Those of us with tender / arthritic / aged knees tend to use knee pads for HFT, but I find they can be somewhat awkward, unwieldy and prone to slippage. I have just bought some Aldi WorkWear trousers that have pockets at the knees designed specifically for the wearer to insert their own padding (the opening is at the bottom of the pocket, rather than the top), or indeed just to enjoy the double thickness material. And at a mere £8.99 they are probably cheaper than just buying knee pads on their own. Furthermore the cut of the trousers is comfortably generous and there is a plentiful compliment of useful pockets.
They have just gone on offer today (Thursday 10th), so I guess once they're gone they won't be around for another few months.