I don’t own commercial extenders, the sole extender I used is a homemade one similar to the Maxxtender (see pic) which has no bars neither screws; I guess with an extender like the Proextender, jes etc., I would add bars more than use the screws, because bars add length where screws raise the tension. I could be figuring wrong the mechanics of these devices though.
I think for length upward stretches will not add much; I think if one wants really to have maximum gains from manual stretches he should do a total of 30-60 minutes per day of manual stretching in these proportions: 2/5 simple stretches, 2/5 side fulcrum stretches, 1/5 downward fulcrum stretches. This could be done in several small sessions. I heard many complaining that they don’t get gains from manual stretching and want to go through an extender or hanging; but they are eager to spend at least 3 hours per day on hanging (2 hours hanging + rest between sets) when they don’t wan’t spend 30 minutes on manuals stretching. How strong they want gains? Most of people will get more gains doing 1 hour of manual stretching per day than 3 hours of hanging per day IMO.
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‘Regarding hanging (vac) ….’ etc. etc.; hanging relies on creeping, not stress relaxation. Of course the difference between techniques is not complete: hanging will allow some stress relaxation, if a mild weight is used, and prolonged progressive stretching will cause some creeping. The difference is that creeping has the side effect of toughening the tissue faster than progressive stretching. So in the long run hanging will carry less gains.
But think to this: instead than using a weight, you could use an elastic band attached to the hanger; when tissue relaxes into the stretches, the band will start pulling with less force. So you can just move a tiny bit to raise the band tension to the starting tension, and keep the stress relaxation going.