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Muscles / PSSM

Principal Body System: Muscular

Definition: Specifically refers to skeletal muscle tissue

Function: Participates in bringing about movement; maintains posture; produces heat

There's muscle, and there's muscle. We've got muscle tissue, which refers to all the contractile tissues of the body, cardiac muscle (part of the cardiovascular system) and smooth muscle (part of the relevant system, i.e. digestive or urinary).

The muscular system, however, refers to the skeletal tissues that make up individual muscle organs, and that's what we're talking about on this page, as this tends to be what us horse-people are concerned with. As in, how they make the equine body move, or not, as the case may be. And when you think how huge the equine skeleton is, with every single bone covered in a muscle, that's a whole lot of muscle.

To quote Dr Eleanor Kellon, "Muscle is the engine which drives all types of work. Skeletal muscle makes up an average of 45% of the weight of a horse with a normal body condition score, which is even more than bone. In addition to initiating movement, implementing fine motor-control of intricate manoeuvres and controlling speed, muscle stabilises and protects the skeleton and joints."

Keeping our horses' muscles healthy

I don't think any of us would disagree that equids have evolved to move, sometimes many miles each day, and no doubt most of us, if not all, know that a sedentary or box-rest horse is not in a healthy muscular state at all.

Muscles respond to exercise, simple as that, both during and after a workout. Exercise improves muscle strength, speed, power and endurance, and changes start to happen with resistance exercise; this means that the workout must be greater than what the body typically does during the day, which activiates the process of muscle building, aka hypertrophy.

However ... top tip - too much intensity and you’re looking at trauma to muscle fibres, resulting in small microscopic tears in the tissue – the severity of the tear depends on the intensity of the activity. These fibres are bundled together into fascicles. Individual fibres are cross-linked so they slide inside the fascicle. Near the end of each muscle these fibres turn into tendon and then attach to bone. Damage develops when a muscle or tendon is overstretched, pulling the fibres apart and losing the ability to adequately contract.

Here are several factors which can contribute to an increased risk of injury:

  • Lack of conditioning - this can leave muscles weak and more likely to sustain an injury from low degrees of force.
  • Fatigue - muscles that have been worked to fatigue are less likely to provide good support to the joints.
  • Tight muscles - improper warmup or lack of stretching reduces range of motion in the joint and makes the muscles prone to trauma and tears. Properly warming up before activity helps to loosen muscles and increases range of motion.
  • Environmental conditions - slippery, uneven surfaces may increase the risk of injury, i.e. sloped surfaces can increase the risk of muscle strain as one leg is hitting the surface lower than the other leg due to the slope.

But all is not lost - the damaged muscle tissue activates satellite cells which rush to the damaged area to replicate and fuse to the torn fibres. This process forms new muscle protein strands which increases the strength and and size of the muscle, resulting in overall muscle growth.

In human-world, exercising specific muscles regularly can increase their size by up to an astonishing 60%, but we're not talking about our own gym membership here, nor can I see our horses heading to a Body Pump class. With our horses it's more about coordination and control rather than physical power and strength, but still on a frequent basis, as this leads to improved muscle coordination which trains muscles to work efficiently together.

Long term, regular exercise increases blood supply to the muscles which improves the delivery of nutrients, minerals and vitamins to the muscles, which makes them able to regenerate after injury or exercise more efficiently. Plus, after regular exercise muscles can store large amounts of glycogen for energy. Size and quantity of the muscle cells’ mitochondria (the power source of each cell) also increase, which also results in an increased rate of energy production.

Active muscles deal with food energy from glucose and fat very efficiently, with enzymes involved in energy production becoming more efficient with enhanced metabolism response. The long and short of it all? Regular resistance exercise leads to healthy musculoskeletal function.


Now for the big one. Everything I've said above relates to that mysterious Perfect World. However, there's a challenging condition (not to put too fine a word on it) that's becoming all the more prevalent out there - PSSM.

