Clinical signs of PSSM range from mild to severe. They include sweating, lameness, sore muscles, undiagnosed lameness, poor performance, and muscle tremors (“tying up”). These may occur with or without exercise. Under saddle, affected horses may be reluctant to go forward or collect. Some affected horses, however, do not exhibit any clinical signs.
Source: https://ceh.vetmed.ucdavis.edu/health-topics/polysaccharide-storage-myopathy-pssm
