  We study a marginally stable circular orbit (MSCO) such as the innermost
stable circular orbit (ISCO) of a timelike geodesic in any spherically
symmetric and static spacetime. It turns out that the metric components are
separable from the constants of motion along geodesics. We show also that a
metric component $g_{rr}$ with a radial coordinate $r$ does not affect MSCOs.
This suggests that, as a test of gravity, any ISCO measurement may be put into
the same category as gravitational redshift experiments. MSCOs for exact
solutions to the Einstein's equation are also mentioned.