As wiki describes it, "Equine polysaccharide storage myopathy (EPSM, PSSM, EPSSM) is an inheritable glycogen storage disease of horses that causes exertional rhabdomyolysis (the destruction of striated muscle cells; azoturia). It is most commonly associated with heavy horse breeds and the American Quarter Horse. While incurable, PSSM can be managed with appropriate diet and exercise."

I can't claim to have my own personal experience with a PSSM horse. However, in 2014 I had my first client, Samantha, who contacted me with her PSSM horse, Star, followed by Sarah and her horse Splodge, and since then, several more. Thanks to Samantha and Sarah, together over the years we've immersed ourselves in trying to fathom this extraordinarily complex and fragile equine syndrome.

Whilst not finding that elusive 'cure', we now have a bit of an understanding of PSSM. After considerable research we've put together our take on PSSM, so if you've recently been given the PSSM diagnosis or you're struggling with which direction to follow, I can't say we're the oracle of all things PSSM, but this page may help guide you with some food for thought.

Meanwhile, there are a couple of excellent Facebook pages packed full of information, latest updates and client stories. Both Samantha and Sarah are regulars on the page.

PSSM Forum - https://www.facebook.com/groups/202978353056065/

PSSM Forum & 5 Panel Genetic Testing & Discussion - https://www.facebook.com/groups/432915936807292/

PSSM UK - https://www.facebook.com/groups/420056078181332/

*NB - Please take this page exactly as it is - this is simply my findings over the years, supported with clients’ updates and feedback, so you may already know everything on here. If I've learned anything over the years re PSSM, it's that the human carer of a PSSM horse has researched the life out of this condition and knows a whole lot more about it than me! Hopefully, though, there’ll be some pointers to take away here so here we go.

Going back to 2017, as far as we knew then, there were/are currently two PSSM types, with Type 1 - the genetic form of the disorder - caused by a mutation in the GYS1 - muscle enzyme glycogen synthase 1 - gene. This gene has what is called a 'gain in function', which means it's more active than normal. GYS1 enzyme starts the process of storing muscle glucose in the form of glycogen, but because other enzymes needed to build glycogen’s structure cannot keep pace, the glycogen produced is abnormal and resists breakdown to release the glucose.

The gain in function in the enzyme also steals glucose away from other processes, such as energy generation - see the Acetyl L Carnitine paragraph below.

However, not all cases of PSSM diagnosed by muscle biopsy are caused by the GYS1 mutation which means there are at least two forms of PSSM.

Type 2, which is the term currently used for the (cause yet to be identified) other PSSM cases, shares many of the clinical signs of Type 1, i.e. excessive glycogen in their muscles. However, the underlying genetic mutation of Type 2 has yet to be identified.

PSSM1 - a workhorse legacy?

PSSM1 is an inherited muscle disorder due to a mutation in a gene called glycogen synthase 1, or GYS1 for short. It’s believed that this gene was mutated 1200-1500 years ago, and the degree of myopathy will depend on whether it was inherited from both, or just one parent.

In the US, around 87% of workhorses and 72% of quarter horses are thought to be carriers of PSSM1, but it’s also strongly represented in the European coldblood breeds, i.e. the Belgian draft horse represents 92.1% of cases, with warm-blooded haffies coming in at 23%.

Normally, glucose is stored in the muscles as glycogen which is then used later for energy production. In the case of PSSM1, too much glycogen is stored in the muscles of affected horses that then doesn’t get broken down.

Cue muscle tissue breakdown, known as rhabdomyolysis, usually in the the back and croup muscles, and is very uncomfortable.

Symptoms usually begin shortly after starting work and can vary from reluctance to move, muscle stiffness and lameness, to complete inability to move. In severe cases, the horses try to get relief by stretching their legs out like a rocking horse - front legs forward and hind legs backward.

Diet is a key factor - low-sugar/low carbs and regular exercise – low sugar hay with balanced minerals, and absolutely no concentrates. Movement is absolutely key, as crucial as the feed plan.

