Quick answer: if one back leg is thinner than the other, your dog has almost certainly been using that leg less, usually because something in it hurts. In the back leg, cruciate ligament disease is a common cause, followed by hip, knee and ankle problems; less often it's a nerve problem. Unlike all-over muscle loss in an older dog, one-sided wasting is not "just age", so it always needs a vet check.
Transparency note: this page is written by MedPaw, which makes a joint supplement. Sources are linked so you can check them. Checked 9 October 2026.
Why muscle goes from one leg only
Dogs are good at hiding pain. Rather than limp badly, many quietly shift their weight onto the other three legs. Over weeks, the unused thigh and rump muscles on the sore side shrink, while the other legs may even bulk up from doing extra work. That's why owners often notice the thin leg before they notice a limp. The American College of Veterinary Surgeons lists "decreased muscle mass in the affected leg" as a sign of cruciate ligament disease, alongside difficulty sitting and getting up from a sit.
Likely causes
Why one back leg may be losing muscle
| Cause | What you might notice | What to do |
|---|---|---|
| Cruciate ligament disease | Limping, sitting with the leg out to the side, stiffness after rest, muscle loss in the affected leg | See your vet; read about cruciate injuries |
| Hip dysplasia or hip arthritis on one side | Stiff getting up, shorter stride, swaying hips, thinner thigh and rump | See your vet; read about hip dysplasia |
| Kneecap problems (luxating patella) | Skipping or hopping for a few strides, then walking normally | See your vet; read about luxating patella |
| An old injury, fracture or operation | Thinner leg since an injury or surgery; may still favour it | Ask about physio; see rebuilding muscle after surgery or rest |
| Arthritis in the hock (ankle) or toes | Stiffness, swelling, shortened step | See your vet; pain relief and controlled exercise |
| Nerve problems (injury, a disc pressing on a nerve root, rarely a nerve tumour) | Scuffing or knuckling the paw, weakness, muscle loss that keeps progressing | See your vet promptly; read about nerve damage |
How to check your dog's legs
- Compare by hand. Stand your dog square on a non-slip floor. Run both hands down the outside of both thighs at the same time. A wasted leg feels flatter and softer, and the bones feel closer to the surface.
- Look from behind. Take a photo from directly behind your dog standing square. A thinner thigh or a hip bone that stands out on one side is often easier to see in a photo.
- Watch the sit. Many dogs with a sore knee sit with that leg kicked out to the side instead of tucked under.
- Watch them walk and stand. Look for a head bob, a shorter step, a toe that touches down lightly, or weight shifted forward.
- Measure. A soft tape around the middle of each thigh, written down with the date, gives your vet something concrete.
Red flags: when to see a vet
Book a vet appointment for any one-sided muscle loss. Go the same day if:
- Your dog won't put the leg down, or is suddenly much lamer
- The leg is swollen, hot or very painful to touch
- Your dog is knuckling over, scuffing the paw, or the leg seems weak rather than sore
- There's a lump, or the muscle loss is getting worse quickly
- Your dog cries out, is off food, or seems unwell
A vet will feel both legs, test the knee for instability, check the hip and nerves, and may recommend X-rays. Read more in why is my dog limping? and a limp that has lasted days or weeks.
What you can do at home
- Rest from high impact until your dog has been seen: no ball throwing, no jumping in and out of the car, and lead walks only.
- Keep walks short and regular rather than cutting them out completely, unless your vet says otherwise.
- Use rugs on slippery floors so the sore leg doesn't slide out.
- Watch the weight. A lighter dog puts less load on the sore leg.
- Once diagnosed, ask about a physiotherapy referral. Rebuilding one leg needs exercises that make your dog use it, such as slow walking, gentle hills and weight shifting. See what to expect from canine physiotherapy.
Can a supplement help?
A supplement won't fix a torn ligament, a slipping kneecap or a nerve problem, and it isn't a painkiller. Those need your vet. Where it can help is alongside the vet's plan: once the cause is known and your dog is building the leg back up, supporting the joints helps your dog stay comfortable enough to keep using it. This is especially relevant for dogs managed without surgery or with arthritis in the joint; see supplements for a cruciate tear without surgery. We'd suggest MedPaw Hip & Joint Chews: a soft chew with all six key joint actives and a 60-day money-back guarantee. It contains green-lipped mussel and brewer's yeast, so it isn't suitable for dogs with a shellfish allergy; check with your vet if your dog takes medication.
For muscle loss more generally, read the best supplement for a dog losing muscle.
Frequently asked questions
Why is one of my dog's back legs thinner than the other?
Because your dog has been using it less, usually due to pain. Common causes in the back leg are cruciate ligament disease, hip problems, kneecap problems and old injuries. Less often it's a nerve problem. A vet check will find out which.
Is muscle loss in one leg serious?
It always needs a vet check, because it means something is wrong with that leg. Many causes, such as cruciate disease, are very treatable, and the sooner they're found the more muscle your dog keeps.
Can my dog's leg muscle come back?
Usually, yes, once the cause is treated and your dog starts using the leg properly again. A physiotherapy plan with controlled exercise helps a great deal. Recovery takes weeks to months.
My dog isn't limping but one leg is thinner. Should I worry?
Yes, enough to book a vet appointment. Many dogs shift weight off a sore leg so smoothly that the limp is hard to spot, and the muscle loss is the first clue. Watch how your dog sits and stands, and take a photo from behind to show your vet.
Can a joint supplement help a dog with one weak back leg?
It can support the joints alongside your vet's plan, particularly where arthritis or a cruciate problem is involved, but it won't treat the cause or replace pain relief. Get the leg checked first.
