Strong spine. Determined by the skeletal muscles that are attached to the bones.
Therefore, stretching those muscles will ensure a strong spine.
For example, yoga is one form of exercise that stretches (forcefully) all the muscles in our body. The same concept applies to tai chi. Or static stretching exercises.
If life force travels through certain paths in our body (ancient masters call them meridians or chakras), stretching the right muscles will minimize blockage and ensure better travelling of absorbed life force via proper breathing.
One obvious effect from a strong spine is a radiant face. Clear, healthy, flushed, pink, red and ruddy face. Just compare how a yoga practitioner, a tai chi master or a gymnast looks vs a sick person.