Strengthening the Spine Without Direct Exercise

Contrary to popular belief, I still exist...and so does Health-Actualization.  Thank you for your patience.  I have spent the last few weeks between vacation in New York and working on a few health-related projects for various companies. Below is a picture I took during my vacation (hiking the woods of northern New Jersey).

The Spine-Strengthening Riddle

Women between the ages of 65 and 75 strength trained three times per week for a year using full body routines and a slower than normal speed (five to seven seconds per rep) [1].  The routine focused on large muscles using exercises like the leg press and bench press.  At the end of the study, the trained women increased bone density in their thighs, hips, and lumbar spine, as opposed to the control group, which experienced no change or loses in each of those areas.  In summary, strength training increases bone density in the elderly...what else is new?

This: the lumbar spine experienced a gain in bone density despite no direct exercise!  The routine included no lower back exercises.  How did this change happen?  I have two explanations:

1. The leg press places a beneficial force on the lumbar spine that leads to increased calcium deposit in the vertebrae.  The researchers conducting this study speculated this as the prime reason.  The leg press features hip flexion and extension.  Each of those movements involve the psoas muscles (see above), and their tendons connect to the lumbar spine.  Therefore, as you perform the leg press, the psoas pull on the lumbar spine, and the body reacts by strengthening those bones.  
2. Indirect effect.  Legendary strength training innovator Arthur Jones created this term, which describes how training one area/muscle group sends a ripple effect that benefits other areas.  The size of the "ripple" is related to the size of the muscles used, and the areas closest to the exercised muscles receive the greatest benefit.  In addition to the psoas effect discussed in number one, the exercises performed by the senior women could have strengthened the the lumbar spine through the indirect effect.  The leg press, row, and pulldown are exercises that would create the largest ripples due to their target muscles' locations and sizes.  

If you take anything from the experience of these senior women, it's that you should use the leg press if strengthening your spine is a priority, and focusing on the large muscles (hips, thighs, and upper torso) in your strength training workouts will provide the best bang for your buck.