It's not as simple as saying that damage to the tissues of the spine are the cause of back pain. for one research example of many, see:
http://www.ncbi.nlm.nih.gov/pubmed/8747239
Studies like this have shown that herniated discs and other types of damage once thought to be the cause of back pain are present in pain-free people with a high rate of incidence.
Further, it's not known what the tradeoffs are in terms of risking damage vs the benefits of increased strength.
I do think extra research is needed.
FWIW your definition of heavy and mine seem to be about the same: A weight the subject can lift with perfect form for a small number of reps, e.g. <= 5. With training, likely to be in the range of bodyweight x 2 and up.
I never claimed damage to the tissues of the spine is the sole and only cause of back pain, I only said that if you do have damage to the tissues of the spine which is not uncommon you clearly risk making it worse by putting your tissues under huge forces - it is clearly not a health beneficial thing whether it happens to cause pain at the moment or not.
It would be strange if having the strength to lift twice your bodyweight or more would somehow be necessary for a healthy back. There are safer ways of increasing strength of the weakened muscles, but first it would be good to identify whether indeed weakened muscles are the cause of one of the causes of your problem. That's all I am saying - deadlifting is not in general a safe recommendation for undiagnosed people with back issues.
Studies like this have shown that herniated discs and other types of damage once thought to be the cause of back pain are present in pain-free people with a high rate of incidence.
Further, it's not known what the tradeoffs are in terms of risking damage vs the benefits of increased strength.
I do think extra research is needed.
FWIW your definition of heavy and mine seem to be about the same: A weight the subject can lift with perfect form for a small number of reps, e.g. <= 5. With training, likely to be in the range of bodyweight x 2 and up.