Travell & Simons proposed using a forefoot correction in the form of a wedge under the first metatarsal to address a condition called Morton’s foot structure. This is a configuration of the foot in which the second metatarsal is longer than the others, resulting in a “V” shape in the joints between the metatarsal and the small phalanx bones of the toes. With this configuration, as the person moves weight into the forefoot, instead of having a stable tripod formed by the 1st and 5th metatarsals along with the heel, the head of the second metatarsal forms an additional vertex. As the weight moves into the forefoot, this point creates instability, and will lead to overstabilization in the muscles of the lower leg as well as the glutes. Other researchers have identified a similar condition, First Metatarsal Deficit, however I believe this arises due to repetitive excess pronation, since the metatarsals are highly mobile, and this might in fact be another face of the same condition.
The net result, however we conceptualize the configuration in the forefoot, is that the central nervous system will sense instability during the pushoff phase of gait, when the weight is entirely on one leg and the glutes and adductors must stabilize the foot. If the body doesn’t proprioceptively sense contact with the ground under the ball of the foot, the choice becomes letting the foot pronate to firmly plant the first metatarsal head, thereby causing valgus knee and other distortions, or forcing supination by abducting the leg and inverting the ankle, which stresses the glutes, tibialis anterior and other key muscles. Most individuals seem to choose the greater degree of control inherent in the latter strategy. For this reason, we always correct with a lift under the first metatarsal if we’re also correcting the midfoot. The degree of correction will correspond to the amount of correction in the arch.