Why should I switch frequently between practicing different things?

Over the summer I "took" one of those free Coursera courses on learning theory.  Quite interesting - it reinforced something I lucked into when I last studied for exams (I think it was in the dark ages but it may simply have been so long ago I have difficulty remembering).   I had a busy first job when I sat for specialty Boards so my learning schedule involved studying in the evening and reviewing it at 6 am the following morning before work.  I passed the exam, and it so happens modern learning theory seems to be confirming this is the best way to make things stick. i.e. learn something, review it when it's starting to disappear.  The course has some other interesting things to say about learning - such things as highlighting is a waste of time, constantly testing yourself works best, and summarizing in your own words is next best.  I can't see that highlighting or summarizing are useful techniques for the guitar, but testing yourself (look away from the music, can you play it?) is certainly a time honored way to get things into memory.  However what I'm particularly interested in at this point is this concept of 'interleaving' - very ably explained by Noa Kageyama in his blog - basically spending practice time switching between different learning activities.  It does what my serendipitous learning schedule did - making you keep pulling things back when they are not quite there, but in the course of a single practice session instead of overnight.... It certainly can be quite frustrating when you are doing it, but there is a lot of research indicating it speeds up the learning process.

Quite apart from learning theory, I now discover there is another reason that its a good idea to switch often between activities, particularly those that are stressful for the hand - and especially if your joints and tendons are not quite as young and resilient as they once were (let's get real here, mine aren't at all young and resilient any more).   Having successfully badgered my guitar teacher for exercises,  I was determined to apply myself. Indeed I thought I'd work in the  "do it 10x without mistakes before speeding up" system at the same time.   Boy was that a bad idea!  Spent way too much time trying to get to the next level, and after just a couple of days I started to get twinges in an entirely new place on my fretting arm.  I have been known to overdo it in the past, but usually ending up with just a slightly sore muscle,  but this was something different...  At least age comes with some degree of common sense, so I abandoned the exercise for a couple of weeks and limited my guitar playing to non stressful things that caused no pain.  Didn't do much to advance my technical skills, but thank goodness the arm pain disappeared.

Now I'm back to practicing the exercise - however I'm limiting the time I'm spending with a timer, and then moving onto something else. I'm even thinking of applying the timer method to all the things I'm working on ... I'll have to let that one sink in for a bit though before exposing myself to the mental pain.  It's a lot more satisfying to work on something for half an hour than it is to pull yourself away when you're just getting it!