How Much Rest Time Do You Need Between Sets

It is common wisdom that we need to split up our training so we get at least a day of rest between training the same muscle or lift again, since it takes a few days to recover from a tough workout [3], but it doesn’t seem to matter, at least all that much.
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Allocate your volume in the smartest way possible that your schedule allows. But, if you can only train Mon, Tues, and Wednesday; and you want to do a full body split hitting each major muscle each of those days, nothing says you can’t do that. In a recent study [1], subjects either trained on 3 non-consecutive days (like Mon-Wed-Fri), or 3 consecutive days (like Mon-Tue-Wed). They trained the same exercises each of those days. Both groups gained muscle and got stronger to the same degree.
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Some athletes feel like they were leaving some gains on the table when they trained a muscle back to back. But fortunately, we can probably rest easy knowing our efforts aren’t being wasted.
However, if you want to recover rep performance; a recent study [4] shows that 48-72hrs is FAR better than 24hrs. Albeit the subjects were doing a very demanding workout of 12 sets to failure of chest pressing. So for a fairly low volume-moderately hard workout, 24hrs might not be so bad, I’ve done it, I seent it.
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So while 48hrs is a good recommendation, it seems that back-to-back training doesn’t compromise training adaptations. How you design your training frequency though will depend on your individual recovery rate, which is partly influenced by training effort, intensity, the exercise, sleep, and nutrition.

References
[1] https://www.ncbi.nlm.nih.gov/pubmed/29967584
[2] https://www.ncbi.nlm.nih.gov/pubmed/26332783
[3] https://www.ncbi.nlm.nih.gov/pubmed/28965198
[4] https://www.ncbi.nlm.nih.gov/pubmed/28195973