An article of footwear, comprising: a sole including a midsole and an outsole, the outsole having a lower portion configured to contact a ground surface; the midsole including a first sidewall disposed on a lateral portion of the sole and a second sidewall disposed on a medial portion of the sole; the second sidewall including a plurality of vertical grooves in a heel portion of the midsole for increasing the flexibility of the second sidewall, at least one vertical groove extending from the second sidewall to the lower portion of the outsole and comprising a first end portion (401, 421, 431, 441) disposed on the second sidewall, a second end portion (402, 422, 432, 442) disposed on the lower portion of the outsole and extending in a lateral direction, and an intermediate portion (403, 423, 433, 443) disposed between the first end portion (401, 421, 431, 441) and the second end portion (402, 422, 432, 442), the first end portion (401, 421, 431, 441) having a first height (H1, H2, H3, H4) in the vertical direction and the second end portion (402, 422, 432, 442) having a first width in the lateral direction, wherein a first vertical groove of the plurality of vertical grooves is disposed between a first laterally extending portion of the sole and a second laterally extending portion of the sole, said first and second laterally extending portions (601, 602) extending substantially laterally across the heel portion, the first laterally extending portion includes a first end portion associated with the first sidewall of the heel portion and a second end portion associated with the second sidewall of the heel portion, the second laterally extending portion includes a third end portion associated with the first sidewall and a fourth end portion associated with the second side wall, and the second end portion of the first laterally extending portion is configured to move substantially independently of the fourth end portion of the second laterally extending portion at the second sidewall, characterized in that the first height (H1, H2, H3, H4) is bigger or similar in length to the first width, and the first end portion of the first laterally extending portion is integrally formed with the third end portion of the second laterally extending portion at the first sidewall so that the first end portion and the third end portion move together. a sole including a midsole and an outsole, the outsole having a lower portion configured to contact a ground surface; the midsole including a first sidewall disposed on a lateral portion of the sole and a second sidewall disposed on a medial portion of the sole; the second sidewall including a plurality of vertical grooves in a heel portion of the midsole for increasing the flexibility of the second sidewall, at least one vertical groove extending from the second sidewall to the lower portion of the outsole and comprising a first end portion (401, 421, 431, 441) disposed on the second sidewall, a second end portion (402, 422, 432, 442) disposed on the lower portion of the outsole and extending in a lateral direction, and an intermediate portion (403, 423, 433, 443) disposed between the first end portion (401, 421, 431, 441) and the second end portion (402, 422, 432, 442), the first end portion (401, 421, 431, 441) having a first height (H1, H2, H3, H4) in the vertical direction and the second end portion (402, 422, 432, 442) having a first width in the lateral direction, wherein a first vertical groove of the plurality of vertical grooves is disposed between a first laterally extending portion of the sole and a second laterally extending portion of the sole, said first and second laterally extending portions (601, 602) extending substantially laterally across the heel portion, the first laterally extending portion includes a first end portion associated with the first sidewall of the heel portion and a second end portion associated with the second sidewall of the heel portion, the second laterally extending portion includes a third end portion associated with the first sidewall and a fourth end portion associated with the second side wall, and the second end portion of the first laterally extending portion is configured to move substantially independently of the fourth end portion of the second laterally extending portion at the second sidewall, characterized in that the first height (H1, H2, H3, H4) is bigger or similar in length to the first width, and the first end portion of the first laterally extending portion is integrally formed with the third end portion of the second laterally extending portion at the first sidewall so that the first end portion and the third end portion move together.