Adaptation

I was reading some literature on the compressive loads experienced by the spine during deadlifts when I came across some interesting data from a study by Granhed et al. (1987). They calculated loads on the L3 vertebrae of experienced powerlifters (all of the lifters in the study were pulling over 200 kg, with a few over 300 kg). What I found interesting was that the authors reported the the annual lifting tonnage from the lifters' training records. I've re-plotted the data below:
This is essentially Granhed et als third figure, although I've switched the axes to reflect my belief in the causal variable, and included the weights for the lifters and data for control subjects.

The data isn't exactly surprising, and there's good evidence that resistance-exercise makes bone stronger (Suominen, 1993). Of course, the study was cross-sectional, so it's unclear how much increase in bone mineral content these lifters saw over the course of their individual training. It's entirely possible that only lifters with strong spines ever manage to lift a lot of weight over the course of a year, and that they experienced little or no adaptation in bone density from lifting heavy weights. Undoubtedly, this bias exists to some degree; the deadlift is an incredible selective pressure after all (I can count on one hand the number of people I've seen deadlift in my gym over 2 years; ok, that's true only if I exclude the Crossfitters, otherwise I would need two hands). Nevertheless, data from longitudinal studies in athletes suggests that increases in bone density are indeed related to training, suggesting that the above plot isn't entirely correlational.

Oh, and the forces on the lumbar spine during the conventional deadlift? No surprise, they are huge, and the worse your form, the more force you are exposing your spine to. Calculated loads for competitive powerlifters exceed 20 kilonewtons. Climbers will recognize that this is close to the rated strength of many carabiners.