Abstract: An improved construction of sole unit for running shoes features a longitudinally asymmetrical recess across the width of the heel area of the midsole and interchangeable inserts, differentiated by their durometer hardness, inserted to fill the recess, such that the shock absorption characteristics of the midsole can be suited to the user and the running surface and any tendency to over-pronation can be corrected, a larger anti-pronation insert being provided for insertion on the inner side of the heel and a smaller anti-supination insert being provided for insertion on the outer side of the heel and the two inserts being arranged to interlock with each other.