The Power Fit Index (PFI) indicates the overall level of gradual pressure distribution woven into each pair of BootDoc's Power Fit Socks. PFI 30 designates a gentle pressure distribution while PFI 90 classifies as strong and is therefore only found in BootDoc's performance series.
Socks work great. Thin enough to fit in my plug race boots with lace up liners. You don't feel heat, but your toes just don't get cold. The App is excellent. Very easy to setup and use!
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Clifford Weiss —
Over the years, I have tried various devices to keep my feet warm during outdoor activities in NH in the winter. I like the socks because I can use them in any of my boots, so I bought a spare pair so I can wash one and wear one. Changing the heat setting on the battery pack isn't easy if you are wearing layers, so I usually set it to Level 2 for most conditions.