Pedal-crank assembly for bicycles, the crank comprising: - an end with a threaded hole, and - a bush having an external thread screwed in said threaded hole, and an internal housing with annular groove with V-shaped cross-section, the pedal comprising: - a frame with tubular housing, said frame being shaped as a basically rectangular platform that comprises an upper side (1 a), a lower side (1 b) and two opposite transverse walls (1 c, 1 d) comprising a first transverse wall and a second transverse wall (1 d), the first transverse wall being an internal transverse wall facing the crank wherein the pedal is mounted, and the second transverse wall (1 d) being an external transverse wall facing the opposite side of the crank, - a pin inserted into said tubular housing of the frame, so that said frame can rotate freely around said pin; the pin having an end section protruding from the internal transverse wall (1 c) of said frame and said end section of the pin is housed in the housing of the bush of the crank; said end section of the pin is shaped so as to provide male-female prismatic coupling with said housing of the bush of the crank; said pin being of tubular structure and internally hollow, - through holes obtained on the end section of said pin, - a rotary shaft having a first end section inserted inside the end section of the pin of the pedal and a second end section, - one or more housings obtained on the first end section of said shaft along a circumference of the first end section, - fast coupling-uncoupling means to lock the end section of the pin inside said internal housing of the bush, said fast coupling-uncoupling means comprising spherical bodies partially housed inside said housings of the first end section of the shaft and partially housed inside side through holes obtained on the end section of the pin, wherein said annular groove of the bush surrounds the spherical bodies when the end section of the pin is inserted inside the housing of the bush, and - a key lock engaged inside one end of said tubular housing of the pedal, the key lock associated with a ring fixed to the second end section of the shaft and contained inside said tubular housing of the pedal, said key lock is configured to be actuated to move said ring in order to drag said shaft into a short rotation or axial translation travel. - an end with a threaded hole, and - a bush having an external thread screwed in said threaded hole, and an internal housing with annular groove with V-shaped cross-section, - a frame with tubular housing, said frame being shaped as a basically rectangular platform that comprises an upper side (1 a), a lower side (1 b) and two opposite transverse walls (1 c, 1 d) comprising a first transverse wall and a second transverse wall (1 d), the first transverse wall being an internal transverse wall facing the crank wherein the pedal is mounted, and the second transverse wall (1 d) being an external transverse wall facing the opposite side of the crank, - a pin inserted into said tubular housing of the frame, so that said frame can rotate freely around said pin; the pin having an end section protruding from the internal transverse wall (1 c) of said frame and said end section of the pin is housed in the housing of the bush of the crank; said end section of the pin is shaped so as to provide male-female prismatic coupling with said housing of the bush of the crank; said pin being of tubular structure and internally hollow, - through holes obtained on the end section of said pin, - a rotary shaft having a first end section inserted inside the end section of the pin of the pedal and a second end section, - one or more housings obtained on the first end section of said shaft along a circumference of the first end section, - fast coupling-uncoupling means to lock the end section of the pin inside said internal housing of the bush, said fast coupling-uncoupling means comprising spherical bodies partially housed inside said housings of the first end section of the shaft and partially housed inside side through holes obtained on the end section of the pin, wherein said annular groove of the bush surrounds the spherical bodies when the end section of the pin is inserted inside the housing of the bush, and - a key lock engaged inside one end of said tubular housing of the pedal, the key lock associated with a ring fixed to the second end section of the shaft and contained inside said tubular housing of the pedal, said key lock is configured to be actuated to move said ring in order to drag said shaft into a short rotation or axial translation travel.