For some time it has been the view of a number of researchers that both the intrinsic foot muscles and the extrinsic foot muscles contribute to balance but by different modalities . That the extrinsic foot muscles affect balance by altering the centre of pressure under the foot is well know but the exact contribution of the intrinsics has been harder to nail down .
The previous posts in this thread outline the mechanism by which I believe the intrinsic provided fine tuning of balance and postural stability .A fine tuning system within a larger overall system if you like .

Gerry