A recent research study using a mouse model was published in the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology comparing the soleus muscle in mice with Down syndrome and mice without Down syndrome.
The results indicated that there was not a significant difference between the two groups of mice with regards to the force production of the muscle, muscle fatigue and muscle fibers. There was not a clear indication of mitochondrial limitation between the two groups either.
There was a significant difference in muscle recovery between the two groups. There was also numerous altered pathways in Down syndrome muscle including the breakdown of proteins, metabolism of glucose and fat, and neuromuscular transmission.
The researchers concluded that the muscle itself is not responsible for muscle weakness in mice with Down syndrome and the perhaps the motor systems in the brain or the neuromusclar junction play a role in muscle weakness in Down syndrome.
Reference: Krupa, D. New Study Offers Insights Into Role of Muscle Weakness in Down Syndrome. Retreived on 12/28/12 from the American Physiological Society at http://www.the-aps.org/mm/hp/Audiences/Public-Press/For-the-Press/releases/12/44.html