For the PSSM horse there are a few general rules of thumb:

  • Feeding a clean (non GM and organic wherever possible) forage diet.
  • Emphasis on ad-lib 24/7 hay.
  • Check what chaff you’re feeding as chaff is usually 7-10% starch, if not more. However, you can get this right down to 2% if you go for the Agrobs Leitchgenus. Thunderbrook do their own low starch 'Herbalite' chaff as well.
  • UK-grassland-appropriate forage-balanced minerals are also vital to balance the body’s chemistry, including magnesium as it calms muscles as well – see our EquiVita mineral balancer range.
  • Keep an eye on the microbiome - hay is your saviour here to feed the beneficial hindgut biome microbes.
  • It’s also recommended to avoid alfalfa as anecdotal evidence shows that horses with muscle issues such as PSSM/Stringhalt seem to manage better when not fed alfalfa.
  • Make sure muscles are always warm in winter and as far as exercise goes, the horse must be able to walk around freely but avoid yeehah running with a herd.

NB - the thinking used to be that feeding a high fat diet was beneficial, but as at 2021 this is now not the case - see the Page Updates at the bottom of this page.

Our PSSM client input

I first got involved with PSSM when Samantha, my first PSSM client, contacted me in 2014. Here’s a copy of her email (Samantha's very happy for me to quote her):

"Hello, you may remember I contacted you last year possibly around September time where I discussed with you my suspicions of our Appaloosa, Star. I just kept getting him scoped for ulcers, however I was convinced any gut issue was secondary to a primary cause.

I guess after contacting you I really did start to take Star’s issues more seriously that something was amiss and compromising him. Long story short, I researched and researched and came across PSSM, a genetic muscle condition. One £30.00 genetic test later and we had a positive diagnosis for PSSM type 1, (end of November). The vet finally took me seriously and is being supportive although the knowledge of this disease in the UK is wafty!

My question is, do you know anything of this disease?”

This was enough to pique my interest, so after a long chat with Samantha, the two of us started swatting everything we could about PSSM and exchanging notes. Took about a month, and I eventually came up with the following, which formed the bulk of my original collective research and my email reply back to Samantha. Apologies if you already know the following, and remember to bear in mind that this was based on very limited 2014/15 data.

Equine polysaccharide storage myopathy (EPSM) is a genetic disorder that affects a horse’s carbohydrate (starch) metabolism, and is by far the most common cause of tying up in horses.

Irrespective of whether we’re talking PSSM Type 1 or 2, affected horses store too much glycogen in their muscles, which they cannot break down to produce carbohydrates. Without these carbohydrates as energy sources, the muscles lack the necessary fuel during exercise. As a result the muscles must use energy from less efficient energy pathways, which produces damaging byproducts, such as lactic acid.

High-starch carbs, i.e. corn, wheat, oats, barley and molasses (none of which we should be feeding anyway, PSSM or not!), as well as the high protein in alfalfa, appear to exacerbate both PSSMs. Hence they should be avoided at all costs alongside short periods of a slow build-up exercise regime as PSSM horses will tire more quickly than unaffected horses. This will improve glucose utilisation and energy metabolism in skeletal muscle.

Low-starch diets produce low blood glucose and insulin levels after eating, which may reduce the amount of glucose taken up by the muscle cells.

Chronic symptoms include (but not exclusively):

  • tying up
  • abnormal hind leg gaits
  • exercise intolerance
  • muscle wasting
  • back soreness
  • difficulty lifting hind legs
  • spasmodic type colic
  • elusive lameness

Acetyl L Carnitine, aka Alcar

Now we come to important nutrients and one in particular is becoming a no-brainer for our PSSM horses. The amino acid Acetyl L-Carnitine, aka Alcar, acts as the fuel pipe needed to transport the energy fuel into the cells, and has been noted to be an essential for muscle support/repair. It works by transporting the essential fatty acids (EFA’s), aka omega 3 & 6, into the ‘mitochondria’ (the energy-creating engines inside each muscle cell), where these fuels, the EFA’s, are burned.

The mitochondria are the power plants where oxygen combines with metabolites of glucose or fat to produce energy in the form of ATP. Converting locked energy from feed into energy that the cell can use is the mitochondria’s most crucial role, and all fatty acids need L-Carnitine to carry them into the mitochondria. In PSSM horses, the number and function of their mitochondria are impaired.

