It's actually pretty amazing that I haven't written one of my trademark rants about trekking poles already, but, as far as I can recall, I never have. Truthfully, I don't really remember people in Canada being quite so fervent as the Australians are with trekking poles. Occasionally, when people were carrying big mountaineering packs off-trail you'd see poles, but it was not that common, at least among my friends and climbing companions. In Australia, folks seem to use trekking poles to hobble from their caravan to the outhouse. In all the 223 km and 13 days of the Larapinta Track I saw exactly two people walking without trekking poles, and to see one of those, I had to look in the mirror.
I do recall, nearing the end of a 30 km day on the Larapinta Track meeting a hiker coming in the other direction who was burdened down with a huge pack, dressed head to toe in new gear - thick pants, heavy boots, gaitors, long sleeved shirt - and bent virtually double over a pair of trekking poles. I felt under-dressed in shorts, tee shirt and a pair of running shoes as I scampered by, but, couldn't help thinking that folks would enjoy hikes more if they got into tolerable shape for their hike by working out in the gym and building a strong core and legs before they went out walking rather than thinking the walking would get them in shape.
This seemingly irrelevant story is not solely presented to cast me in a favourable light, but is meant to set the stage for an alternative view of trekking poles to the standard "rah, rah" trekking poles are great. I actually think trekking poles are a bit like bands used in Crossfit to help people get their first pull-ups. They work, but there are better ways. Doing negatives is a much better way to get your first pull-up and strengthening the weak musculature and poor balance/proprioception that is the reason you picked up those trekking poles in the first place is a infinitely preferable solution to using trekking poles.
Doug on Euro Ridge near the east end of the Larapinta Track
The proponents of trekking poles rave about how they help your balance ("four feet are better than two," they say), but, I say, if your balance is a problem, instead of propping it up with an artificial aid, work your balance. Stop wearing big clumpy shoes that don't allow you to feel the ground, walk barefoot, walk on rough ground, practice various balance exercises, walk without poles, do all the things that actually improve your balance. Physiotherapy studies have repeatedly shown that using trekking poles reduces proprioceptive ability which is why it makes no sense to worsen your already poor proprioceptive ability with an over-reliance on trekking poles.
One of the other big reasons people use trekking poles is to help them stay more upright when walking/running. If you can't hold yourself upright while walking/running even with a big pack on, you have weak core musculature and should be strengthening your core, not compromising already flaccid trunk muscles by leaning on poles. If you are so tired from running/walking that you can't hold yourself upright, stop. There is no difference between using poor form when weight lifting and using poor form when walking/running. Once you've reached the leaning over stage, you're done for the day. It is counter-productive to ingrain poor movement patterns simply because you want to run a few extra kilometres or walk a few extra miles. Poor form doesn't count when weight lifting and it doesn't count when running or walking either.
The same goes for taking the weight off your knees, helping you go uphill or down, or even propelling you on the flats. Don't prop up weak leg muscles by using poles, take a step back from all the walking/running and get into the gym and strengthen your legs instead. If you are one of those steady state cardio junkies who has to run, run, run and is concerned about trashing your knees, well, you're probably right to be concerned, but the answer isn't picking up a pair of trekking poles and continuing to run with weak leg muscles. A better solution would be to decrease the running and increase the weight training. You might find that your running improves.
You could train all your trunk muscles, your torso, your upper body and that amorphous thing that yuppies call your "core" simply by going to the gym for half an hour two or three times a week and doing some big Olympic lifts with heavy weights. Two or three months and you'd be strong enough to snap those stupid trekking poles in half, lift your 30 kg pack overhead, and walk/run all day without your trunk collapsing over your knees.