Muscle fatigue is an umbrella term to describe muscles that are temporarily incapable of producing the desired effort. During brief exertions, glycogen is not used up sufficiently and the body doesn’t process the excess lactic acid. It is also possible that your horse has used up its supply of glycogen entirely.

During sustained effort, your horse can become fatigued due to a lack of sufficient glucose. Glucose is necessary to burn the fat that is its source of energy at that moment. When this source drops away, fatigue is the result. Your horse’s circulation may also be suboptimal, restricting the supply of oxygen.

Another muscle disorder is Monday morning disease, also known as azoturia or tying-up. This condition is due to a failure of the horse’s metabolism, triggering abrupt and complete muscle acidification. Common symptoms include stiffness, a shorter stride, rapid breathing and pulse, trembling, sweating, or reluctance to stand up. Causes can include excessive ingestion of sugar or starch or sudden exertion after a rest period without dietary adjustments.