Blümchen, Gerhard (July 2009). "The Maya Ball Game: Comparison of the Physical Load with Modern Ball Games". Cardiology. 113 (4). Basel, Switzerland: S. Karger AG: 231–235. doi:10.1159/000203640. PMID19246899.
Blümchen, Gerhard (July 2009). "The Maya Ball Game: Comparison of the Physical Load with Modern Ball Games". Cardiology. 113 (4). Basel, Switzerland: S. Karger AG: 231–235. doi:10.1159/000203640. PMID19246899.