These two studies measured the “pre-study” foot strength and took MRI’s to measure the girth of the foot muscles. Then, they had half the group train in Nike Free’s and a control group train in conventional shoes. By “training” they meant skipping, running, aerobics etc.
In the end (about 6 months), the Nike Free group had increased strength in the foot muscles and increased size of the muscles in the feet. In other words, wearing a less stiff and bulky shoes, the subjects did, in fact strengthen their foot muscles. Based on these two studies (one funded by Nike) we can begin to form the idea that wearing miminalist shoes or going barefoot can strengthen the foot muscles.
Study #1. http://www.staffs.ac.uk/isb-fw/ISBFootwear.Abstracts05/Foot52.Potthast_ChangesInMorphology.pdf
Studied 100 runners and took strength measurements of the feet and toes and MRI’s of their feet and lower legs (to measure the girth of the foot and lower leg muscles). They were divided into two groups and that all had to do weekly exercises including running, skipping and aerobics for 6 months. The difference was that one group was wearing conventional running shoes while the other group wore Nike Free’s. The results? The Nike Free group ended up with significant increase in the toe flexion strength and on the MRI’s, the flexor hallucis longus (the muscle that pushes the big toe down) was significantly larger after the experiment. Some other muscles trended toward an increased girth, but the difference was not statistically significant.
Study #2. http://www.asbweb.org/conferences/2005/pdf/0553.pdf
This study looked at 50 individuals and the study was very similar to Study #1. They took strength measurements and muscle belly girth measurments via MRi and then split the group in two – one group wearing Nike Free’s and the other group conventional running shoes. This study was 5 months long but again, showed statistically significant changes in muscle strength and girth compared to the group wearing conventional shoes.