninety times per minute per leg! Thus, just like the rotator cuff must be conditioned appropriately to handle the demands of swimming, training for the lower-body triple extensors (the ankle, the knee, and the hip) must include an emphasis on the negative portion of a lift so nothing in the kinetic chain eventually fails. Below is one exercise to include in your strength training program (which should come before the development of power--which running is an expression of).
1. Stand with the balls of your feet on a step and your heels hanging off.
2. Rise up onto toes using both legs.
3. Take the dominant leg away and use a 10-count to control your descent so that your heel ends up below the ball of your foot.
4. At end range of motion, put your dominant leg back on the step and rise back up to the top position again, repeating the process for the designated number of reps.
5. Doing the movement with a straight leg targets the gastrocnemius. Doing the movement with a bent knee (15-20°) targets the soleus. Both versions should be performed.
Many other exercises which can be utilized to enhance the orthopedic integrity of any athlete who runs can be found on the pages of my book: Holistic Strength Training for Triathlon (available in print version or as an instant download here: http://triumphtraining.com/pages/holistic-strength-training-for-triathlon)