Extendable trekking poles have gone from guaranteeing a good laugh at the expense of the user to being all the rage in recent years. They work by taking the strain off the ankle, knee and hip joints and transferring some of it to the wrists instead. They are also particularly good for helping you keep your balance in difficult terrain such as deep snow, river crossings and Galloway bogs.
For climbers, the best sort are three part poles which can be easily stored in your rucksack when not in use (top tip: if storing poles on the side of the sack with wand pockets and compression straps, store them both on the same side. This guarantees that you'll only get them caught under a roof 50% of the time, instead of 100% if you store one on each side!)