Muscle problems can vary from injuries such as bruises or external trauma to cuts as well as myopathy (muscle cramps).
Severe muscle damage such as Exertional Rhabdomyolysis (Azoturia) is quite rare but very serious when it occurs and can lead to severe hindlimb lameness or stiffness along with huge increases in muscle enzyme counts (even up to 100 times the normal level). The muscle pigment can be seen in the urine as a dark red or brown/black colour as a result of the severe muscle cramps or damage.
Occasionally myopathy can be caused by a metabolic problem in muscle such as Polysaccharide Storage Myopathy (PSSM).
Muscle biopsies can be taken from the hamstring muscles in the hindlimbs to diagnose the cause of severe muscle damage. These have to handled very carefully once the biopsy is taken to prevent the subtle metabolic changes in the tissue being altered after the biopsy tissue is sent to the lab.