a shoe having a toe region , a middle region , a heel region , and a multi - layer , multi - density midsole ; the midsole being comprised of at least a shank and a lower layer ; the bottom surface of the shank having at least one longitudinal concavity and at least one longitudinal convexity , the longitudinal concavity typically occupying a substantial portion of the heel region and the longitudinal convexity typically occupying a portion of the middle region . collectively , these elements contribute to making the shoe appropriate for both walking and higher impact activities such as running , and simulating the effect , and imparting the fitness benefits , of use on a sandy beach or on a giving or uneven surface regardless of the actual hardness of the surface .