This is also an important factor in impacting the PSSM horse being able to keep warm. 🤓 Science Alert, and quoted from Dr Eleanor Kellon: "A major strategy for generating body heat is oxidative uncoupling. With oxidative uncoupling, the normal efficient generation of ATP in mitochondria is disrupted so more energy is lost as heat. The deficit in mitochondrial number and function is likely why PSSM horses are more sensitive to cold."

Just as an aside, Alcar is also fantastic for our IR/metabolic horses as it helps manage leptin resistance – all explained in detail in our EMS/IR Metabolic Horse page.

Additional antioxidants such as Vitamin E (at least 2000iu, with some PSSM clients reporting needing to feed up to 8000iu, if not more) and Selenium is said to help support and repair muscle damage. We include both the (non-synthetic) selenium RDA and 2000iu natural vitamin E as standard across our EquiVita/VitaComplete mineral balancer range.

In addition, the nervous system is supported by phosphorous, calcium and magnesium in balanced ratios - again all these nutrients are an integral part of our EquiVita range of forage mineral balancers.

Having also done a ton of research on herbs to help the PSSM syndrome, Viburnum opulis (Cramp bark) is a really effective muscle relaxant that Samantha found very helpful for Star. An incredibly versatile plant - we use it extensively across our range where 'cramping' needs relieving, i.e. arthritic symptoms, mares' seasons, gut spasms - it's also especially beneficial for PSSM symptoms, specifically helping relax muscle pain and inflammation, with overall detox and antioxidant support as well.

Signs of improvement include re-bulking of atrophied muscles, comfort when hind legs are raised, and freer movement of the hind end in general. Many PSSM horses apparently then appear to have higher energy levels, thought to be being mainly due to the freedom from chronic muscle spasm. I’m also reading that they seem much calmer and cheerier as the discomfort reduces, which has to be a good thing 😉

Through Samantha's contributions on the then-Facebook PSSM group page, this then brought Sarah and her horse Splodge to me. Since then, I’ve had numerous more, and what’s becoming more apparent is that it’s no longer breed-specific as previously thought, as in not just Appy’s and quarter horses; I now have a connie, a gypsy cob and a new forest in my PSSM client base to name a few.

Back to Sarah and she’s become a regular corresponder over the years. Here’s a copy/paste from one of her emails with some useful tips on what she feeds her PSSM horse :

“Hi Carol, the FB PSSM page has literally been a lifesaver for me. I’m now using your L-Carnitine and both my horses are better with a scoop of this every day – I also add L-glutamine* for stomach support, which has been brilliant.

Secondly I keep Splodge's diet super low in sugar/starch. I soak hay all year round now which I hate, especially as I have to soak for 2 as they share a bald field. Thirdly I bully her into exercising daily. If she's feeling bad then it is bullying too, but she's better for it."

* Edited to add: Glutamine is actually the most abundant amino acid in muscle tissue, hence why we include it in our JointReflexa blend, in water-soluble form (N-Acetyl L Glutamine) for effective digestion (the protein digestive enzyme, Pepsin, is water-soluble) and therefore greater bioavailability (time taken to absorb into the bloodstream). Our GutAminos blend is also a combo of Glutamine and Cysteine (N Acetyl L Cysteine) with added Marshmallow Root (certified organic) for leaky-gut membrane repair.

Back to me again and that’s pretty much it. I know this was a long one but amongst all the ramblings I hope there's some food for thought here. This area is so grey with very little research available for it, and if I’ve learned anything, especially from my Connemara client, and as Samantha references, a one-size fix definitely does not fit all. With my connie client, we had to tweak the volumes of the various key feedstuffs several times to eventually get him to a comfortable place.

Updated - May'21

April'21 I completed Part 1 of an intense equine nutrition workshop by a new face in nutrition coming over from Germany, a Dr Christina Fritz. Well, not a 'new' face as Christina's a huge presence in Germany, but certainly new to us here in the UK. The workshop was heavy on the latest science, and I have to extend a huge thanks to Lorraine Dearnley, Coppermead Equestrian Services, for pointing me to it.

