Tucker's Diagnosis per MSU on 11-7-08
Postural Repetitive Myoclonus involves muscle activity and therefore could be considered as action related, but this form appears limited to postural muscles involved with weight support and is absent during voluntary movements. It occurs primarily in 2 forms, one where it affects the head and neck postural muscles of relatively young dogs, while the other occurs in the pelvic limbs of older dogs. In older dogs, postural repetitive myoclonus tremor occasionally develops in the pelvic limbs. Rarely, all 4 limbs are involved. This tremor is evident only in the relaxed standing dog and disappears or is completely masked during voluntary movement and disappears in the recumbent dog that is therefore no supporting weight. There is no evidence of tremor when the dog is walking or running, and the dog does not become fatigued. The tremor disappears when the standing dog is picked up and therefore is not supporting weight. More info on this condition can be found by Google.