A dynamic platform with extending struts has fastened thereto a bi-directional torsional power unit to selectively deliver force opposing either extension or flexion, and to provide assistance in a respective opposite direction. The power unit is threadably mounted on a hinge pin (spline) centrally located on the platform, and is latched to a catch assembly radially located thereto, the hinge pin of the platform communicating with one end of a torsion spring of the power unit and the catch assembly communicating with another end of the torsion spring, to selectively deliver the extension/flexion force. The power unit can be detached, with simple manual operation of the catch assembly, without tools, flipped over and reattached to the same platform attachment points to switch (reverse) extension torque to flexion torque and vice versa.