Here's a recent review in Clinical Epidemiology:
http://www.dovepress.com/risk-factors-fo...ticle-CLEP
Here's the section on sports related factors:
Athleticism, chronic traumatic encephalopathy, and physical exercise
Historically, the most famous ALS patient was Lou Gehrig, the renowned US baseball player. Several studies have demonstrated increased risk of ALS among football or soccer players,111–114 other athletes,115 and individuals who engage in vigorous physical activity,116 but inconsistent results have also been reported.117–120 Strenuous physical activity, repeated head injuries, use of illicit performance-enhancing drugs, or chemicals used to treat football fields have all been discussed as potential explanations for such risk elevations.111,121 Chronic traumatic encephalopathy, a newly defined neurodegenerative disease, often resulting from repeated head injuries, has been proposed as the underlying reason or the “correct” diagnosis for ALS cases observed among professional athletes and perhaps also among military veterans.122 Different levels of physical exercise (professional versus recreational) may have very different biological effects on neurodegeneration. This is in line with previous findings of an increased risk of ALS among professional football players,111–114 although not among high school players.119 Similarly, a large European case-control study showed a 51% lower risk of ALS for organized sport, but a 59% higher risk of ALS for professional sport.123
Further efforts to disentangle the different exposure patterns involved in professional sports as compared with recreational sports will be needed to better understand these findings. Although the hypothesis that athleticism contributes to ALS is intriguing, caution should be exercised in interpreting these findings, given the fact that the vast majority are based on small numbers of ALS cases.
Here's the section on head trauma in general:
Head trauma
Early case-control studies reported a significant association between history of head trauma and ALS.184 Aiming to rule out the possibilities of recall bias and reverse causality (ie, trauma as a result, rather than a cause, of ALS), later studies generally used more objective assessment of head trauma history and excluded traumas experienced during the years immediately before the diagnosis of ALS.185,186 Severe head traumas that were hospitalized were not associated with a higher risk of ALS in Sweden.186
A possible association of ALS with milder head traumas, perhaps specifically with repeatedly experienced mild traumas, has not been thoroughly addressed.
Reading the whole article, I think it is safe to say that the jury is out on most of the non-genetic risk factors for ALS