Electromyography (EMG) aids in differential diagnosis of neurogenic (controlled by the nervous system) versus myogenic (taking place or functioning in ordered rhythmic fashion because of the inherent properties of cardiac muscle rather than specific neural stimuli) disorders in the muscle. The technique involves systematic placement of a needle into muscle beds and examination on a monitor of resulting activity of muscle fibers and surrounding nerves, called motor unit action potentials (MUAP). In humans, patterns produced on the EMG monitor, including duration, amplitude, and number of phases of the MUAP signal, are lowest in infants and children, increase and plateau between ages 15-65, then increase further after age 65. This latter increase is an indication of loss of small motor units and nerves that normally occurs with aging. A study was recently conducted at Utrecht University in the Netherlands to determine whether these normal aging changes occur in horses as well.

Three age groups of Dutch Warmbloods were compared–13-18 months old, four to 10 years old, and 18-21 years old. The results of EMG analysis on muscle groups in the shoulder revealed that MUAP duration and amplitude were significantly lower in the youngest horses compared to the adult and senior groups. When adults were compared to senior horses, MUAP duration and number of phases of the signal were significantly decreased in the adult group, mimicking human subjects. Therefore, senior horses could be misdiagnosed with a neurogenic abnormality based on EMG findings alone. Since Dutch Warmbloods routinely perform at high levels well into the senior years, any EMG analysis should be interpreted in light of the horse's age.

Wijnberg, I.D.; Franssen, H.; van der Kolk, J.H. American Journal of Veterinary Research 64 (1), 70-75, 2003.