Quite remarkably, Herrick managed to describe the world’s first heart attack in 1912 and then, without missing a beat, he immediately knew that strict bed rest was an essential form of treatment – for a condition never before described: "The patient is to be guarded by day and night nursing and helped in every way to avoid voluntary movement, or effort."
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Why six weeks? I suspect this time period slipped over from orthopaedics, where it generally takes a broken bone about six weeks to heal. Bones and heart muscle, pretty much the same thing aren’t they?

The reality is that strict bed rest killed millions of people over a fifty year period from 1912 to around the mid 1960s.

You may wonder how bed rest is so deadly. Well, there are two main ways it can kill you. First, lying in bed stationary for six weeks means that there is a damned good chance of developing a deep vein thrombosis (DVT) in the legs. Secondly, without any exercise, and especially after a heart attack, the heart degenerates very rapidly. It becomes weaker, and deadly heart rhythms develop.