Closed chain exercises are those rehabilitation exercises where the moving limb is continually in contact with the surface of the exercise equipment, eg cycling.

Closed chain exercise
Closed versus open chain
In contrast, walking - where the foot is free of any surface for some time - is an open chain exercise.
-
-
CLINICAL PUBLICATIONS
Comparison of the Clinical Effects of Open and Closed Chain Exercises after Medial Patellofemoral Ligament Reconstruction. Zhang F, Wang J and Wang F. J Phys Ther Sci. 2014 Oct; 26(10): 1557–1560.
The Effect of Open and Closed Kinetic Chain Exercises on Dynamic Balance Ability of Normal Healthy Adults. Kwon YJ, Park SJ, Jefferson J and Kim K. J Phys Ther Sci. 2013 Jun; 25(6): 671–674.
See also -