We demonstrate that a finite-length (10,0) carbon nanotube (CNT) with two fullerene caps, namely D-5(450)-C-100, is an ideal prototype to study the mechanical responses of small CNTs upon endohedral metal doping. Encapsulation of a large La2C2 cluster inside D-5(450)-C-100 induces a 5% axial compression of the cage, as compared with the structure of La-2@D-5(450)-C-100. Detailed crystallographic analyses reveal quantitively the flexibility of the [10]cyclacene-sidewall segment and the rigidity of the pentagon-dominating caps for the first time. The internal C-2-unit acts as a molecular spring that attracts the surrounding cage carbon atoms through strong interactions with the two moving lanthanum ions. This is the first crystallographic observation of the axial compression of CNTs caused by the internal stress, which enhances our knowledge about the structural deformation of novel carbon allotropes at the atomic level.