A horse will alter the way it carries or balances itself to compensate for an injury (however slight), poorly fitting tack, bad rider position, a bump, strain, landing heavily while jumping, a twist or even if he has just "tweaked" something out in the field.
This often results in excess strain to parts of the horse's back and, even after the original cause has gone, can show up in a change in the horse's muscular structure, gait, performance or behaviour.
Signs and symptoms of possible back problems The following are guidelines to help you assess whether your horse seems generally comfortable and is moving to the best of his ability within the limitations of his conformation.
  Obviously all horses are different and some are far more sensitive than others, so the secret is to know what is "normal" for your own horse.
  Any deviation from his normal behaviour, performance or posture should be looked into to.
  When trying to get to the bottom of any problem you are having with your horse you need to eliminate all possibilities, and, you should consider structural changes such as arthritis, other lameness problems, the foot and shoeing, dental or biting problems, saddle problems, training, rider error and general management.
  You may therefore need to enlist the services of your vet, farrier, therapist and other professionals in order to get to the root of the problem.
General behaviour

• Is your horse grumpy, fidgety, does he swish his tail or pin his ears back when you groom him or tack him up?
• Does he try to move away or turn his quarters towards you, tighten his back muscles, or dip his back when you put a saddle on?
• Is he difficult to mount or "cold backed"?
• Can you see or feel him tighten up his muscles or brace himself?
• Does he sink or lower his back when you mount?
• Are there any rub marks, white hairs, lumps and bumps under the saddle area?

Remember that you are looking for a change in his behaviour, if your horse has always been ticklish and fidgety some or all of this behaviour may be normal.
Musculature and Asymmetry What does it mean when a horse is described as "asymmetrical?" It's when the size and shape of the muscles and/or the position of the horse's body on one side is different from its corresponding part on the other side.
  Like us, horses tend to be right- or left-handed, and this "handedness" can lead to uneven development in the horse's body.
  However, injuries and other outside influences can also contribute to compensatory movement from the horse which then becomes a habit, causing unequal building of the muscles.
Asymmetry may create lameness and/or movement problems and may also masquerade as a training, behavioural, or attitude issue.
  Asymmetry can be caused or aggravated by rider interference, poor saddle fit, hoof imbalances, the need for dental work, injury etc.
Things to look for:

• Muscles evenly developed on both sides of his body.
    The muscles should look equal in size on both sides of the body.
    Easily spotted differences are the shoulders, gluteals (over rump), hamstrings (point of the buttock either side of the tail) and second thigh.
  
• The hindquarters should match the front end.
    Well-developed shoulders and necks but weaker hindquarters that don't seem to match could point to difficulty in using the hindquarters.
  
• Has he got a sunken or sway back?  Do not assume that because your horse is older his back should naturally sink.
  
• When compared to his workload has he got a "topline"?  Horses who use themselves correctly and work in an outline should develop muscle from poll to tail.
  
• Any hollows behind the shoulder?  This usually suggests that he is wearing, or has recently worn, an ill-fitting saddle.
  

Other symptoms Things to note when handling the legs:

• If you start to have trouble picking your horse's hind feet out.
  
• If your farrier reports that he's having trouble holding one or the other of your horse's hind legs up.
  
• This can be a sign of hock trouble or it may be a sore back.
    Hock problems cause referred back pain and soreness can make the back muscles go into spasm when the horse tries to keep his hind leg up.
  

Equitation and ridden problems

• Does your sweet natured, relaxed, happy horse turn into a raving lunatic as soon as you sit on his back?
• Does he have difficulty engaging his hindquarters or stretching his neck and going 'long and low'?
• Does his back swing with his stride?
• Does he resist bending on one particular rein?
• Is he slow to warm up or relax?
• Is he resistant to work, difficult to ride in an outline or has difficulty maintaining an outline?
• Does he have difficulty changing the rein and/or changing the lead in canter?
• Does he consistently go on the wrong canter lead on one rein?
• Does he go disunited in canter?
• Collection and maintaining impulsion is difficult.
  
• Does he swish his tail, pin his ears back, grind his teeth, grunt or toss his head around?

One-sidedness

• Is he particularly stiff on one side - ridden or lunged?
• Does he have problems with incorrect bend or always looking to the outside on one particular rein?
• Is he difficult to turn?
• Does he only ever roll on one side?  Horses normally roll both sides, if it's only ever one side that's muddy he may be uncomfortable.
  
• Does he carry his tail to one side?

Trouble going up and down hills

• Does he rush downhill, go very slowly downhill or regularly catch or drag one or both hind toes?
• Does he pull uphill with his front end, unable to use the back or hindquarters properly?

Jumping

• Does he rush to or from fences and/or refuse jumps?
• Does he uncharacteristically knock jumps down?
• Is he unable to travel straight, twists over fences or always veers off to one side?
• Is he unwilling or unable to round the back and/or neck properly and athletically?
• Does he have difficulties with combinations?

If your horse has or develops any of the above symptoms, get him checked by a professional therapist who will work with your vet to get to the bottom of the problem.
Find out more information: check out - looking after your horse's back.