A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton
supported by fluid pressure... As its name suggests, containing hydro
meaning "water", being hydrostatic means that the skeleton or organ is
fluid-filled... As a skeletal structure, it possesses the ability to
affect shape and movement, and involves two mechanical units: the
muscle layers and the body wall. The muscular layers are longitudinal
and circular, and part of the fluid-filled coelom within.
A hydrostatic skeleton consists of one or more elongate organs containing fluid. The rigidity of the organ can be altered by muscular compression of the fluid within, or admission of fluid from an external pressurized reservoir. Lots of marine organisms use hydrostatic skeletons. Depicted - the velvet worm, a terrestrial organism which uses hydrostatic organs in its appendages to walk.
Your humanoid could be like a velvet worm, with either the entire appendage or an internal skeleton comprised of hydrostatic organs with variable rigidity.
Real humans actually have organs with variably rigid internal hydrostatic skeletons. These organs have played a major role in the propagation of our species to date.