Christina has only ever published in german but apparently her works are due to be translated to English soon. Her bio says, “Dr Christina Fritz, German Biologist with a PhD in Animal Physiology/Neurobiology, has been treating horses since 2006 focusing on metabolic therapy using holistic feeding methods.”

Part 1 of the workshop was all about Feed, Parts 2 & 3 coming in September and more about the internal physiological structure of the horse. While there wasn’t that much in Part 1 that I wasn’t aware of already, her knowledge and presentation went way deeper than any other thinking out there that I’ve come across, plus she’s bang up to date on the current research, so I took copious notes throughout. It was also pretty impressive that she presented the whole thing in very fluent English. 😉

Now to the PSSM connection, because one of the Feed topics was … feeding oil, as in liquid/pouring oil, to the horse, and the message was that we shouldn’t – ever – because the horse hasn’t evolved to digest fats other than the naturally sourced, and very limited, EFAs (omegas) in their forage/roughage (I’ve already added a page on the website in the 'Feeding our Horses/Why what we feed' page that explains this particular ‘Why’ in depth).

So, now cut to the live Q&A session at the end of Christina's workshop and with my PSSM clients in mind, one of my questions was:

“For PSSM horses the current thinking here in the UK is a low starch, high fat diet to provide muscle energy. You've covered on the course that it’s now clear we shouldn’t ever add oil to the feedbowl, so, what are your thoughts on an alternative for the PSSM horse?”

(Bear in mind my Q&A question didn't differentiate between PSSM Type 1 and 2.)

Here’s her reply, literally word for word as I copy-typed it all down from the Q&A recording. Christina apparently has many PSSM horses as clients, so … stand by your guns and here's what Christina said:

“Yes, it’s quite a weird thinking to give oil to a PSSM horse - the PSSM horse is not energy deficient, just the contrary – they derive a lot of energy from fibre as well as glucose and starch. So you only get PSSM symptoms when there’s too much sugar/starch and not enough work cos the muscle cells are highly sensitive from taking up sugar from the bloodstream, and they’re taking up too much sugar from the bloodstream which is why it accumulates in the muscle cell which then gets into trouble and dies, which we see in tying up or colic symptoms in horses.

I don’t know where it came from to feed high protein/fat to PSSM horses – probably cos over the last 30yrs or so ago the digestion research ended at the small intestine but never looked at the large intestine, so when you can’t feed sugar/starch to humans as an energy source you have to switch to feeding protein/fat to bring energy into the body cos we can’t digest fibre to get energy from that, but for horses it’s different.

The most important part of digestion is the hindgut and even PSSM horses are great digesters of cellulose, from which they’ll get propionate, butyrate, acetate (the three energy-forming volatile fatty acids produced by the hindgut microbiome) from their hindgut microbiome which can be used for their muscle work. The thinking used to be that horses have to make glucose from the propionate to use it but latest research shows that they’re able to change propionate to glucose but they can also use propionate directly in the cells as fuel so there’s no need to use high fat/protein diets to horses.

I always recommend low-sugar hay, a good mineral supplement, and daily exercise. When you feed high fat/protein rations you’ll get kidney problems cos excess protein has to be degraded and excreted by kidneys as urea, so the more protein you feed the more urea you produce. Horses don’t use protein for energy but as building blocks for their body, so with 10yrs of high fat diet you’ll send the kidneys into kidney failure. It should always be good quality hay only.”

Meanwhile I contacted Sarah with the update as I was super-keen to get her thoughts. Here's her email reply back to me:

"Oh interesting!! So Splodge damaged her DDFT last year and given she's known as Podgy Splodgy for a reason I dropped all oil out of her diet - no need for extra calories if she's just walking in a paddock. I have dithered - a lot - over whether to add it back in but I think she's looking as good and as energetic without it. Rather too energetic at times 😩 She could easily do any sort of normal leisure riding on her miniscule amounts of feed with no oil.

