I would like to look at the whole paper and whether the relationship is linear. The association is only among old people and even more so among the “oldest old subjects” so it might be that a subgroup of frail people with hypotension creates that relationship while the relationship doesn’t exist among those with normal or high BP.
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They show a bar chart with grip strength versus blood pressure with pressure broken down into tertiles – “lowest tertile”, “medium tertile”, and “highest tertil”. Grip strength increases from lowest to medium to highest tertiles, but it’s not enough information to see that the trend is linear.
They offer a suggestion as to the cause of the relationship in the paper:
How can we explain the associations between higher BP and better muscular function in the oldest old? Peripheral vascular resistance increases with chronological age due to a reduced sympatholysis, which results in an elevated sympathetic tone.[31] [32] Second, morphological changes to the arteriolar network contribute to higher vascular resistance.[8] Third, aging is associated with a reduced capacity in vasodilation caused by changes in endothelium-dependent pathways in animal models.[33]–[36] Maybe increased vascular resistance during the aging process requires higher pressure as a mechanism to maintain tissue perfusion as a means to prevent further ischemic end organ damage in kidney, brain, and skeletal muscle. Another cause for higher BP in elderly could be the age-associated increase in cortisol levels.[37], [38] But as higher cortisol levels associate with lower handgrip strength, this does not explain our finding of higher handgrip strength.[38]
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