I doubt you'll find any direct sources (research) linking spinal loading with systemic hypertrophy increases. However, you could probably take a collection of studies and link them together to figure it out. Example: spinal loading and systemic responses, and those systemic responses regulating systemic growth. Check out google (scholar.google.com) and/or pubmed. If anyone has researched this in the past, I would love to hear about it. But I'd imagine it's one of those "the research is ten years behind practice" things.