So I've been strength training my cat.
I started mixing extra whey into its food, small amounts at first, just to get it used to the taste, and slightly more every meal. Right now, it's eating about 4 meals of 50% meat and 50% whey powder a day, about 50g protein total, which, for a 5kg cat is the equivalent to a human eating about 750g of protein a day.
The last 3 months I got it started on creatine, which is hasn't seemed to notice. 2-3g per day, sprinkled on top of its food.
My cat had this knitted sweater thing (I live in a cold climate, so cats often wear these things outside), which I attached small weights to. Mainly fishing sinkers. I attached them one by one, so that it wouldn't notice the additional weight. By now, my cat is wearing this 2.5kg knitted sweater around outside, climbing trees, running around, normal cat stuff.
I also feed it Vitamin D, fish oil and a multivitamin, crushed up into its water bowl.
In addition to the weighted vest training, I've started my cat on climbing this large tree in my back yard (3 sets of 5 climbs up and down, enticing it with food), and I managed to figure out a way to make it do a sort of crude bench press by turning it upside down and pushing down on its paws (3 sets of 8-10).
Cat has made decent gains so far, forelimbs are noticeably bigger, and chest has increased in size markedly so. A little extra fat (probably around 15% atm) but I plan on putting it on an EC stack and maybe Yohimbine HCL.