Some neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD), characteristically present with asymmetrical symptoms, and previous studies have shown that there is concordance for side of onset and handedness.1 ,2 Separately, the markedly higher incidence of ALS has been reported in athletes, particularly in professional soccer players in Italy, and American football players in the USA. These observations have led to debate about the possible role of physical exercise in the pathogenesis of ALS, and direct effects mediated by excitotoxicity, oxidative stress and mitochondrial dysfunction.2 ,3 Further evidence to support the role of exercise or overuse is the split hand syndrome in ALS, whereby the thenar muscles are more preferentially affected than the … [Full text of this article]