I wonder if an organism took over a human brain, what kind of motor function would it have. I think it is more likely to be slow and awkward.
In 'I am Legend' they were fast and abnormally strong but that was a strain of Rabies that did that. It was almost a vampire like transformation, which I don't think could happen. The bodies would have to go into a cocoon of sorts and metamorphosis if the physical properties were to change. Otherwise standard bone density would only allow for so much stress, as would ligament, tendon, and muscle limitations. If a basketball player can break a femur on a jump shot then why should a zombie be able to jump 50' feet. And a damaged decaying body would have a reduced muscle function and coordination the same as an ill person or a malnourished person. Besides, rabid animals can become vicious but are still governed by their original physical limitations.
Now if the idea was that the organism that overtook the brain was somehow able to enhance the synapse and increase the speed of the electrical signals from the brain to the muscles then maybe it would make sense for running zombies, but the whole idea is that the diseased human body would be functioning solely on the brain stem, which seems to me would reduce coordination.
