This is also known as tying up although the more correct name is exertional rhabdomyolysis. It is usually seen in fit horses and develops when the horse is working or shortly afterwards. The horse's paces tend to become very short and stiff and the horse becomes less willing to work. If you feel the horse's hindquarter muscles they are extremely hard to the touch and can be very painful. Some horses may even lie down and refuse to move.
You should not make the horse move more than essential as you could cause permanent muscle damage and there's the possibility of the kidneys failing. If you are a distance from home arrange for a trailer or lorry to be brought out so the horse can be transported home.
Rug up your horse to keep his muscles warm - he may be sweating but if his muscles become chilled further pain will result. Your horse's muscles will in effect be cramping because of an imbalance of electrolytes in the body of the muscle. Offer your horse water, as you need to try to flush the body of the toxins being produced by his cramping muscles.
Call your vet immediately. His treatment will depend upon the severity of the case.
Your vet and a nutritionist should be able to advise on changes to your horse's training programme and diet to reduce the likelihood of recurrence.