Just finished up with 18 months of rotating 2 pairs of orig. zigkicks. After 18 mon. I am still getting blisters. I have 9 years in the Army, the Zigkicks added additional propulsion to my 2mile run. However, I have retired my pair of Physical Training shoes and started wearing my COB and weekend shoes for PT, Blisters around back of ankle are still appearing. Pushing oneself moderates the pain from blisters the size of eraser heads on the back of the Achilles tendon. I have noticed the Alpha version provides mesh around this area and flex where there should have always been.