A sole structure (12; 112; 312) for an article of footwear (10; 110; 210; 310; 410) comprising: a unitary midsole (14; 114) having a first portion and a second portion rearward of the first portion; wherein a bottom surface (34; 134) of the unitary midsole (14; 114) defines a groove (36; 136; 536; 636; 736) extending from a medial side of the unitary midsole (14; 114) to a lateral side of the unitary midsole (14; 114), and a top surface (38; 138) of the unitary midsole (14; 114) defines a slit (40; 140; 340; 540; 640; 740) disposed over the groove and extending from the medial side to the lateral side; wherein the unitary midsole (14; 114) forms a living hinge (16; 116; 316; 516; 616; 716) at the groove and the slit (40; 140; 340; 540; 640; 740), with the living hinge (16; 116; 316; 516; 616; 716) connecting the first portion to the second portion so that the first portion and the second portion are selectively pivotable relative to one another at the living hinge (16; 116; 316; 516; 616; 716) between a first orientation and a second orientation; and wherein the groove is wider in the first orientation than in the second orientation, and the slit (40; 140; 340; 540; 640; 740) is wider in the second orientation than in the first orientation, and wherein the slit (40; 140; 340; 540; 640; 740) includes: a main portion (60, 64; 140A; 540A; 640A; 740A) extending downward from the top surface (38; 138) of the midsole toward the groove and having a distal end spaced above the groove; a front branch (140B; 540B; 640B; 740B) extending from the distal end of the main portion (60, 64; 140A; 540A; 640A; 740A) into the first portion and terminating above the bottom surface (34; 134); and a rear branch (140C; 540C; 640C; 740C) extending from the distal end of the main portion (60, 64; 140A; 540A; 640A; 740A) into the second portion and terminating above the bottom surface (34; 134). a unitary midsole (14; 114) having a first portion and a second portion rearward of the first portion; wherein a bottom surface (34; 134) of the unitary midsole (14; 114) defines a groove (36; 136; 536; 636; 736) extending from a medial side of the unitary midsole (14; 114) to a lateral side of the unitary midsole (14; 114), and a top surface (38; 138) of the unitary midsole (14; 114) defines a slit (40; 140; 340; 540; 640; 740) disposed over the groove and extending from the medial side to the lateral side; wherein the unitary midsole (14; 114) forms a living hinge (16; 116; 316; 516; 616; 716) at the groove and the slit (40; 140; 340; 540; 640; 740), with the living hinge (16; 116; 316; 516; 616; 716) connecting the first portion to the second portion so that the first portion and the second portion are selectively pivotable relative to one another at the living hinge (16; 116; 316; 516; 616; 716) between a first orientation and a second orientation; and wherein the groove is wider in the first orientation than in the second orientation, and the slit (40; 140; 340; 540; 640; 740) is wider in the second orientation than in the first orientation, and wherein the slit (40; 140; 340; 540; 640; 740) includes: a main portion (60, 64; 140A; 540A; 640A; 740A) extending downward from the top surface (38; 138) of the midsole toward the groove and having a distal end spaced above the groove; a front branch (140B; 540B; 640B; 740B) extending from the distal end of the main portion (60, 64; 140A; 540A; 640A; 740A) into the first portion and terminating above the bottom surface (34; 134); and a rear branch (140C; 540C; 640C; 740C) extending from the distal end of the main portion (60, 64; 140A; 540A; 640A; 740A) into the second portion and terminating above the bottom surface (34; 134).