Forefoot supinatus is defined as a soft tissue contracture in which the forefoot is inverted on the rearfoot when the rearfoot is in it defined neutral position and the midtarsal joint is maximally pronated. This looks similar to forefoot varus, but it is very different from it.
The cause of a forefoot supinatus is most widely assumed to be an ‘overpronated’ foot when the rearfoot everts past vertical during stance. As the rearfoot is everted, for the forefoot to remain flat on the ground it is in a relatively inverted position relative to the rearfoot. Over time, the soft tissues contract in this position, so that when the rearfoot is vertical or in its defined neutral position, the forefoot develops into the inverted position.
An alternative proposal for the development of a forefoot supinatus is that it is the result of a failure of the foots windlass mechanism. If that windlass is not working well, then when the medial side of the foot is loaded in gait, the medial column dorsifexes from the ground reaction forces as the plantar fascia is not capable of plantarflexing or stabilizing the medial column on the ground. Over time, the soft tissues adapt to the dorsiflexed position of the the medial column resulting the inverted position of the forefoot, resulting in the forefoot supinatus.