Saying all that though the metabolic team at Liverpool uni are still recommending high oil and a lot of the competing PSSM horses are having neat oil alongside rice bran, linseed etc - in high quantities too. I wonder if the oil-free diet is ok for leisure riding or working all day at the walk, but when we ask more of them they run short. My new vet said she'd had it described to her as an energy crisis in the muscles - they cramp up because the body is trying to pull glucose out but it can't.

The PSSM horses do have to become oil adapted and i have no idea what that means for the gut biome. Intuitively feeding large amounts of oil has never felt healthy to me!

I think Christina is probably right although I'd be interested to find out if she has any competing pssm horses on her books and what / how they are fed. Can low NSC fibre alone do sponsored rides, eventing, hunting etc?

Thanks for sharing this! I'm going to Google her and see if I can dive deeper too! There's so many horses being diagnosed now. Depressing for owners but in many cases giving them answers to the 'not quite right' gut feelings they've had for ages.

Keep me posted!"

Update - Aug'21

Message from Christina:

"Hi Carol, so we really should do a webinar on PSSM …

Calculating oil into the energy ration of a horse has never made any sense. Since we have studies showing the the horses gain their energy first and most out of fibre. When you start increasing training, they are able to get even more energy from their fibre. Nobody knows how they do it, but fact is: they can. Only when you increase training further, the metabolism starts to switch to other nutrients for energy and those are branched-chained amino acids. So they will degrade proteins and gain energy from those rather then using oils. Only as a last resort, the metabolism starts to degrade fatty acids for energy.

So this means the body uses oils (you have around 1-2,5% crude fat in hay = 10kg hay = 100 - 250g crude fat) as building bricks for regenerating tissue (all cell membranes are built from fatty acids), producing hormones (e.g. steroids) etc., but not as energy source.

Most people think nutrition in horses from the wrong end: they just look at what get’s digested in the small intestine. For most people, even vets and some scientists, the horse kind of ends with its small intestine. This would be suitable if we were talking human nutrition (or dog, cat, mouse, rat…), since they are small-intestine-digesters. Neither man nor dog can live on plant fibre, they would simply starve to death. Unlike horses who are perfectly fine living on plant fibre.

So you have to think large intestine first. This means in all horses (not just PSSM): take care that the microbiome is healthy and working properly. Feed the hindgut with good quality hay, avoid anything that disturbs the fermentation process there (like haylage, beetpulp, apple pomace, small-cut fibres from some muslis…). Make sure the microbes are provided 24/7 with cellulose (straw does not contain cellulose!). Offer access to green pasture or fresh cut grass during the summer to provide vitamins, as well.

Only then and if necessary add some nutrients like protein or minerals (depending on training level, quality of forage…).

Yes, I do have PSSM horses among my customers that are fed according to my advice, that are competing. They get 24/7 low-sugar, good quality hay, minerals and sometimes a little protein through esparsette / sainfoin. They are top healthy and full of power. Energy comes from the hindgut and good training, not the foregut or the feed bucket.

When you start to change the feeding of competition horses, I usually recommend to start with the end of the competition season (=fall) or just skip one season. In the beginning, they have a hard time, since the metabolism has to get rid of all the excess fat and toxins that were stored in the body. You have to help the body by some therapeutic measures, here. Also you have to re-establish a working microbiome, in most competition horses, you find severe dysbiosis due to wrong feeding in the past. After about 6-12 months, the energy level goes up, the horse looks more healthy than ever, shiny coat, bright eye and you can start training them just like you used to.

Horses fed that way have less hoof horn issues (laminitis, not being able to walk without hoof protection…), tendon issues, muscle cramps, they regenerate faster after hard competitions and they last longer, so you don’t have to put them to a retirement home as early as most other competition horses. By the way, works even with racehorses, here you just need to definitely add protein in form of esparsette / sainfoin and they need more cellulose (hay), so I recommend to offer soaked haypellets instead of grains. Same result: full speed, less injuries, last longer on the racetrack :-)

Yours, Christina"